The association of high birth weight with intelligence in young adulthood: a cohort study of male siblings.

We aimed to explore why, in population studies, the positive association between normal-range birth weight and intelligence becomes negative at the highest birth weights. The study population comprised 217,746 Norwegian male singletons born at term between 1967 and 1976. All had data on birth weight and intelligence quotient (IQ) score at the time of military conscription; 137,574 had data on sibling birth weights; and 62,906 had data on male sibling birth weights. We estimated associations between birth weight and IQ score by ordinary least squares regression for the total study population and by fixed-effects regression for comparisons of brothers. The crude mean IQ score was 1.2 points (95% confidence interval (CI): 0.3, 2.2) lower for those with birth weights of 5,000 g or more compared with the reference group (with birth weights of 4,000-4,499 g). This difference leveled off to 0.0 (95% CI: -0.8, 0.9) in multivariable ordinary least squares regression and reversed to 2.2 points (95% CI: 0.3, 4.2) higher in fixed-effects regression. Results differed mainly because, at a given birth weight, participants who had a sibling with macrosomia had a lower mean IQ score. Nevertheless, within families with 1 or more macrosomic siblings, as in other families, men with higher birth weights tended to have higher IQ scores. Thus, a family-level confounder introduces a cross-level bias that cannot be detected in individual-level studies. We suggest ways in which future studies might elucidate the nature of this confounder.

Life expectancy and cause of death in men examined at medical check-ups in 1964.

BACKGROUND: The purpose of the study was to determine the associations between blood pressure, body mass index and smoking habits, recorded at routine health check-ups with the occupational health officer, and life expectancy and cause-specific mortality through several decades. MATERIAL AND METHODS: Participants in the Linseed Oil Study were followed from 1967 through 2005 with respect to total and cause-specific mortality in the Cause of Death Registry. The population studied consisted of 15,934 men who were born between 1905 and 1914 and were in work in 1967. The analyses used various multivariate regression methods. RESULTS: A total of 15,535 participants (97.5%) had died, and the average age at death was 76.6 years (SD 9.1). Blood pressure, body mass index and cigarette smoking were related to the age of death and mortality due to cardiovascular disease, lung cancer and respiratory system diseases. High systolic blood pressure (≥ 160 mm) was associated with a life-expectancy shortening of 5 years, 15 cigarettes daily with 3.5 years and a confirmed elevated sedimentation rate with a 3.3 year shortening of life expectancy. The excess mortality persisted throughout the follow-up period. The association with blood pressure gradually lessened, while the association with smoking and body mass index did not change over time. The association with smoking was weaker than in most early studies. INTERPRETATION: The results of routine health check-ups in the occupational health service can predict lost years of life through several decades.

Impact of aerobic fitness on musculoskeletal sickness absence 5-15 years later: a cohort study of 227,201 male Norwegian employees.

OBJECTIVES: The aim of the study was to estimate associations between aerobic fitness among men at age 18-19 years and work absence due to musculoskeletal sickness 5-15 years later. METHODS: All 321,975 men born between 1967 and 1976 in Norway were identified and followed up in several national registers. Men who completed an aerobic fitness test at military conscription during 1985-1995 (N=227,201) were followed from 2000 through 2003 with respect to a first musculoskeletal absence. Cox regression was conducted to estimate HRs between aerobic fitness (high, medium, poor) and musculoskeletal absence. RESULTS: A total of 26,061 men had a musculoskeletal absence (absolute risk 0.115). Absence was associated with fitness level. Associations were confounded by other conscript characteristics (intellectual capacity, body mass index, musculoskeletal condition) and parental education level and were restricted to non-injury absence. With high fitness as reference, the adjusted non-injury HR estimates were 1.18 (95% CI 1.12 to 1.24) and 1.39 (1.31 to 1.47) for medium and poor fitness, respectively. Poor fitness men were more likely to achieve low educational attainment and employment in high-absence industries and enterprises. The impact of intellectual capacity and parental education level on absence was considerably larger than the effect from fitness. A subset analysis with fitness data of better quality yielded moderately stronger associations. CONCLUSIONS: Aerobic fitness among conscripts was moderately associated with non-injury musculoskeletal absence 5-15 years later. However, the overall impact of intellectual capacity and parental education appears to be greater than that of aerobic fitness.

Social inequalities in road traffic deaths at age 16-20 years among all 611,654 Norwegians born between 1967 and 1976: a multilevel analysis.

BACKGROUND: Road traffic injury is a major cause of death among youths. AIMS: To estimate mortality differences in family socioeconomic position (SEP) and municipal disadvantage level. METHODS: Data on all Norwegians born in 1967-76, gathered from national registries, were linked by a unique national identification number. The 611 654 participants were followed-up for 5 years from age 16 years. Parental education level, father's income level, and proportion of high-income earners in the municipality served as SEP indicators. Associations between SEP and road traffic deaths were analysed by multilevel Poisson regression. Results Road traffic deaths (n=676, rate 22.2 per 100 000 person-years) constituted a major cause of death, of which 91.9% were motor vehicle occupants. SEP distributions differed according to gender and type of motor vehicle crash (collision, non-collision). There was an inverse relationship between municipal proportions of high-income earners and mortality (population attributable fraction (PAF) 0.43, 95% CI 0.30 to 0.53) in all categories of gender-specific crash types. Family SEP gradients were not found except for male non-collision deaths, where increasing mortality was found in association with decreasing parental education level (PAF 0.94, 95% CI 0.59 to 0.99) and increasing paternal income (PAF 0.25, 95% CI 0.06 to 0.40). CONCLUSION: The different SEP patterns for road traffic deaths across gender and motor vehicle crash type illustrate that heterogeneity of social inequalities in health can be found even within narrow age bands and for similar causes of death.

Influence of early life factors on social inequalities in psychiatric outcomes among young adult Norwegian men.

BACKGROUND: Social inequalities in health can to a substantial degree be explained by social causation. However, indirect selection by early life factors has been suggested. The main aim of this study was to estimate how much adult social gradients in selected psychiatric outcomes depended on parental and individual characteristics in early life. METHODS: The population comprised all males born in Norway 1967-71 (n = 170 678). We compiled data on several social and biological variables from birth onwards from different national registers. Health outcomes were collected from the Cause of Death Register (suicide) and the Labour and Welfare Administration (psychiatric disability, psychiatric sickness absence). Indicator of socio-economic position was education level at the age of 28 years. Men were followed up between 4 and 9 years from the age of 29 years. RESULTS: Crude rates per 100 000 person-years were 21.8 (suicide), 145.7 (disability) and 1164.7 (sickness absence). Social inequalities were strong and consistent for all outcomes. Parental and individual characteristics accounted for a substantial part of the social inequalities in neurosis or personality disorder disability (44.1%) and a moderate role for inequalities in psychiatric sickness absence (25.6%), schizophrenia disability (20.7%) and suicide (17.4%). General ability at the age of 18 years had strongest influence on the social health gradients. Suicide and schizophrenia disability were associated with a combination of high parental and low own education level. CONCLUSION: This study indicates that indirect selection explains a substantial part of social inequalities in certain psychiatric outcomes and that early life prevention is important to reduce health gradients.

Dealing with emigration in cohort studies: follow-up of mortality and cancer incidence among Norwegians born between 1967 and 1976.

Emigration causes loss to follow-up. The study aim was to assess the influence of the choice of handling migration in population-based cohort studies on estimated mortality and cancer incidence in the population of origin. All persons born in Norway between 1967 and 1976 and who were not registered dead before 1992 (N = 614,176) were followed up in national registries regarding migration movements, death, and incident cancer between 1992 and 2004. A total of 40,366 (6.6%) of the study population had between 1 and 13 migration movements and 5,354 deaths and 4,447 first cancer cases were recorded during follow-up. Four different follow-up scenarios concerning migration were analysed: considering only person-time before emigration; considering person-time as national residents both before emigration and after repatriation; disregarding whether emigration took place or not; and excluding all who emigrated during follow-up. Mortality and cancer incidence rates were compared in Poisson regression models. Mortality and cancer incidence were only marginally influenced by choice of follow-up scenario. Mortality was higher after repatriation, in particular during the first year of follow-up (rate ratio 2.03; 95% confidence interval 1.02-4.03). This excess had little influence on total population rates. Cancer incidence was not affected by repatriation status. Mortality rates after repatriation were probably elevated because persons who expected to die shortly were more prone to return to their native country ("salmon bias"). The analytical choice concerning follow-up has little influence on outcome occurrences in populations with rather low migration rates. However, the best solution is apparently to censor out persons at the date of emigration in order to avoid salmon bias.

Educational attainment of Norwegian men: influence of parental and early individual characteristics.

The life course perspective in social inequalities in health research has resulted in an increased interest in status attainment processes. Adult status is commonly measured as occupational class, income level or educational attainment, and the latter was applied in this study. The study objective was to estimate the relative contribution of parental and early individual characteristics on educational attainment. The study population comprised all males born in Norway in 1967-1971, and alive at age 28 years (n=160,914). Data on social and biological variables were compiled from birth onwards in several national registers. Information on educational attainment at age 28 years was derived from Statistics Norway. Mean years of education was 12.62 years (SD 2.24). Educational attainment was strongly associated with general ability score at age 18 years and parental educational attainment. Parental income had more limited influence; all other early factors had only marginal effect. Path analysis results suggest that the direct effect of general ability was of the same size as the combined direct and indirect effect of parental education and income. The results suggest that status attainment in this young male population is mainly dependent on general ability and parental education level.

Influence of physical, mental and intellectual development on disability in young Norwegian men.

BACKGROUND: Few studies have examined the effects of educational level, intellectual performance, mental function, body mass index and height as well as their interrelationship on the risk of disability pension (DP), taking other life course factors into account. METHODS: We linked several national registers, comprising the Medical Birth Registry, the Central Population Register, the Education Register, the Norwegian Labour and Welfare Organisation and the Norwegian Armed Forces Personnel Data Base, providing personal data on biological and social variables from childhood to young adult age. Participants were all males live born during the period 1967-76, followed up through 2003. Men were excluded who died, emigrated or were granted a DP until age 23 years (when follow-up started) and persons who did not become gainfully employed during the study period. Thus, the study population comprised 302 330 men, and the study outcome was being granted a DP after age 23 years. RESULTS: 3651 men (1.2%) were granted a DP. The DP rate was inversely associated with both educational level and intellectual performance. The adjusted population attributable risks (PAR) values for these two variables were 47% and 35%, respectively. The effect of the other variables was modest. Over- and underweight and short stature were associated with DP, but the effect was largely reduced after adjusting for intellectual performance. Impaired mental function seemed to have an independent effect. CONCLUSION: Receiving an early DP is dependent on several factors acting at different stages of life, above all educational level and intellectual performance. High education can modify some of the effects of low intellectual performance.

Associations of birth size, gestational age, and adult size with intellectual performance: evidence from a cohort of Norwegian men.

The influences of prenatal and postnatal growth on intellectual performance are unclear. We examined the associations of birth size and gestational age with intellectual performance and explored whether these associations were influenced by adult body size and social factors. In this nationwide cohort study, the records of 317,761 male infants registered in the Medical Birth Registry of Norway (1967-1979) were linked to the Norwegian Conscript Service (1984-1999). The variation in intelligence test score at age 18 due to birth weight and birth length was evaluated using absolute and standardized (z scores) values. Mean intelligence score increased by gestational age, birth weight, and birth length. However, a decline in intellectual performance was observed for gestational age >41 wk and birth weight >4500 g. There was a strong interaction on intellectual performance between birth size and gestational age (p < 0.0005). Adjusting for adult size strongly attenuated the association of birth size with intellectual performance. The overall R of intellectual performance explained by birth size was <1%; however, adding adult body size and social factors to the model increased R to 12%. In conclusion, the association of birth size with intellectual performance was weak, but still present after adjustment for adult body size and social factors.

Explaining the relation between birth order and intelligence.

Negative associations between birth order and intelligence level have been found in numerous studies. The explanation for this relation is not clear, and several hypotheses have been suggested. One family of hypotheses suggests that the relation is due to more-favorable family interaction and stimulation of low-birth-order children, whereas others claim that the effect is caused by prenatal gestational factors. We show that intelligence quotient (IQ) score levels among nearly 250,000 military conscripts were dependent on social rank in the family and not on birth order as such, providing support for a family interaction explanation.

Maternal epilepsy and offsprings' adult intelligence: a population-based study from Norway.

PURPOSE: We examined if children of mothers with epilepsy had normal intelligence, speculating that either epilepsy or its therapy might affect this parameter. METHODS: In a population-based cohort study in Norway, information on maternal epilepsy reported to the Medical Birth Registry of Norway, 1967-1979 was linked to information on 18-19-year-old men's intellectual ability and anthropometric measures ascertained by the Norwegian Conscripts Service, 1984-1999. The standardized intelligence test (IQ score) was reported as single-digit standard scores with values from 1 to 9. No individual information on antiepileptic drug therapy was available. RESULTS: Mean IQ score was lower in 1,207 conscripts whose mothers had epilepsy reported on the birth notification form, as compared with 316,554 conscripts of mothers without epilepsy; 4.8 (standard deviation 1.8) versus 5.2(1.8), p < 0.001, respectively. This difference remained after adjustment for maternal education, maternal age, birth order, marital status, year of birth, and weight and length at birth. When comparing men with and without reported maternal epilepsy, the odds ratio of having an IQ score < or = 3 was 1.6 (95% confidence interval: 1.4-1.8), which was unaffected by adjustment for confounding factors. Mean height among conscripts of mothers with and without epilepsy measured 178.6 and 179.9 cm, respectively, a difference of 1.3 cm (p < 0.001). CONCLUSIONS: We observed that almost 20 years after birth, maternal epilepsy was associated with reduced IQ score and also shorter adult height in male offspring. We do not know whether these findings will persist when epilepsy is better treated with the newer and safer generation of antiepileptic drugs.

Life course determinants for early disability pension: a follow-up of Norwegian men and women born 1967-1976.

BACKGROUND: Most studies on disability pension (DP) have focused on work conditions, socio-economic status and other contemporary factors. We wanted to study possible determinants of an early DP with a life course perspective within a large register-based cohort, with a main focus on the biological and social factors from childhood. METHODS: We established a longitudinal, population-based cohort of all persons liveborn in Norway between 1967 and 1976. Through linkage between several national registers we obtained personal data on biological/health related as well as social background factors. After excluding persons who died, emigrated or were granted a DP before age 20 years (at which age follow-up started) and persons who did not become gainfully employed during the study period, the study population consisted of 595,393 persons. They were categorized into four strata according to gender and educational attainment. Adjusted hazard ratios (HR) for granting a DP until the end of 2003 and the corresponding population attributable risks (PAR) were computed. RESULTS: A total of 9,649 persons (1.6%) were granted a DP during follow-up. The disability risk was slightly higher among women than among men (1.7% vs. 1,5%). The following PARs were found: birth weight below the mean 5.7%, chronic childhood disease 6.8%, maternal marital status 4.4% and parental disability 8.8%. Low educational achievement was highly associated with DP, with a PAR more than twice as high as the overall PAR for the childhood factors. CONCLUSIONS: Early DP is associated with several biological and social background factors from childhood. It also shows a strong dependency on educational achievement.

Early life determinants of musculoskeletal sickness absence in a cohort of Norwegians born in 1967-1976.

In order to investigate the extent to which musculoskeletal sickness absence was influenced by a range of circumstances concerning family background and health in early life, we established a register-based cohort of all live-born in Norway between 1967 and 1976. Personal data on parental factors and health early in life were recorded prospectively from birth onward in the Medical Birth Registry of Norway, the National Insurance Administration, Statistics Norway, and the Central Population Register. We collected data in the National Insurance Administration on the first spell of medically certified long-term (>16 days) musculoskeletal (International Classification of Primary Care group L) sickness absence in 2000-2003 among 378, 356 participants who were considered to be at risk of sickness absence on January 1st, 2000. The 4-year musculoskeletal absence risk was 0.264 for women and 0.156 for men. Parental education level was associated with musculoskeletal sickness absence, with increasing adjusted relative risks by decreasing educational level for both genders. Associations with other early determinants (birth weight, childhood disease, parental survival, parental disability, parental income, and parental marital status) were all close to unity. Parental education level attributed 36% (95% confidence interval 33-38) to the population risk for women and 67% (64-70) for men. The parental education association was partly mediated through own educational attainment, which was strongly associated with musculoskeletal sickness absence in itself. Our data suggest that mechanisms acting early in life could influence later risk of musculoskeletal sickness absence.

What mediates the inverse association between education and occupational disability from back pain?--A prospective cohort study from the Nord-Trøndelag health study in Norway.

Low education is consistently associated with an increased risk of back pain disability, but the underlying mechanisms for this relationship are poorly understood. In a seven-year prospective observational study of 38,426 employed men and women between 25 and 59 years in Norway, we investigated to what extent occupational class, working conditions and individual lifestyle mediated the effect of formal education on disability pensioning from back pain. Each additional year of formal education was associated with decreased risk for disability pensioning from back pain for both men [age adjusted Hazard Ratio (HR) 0.77; (95% Confidence Interval, 0.72-0.82)] and women [HR 0.76(0.71-0.82)]. Adjustment for occupational class and factors related to working conditions (authority to plan own work, physically demanding work, concentration and attention and job satisfaction) and individual lifestyle (smoking, body mass index, physical exercise and alcohol consumption) reduced the effect of education by 39% [HR 0.86(0.79-0.93)] for men and by 21% [HR 0.81(0.73-0.89)] for women. Working conditions contributed most to the explanation for men, while occupational class, working conditions and life style factors contributed equally for women. Subgroup analyses indicate small differences between full-time and part-time employees, while some differences were found between subcategories of back diseases. The study indicates that there is a strong and unexplained effect of education on back pain disability pensioning, which is not mediated by occupational class, working conditions or individual lifestyle.

Associations of birth defects with adult intellectual performance, disability and mortality: population-based cohort study.

Infants born with birth defects have poorer outcomes in terms of mortality and disability, but the long-term intellectual outcome in children with birth defects is generally unknown. We assessed the long-term associations of various birth defects with mortality and disability, and evaluated whether high mortality and disability were reflected in impaired intellectual performance at age 18. In this nationwide cohort study, records of 9,186 males with and 384,384 without birth defects, registered in the Medical Birth Registry of Norway (1967-1979) were linked to the National Conscript Service (1984-1999). Mortality and disability before military draft, and intelligence test score at conscription were the main outcome measures. Males with birth defects had a relative risk for disability of 6.0 compared with males without defects. Disability was low within categories of birth defects associated with low mortality, and high within defect categories associated with high mortality. The relative risk for not being drafted was highest if maternal educational level was low. Heart defects and cleft palate were the only subgroups in which intellectual performance was lower after adjustment for maternal education, maternal age, marital status and birth order. In particular, intellectual performance was not impaired among those with multiple compared with single defects. We conclude that for the majority of birth defect categories in the present birth cohort, our hypothesis that intellectual performance would be impaired was not confirmed. Thus, there seems to be little reason to fear an adverse intellectual outcome in non-disabled surviving infants with birth defects.

[A follow up of persons who received basic and/or supplemental benefits in childhood]. / Oppfølging av personer som fikk grunnstønad og/eller hjelpestønad som barn.

BACKGROUND: Consequences of chronic diseases in childhood with respect to health, educational achievement and participation in the labour force in young adult age are evaluated. MATERIAL AND METHODS: A total of 14,364 children (2.3%) of the 626,928 born in Norway 1967-76 received basic and /or supplemental benefits for at least one year of the age span 0-16 years. The more common diagnoses included Endocrine diseases (diabetes), disease classified under Mental diagnoses, Neurological diseases and Congenital malformations. All the children were followed up to the age of 27 with respect to mortality and disability pensioning and to the age of 25 with regard to education, participation in the labour force and income, and in addition national service for the men. The study was made possible through the linking of data from several national registers, performed by Statistics Norway. Before the file was released for analyses, all personal identification was removed. RESULTS: Basic and supplementary benefits in childhood predict adverse outcomes in young adult age: mortality, disability, low education, lack of gainful employment and low pensionable income. The diagnosis registered with the benefit strongly influenced the outcomes. Conscripts who had received benefits were judged to have a mean score for general ability of 4.5 compared to 5.2. Adjusted for score for general ability the proportion of those having received benefits achieving higher education was 84% of that of those that had not received benefits. After adjustment for educational attainment, the percentage gainfully employed was 11-12% less among subjects having received benefits in childhood. This negative association between having received a benefit in childhood and gainful employment in adulthood was restricted to the low educational group. INTERPRETATION: Persons with health problems in childhood did not achieve the education, employment and income they should have been capable of judged by their general ability and the education they had actually achieved.

[Chronic diseases among and social benefits to four-to-five-year-old children]. / Kroniske sykdommer og trygdeytelser blant 4-5-åringer.

BACKGROUND: The purpose of this study was to estimate the prevalence of chronic diseases among four-to-five-year-old children in a Norwegian county in order to identify significant prognostic factors and to investigate the use of social benefits. METHODS: In the context of ordinary medical examinations in a Norwegian county, a questionnaire was given to 2430 parents, of whom 1913 (79%) responded. Children with chronic disease and with frequent episodes of disease were selected for further examination. Of the 391 children selected, 263 (69%) attended. The parents answered questions about diagnosis, prognosis, marital status, parents' health, use of social benefits, and the development of the disease. RESULTS: Almost half of the 263 children who met for further examination had airway diseases, nearly all of them asthma. Of the remaining, 17% had congenital malformations, 13% had neurological development disorders, 10% had skin and joint diseases, and 12% various conditions including gastrointestinal diseases and hormonal disorders. One third had recovered, one third had diminishing problems, and one third was unchanged or worse. Parents of 15 children born in 1991 who received basic or supplementary benefits stated that the child had recovered or had a diminishing problem. However, 53 children who had a stable or deteriorating condition did not receive any benefits. In 22 of these, the parents stated that their child's general functioning was significantly affected. It is possible that they would have met criteria for supplementary and/or basic benefits. INTERPRETATION: The results suggest that in 1996, 13% of four-to-five-year-old children in this county had a chronic disease. This finding is in line with other studies. Overuse and, more frequently, underuse of social benefits for these children was disclosed.

Size at birth and gestational age as predictors of adult height and weight.

BACKGROUND: Both birth length and birth weight are associated with height in adulthood and may have independent contributions to adult body size, but the effects of gestational age on these associations have not been fully evaluated. Our objective was to examine the independent contributions of gestational age, and of length and weight at birth, on adult (age 18 years) height and weight, with a special focus on the effects of being born preterm. METHODS: In this nationwide cohort study, records of 348,706 male infants included in the Medical Birth Registry of Norway (1967-1979) were linked to the Norwegian Conscripts Service (1984-1999). Complete follow-up information, including deaths, emigration, and disability pension, was obtained for 94%. We analyzed length and weight at birth using standardized (z-scores) values and stratified by gestational age. RESULTS: The positive association between birth length and adult height was stronger than between birth weight and adult weight (R = 7-9% compared with <0.1%, respectively). The strongest associations were seen among those born at gestational age 39 to 41 weeks. The effects of birth length on adult height, and of birth weight on adult weight, were considerably less among preterm births than among term births. Length and weight at birth each contributed independently to adult stature and body weight. The increase in adult weight per relative birth weight category was greatest for infants who were both heavy and long at birth. CONCLUSIONS: Birth length is perhaps a better predictor of adult height and weight than birth weight, and should be considered as a possible risk factor for adult morbidity and mortality.

Breech delivery and intelligence: a population-based study of 8,738 breech infants.

OBJECTIVE: Long-term intellectual performance in breech-presented infants may be negatively affected by vaginal delivery. We evaluated the effect of presentation at birth and delivery mode on intellectual performance at age 18 years in a nationwide population study. METHODS: We studied 8,738 male infants in breech and 384,832 males in cephalic presentation registered in the Medical Birth Registry of Norway, 1967-1979, and linked to data registered at the National Conscript Service, 1984-1999. Test scores of intelligence testing at conscription were presented as standard nine ("stanine") scores. Mean stanine scores and odds ratios of low score were computed and adjusted for birth order, maternal age, and education. RESULTS: Mean stanine score was slightly higher among breech-presented males than among cephalic-presented males (5.26 versus 5.22, P = .05), whereas after adjustment the difference disappeared (P = .3). Breech-presented infants had lower mean scores if delivered by cesarean compared with vaginal breech delivery (P = .03), and cephalic-presented males scored lower if their mothers had a cesarean delivery instead of a vaginal delivery (P < .001). Comparing cesarean and vaginal delivery in breech births, the odds ratio of having a stanine score less than or equal to 3 was 1.12 (95% confidence interval 0.92,1.36), after adjustment for confounding factors. CONCLUSION: Presentation at birth did not affect adult intellectual performance. Cesarean delivery of breech-presented infants did not improve adult intellectual performance when compared with a vaginal delivery. The excess perinatal hazards of breech-presented infants with a vaginal delivery were not reflected in adult intellectual performance.