Late reproduction at lower risk in Sardinia island: a case of reproductive longevity?

Population of Sardinia Island is characterized by geographical and historical isolation, a deep-rooted tendency to postpone childbearing, and a demographic and genetic structure largely different from that of continental Italy and other European countries. Based on such differences we investigated whether the risk of adverse pregnancy outcome associated with late reproduction was different between the Sardinian population and two mainland areas (North+Centre and South+Sicily), representative of different Italian socio-economic contexts. In particular we suggest that parents, who show aptitude to late childbearing associated with a reduced risk of adverse pregnancy outcome, enjoy "reproductive longevity". Data set come from the National Institute of Statistics and concern all 1990-98 Italian birth records (n=4 830 742). We considered three types of adverse pregnancy outcome: i) stillbirths of the total births, ii) very preterm births (<32 gestation weeks) of the live births, iii) very low birthweight births (<1.5 Kg) of the live births after exclusion of very preterm births. Using logistic regression models we investigated whether the risk of adverse pregnancy outcome associated with maternal or paternal ageing followed differing trends between areas. Moreover we evaluated the Odds Ratio of the three types of adverse outcome as a function of maternal and paternal age and education, and delivery rank. We found that in the three areas the risks increase with parental ageing, but in Sardinia the increase is less rapid than in the mainland. In particular with respect to South+Sicily, in Sardinian mothers=35 years and fathers=40 years the OR is 25% lower for stillbirth and preterm birth, and 19% lower for low birthweight. We suggest that the aptitude to late and successful childbearing may be a peculiar trait of the Sardinian population, indicative of "reproductive longevity". The possibility of identifying populations where a relevant proportion of mothers show the character might promote case-control studies focused on the possible determinants and measures of prevention and/or protection against the adverse effect of late reproduction.

Gestational age shortening in single births at term. Italy 1990-1998.

The increasing incidence of moderate preterm births (32-36 gestation weeks) might reflect a more general tendency toward a shortening of the gestational length occurring also in the term births (37 + gestation weeks). We examined all Italian 1st live born singletons (n = 2,356,365) and found that from 1990 to 1998 the births of 40 + gestation weeks decreased from 60.7 to 51.7% and among term births the average gestation weeks decreased from 39.74 to 39.55. In term pregnancies the effect of low education and advanced age of the mother in decreasing the gestational length persisted over time, but, independently of the maternal factors, the pregnancies experienced a progressive shortening. The finding, if confirmed for other countries, should deserve further investigations on possible determinants, as improved estimates of gestational age through the widespread use of prenatal ultrasound or diffusion of hazardous and stressing working and living conditions.

Delayed maternity and risk at delivery.

We evaluated the effect of the mother's age on the risk of unfavourable pregnancy outcome. The study considered 1348 190 first-born and 957 689 second-born babies born in Italy in 1990-94. The risk of stillbirth, preterm birth in liveborns, and low birthweight in liveborns at term, was first evaluated separately and then globally as a function of maternal age, education and parity. Older (> or = 35 years) mothers were found to run an increased risk, both on single and on global evaluation, with respect to their younger counterparts. Primiparae > or = 35 years of age, of low education, ran a global risk threefold higher (3.14 [3.02, 3.26]) than the young highly educated secondiparae who were the lowest risk (4.64% of babies at risk). From 26 years the global risk rose with each advancing year of maternal age, but parity and education modified the age effect. Linear fitting of the proportion of mothers > or = 35 since 1984 indicates that by the year 2025 about 25% of the mothers may face late child bearing.

Trends in childbearing and stillbirth risk: heterogeneity among Italian regions.

In Italy, as in all Western countries, the almost monotonic decline in fertility observed since the 1960s has been paralleled until the beginning of the 1980s by a decrease in maternal age at delivery. Since then, age at marriage and at childbearing has been increasing and marital fertility has continued to decrease. By 1994 Italy showed extreme values of low total fertility rate (1.22) and of high mean maternal age at delivery (29.7). For the period between 1960 and 1994 we identified five U-shaped patterns in maternal age at delivery corresponding to five geographical areas, which differ socioeconomically and culturally. Since it is well known that an increase in the maternal age is accompanied by an increase in the risk of unfavorable pregnancy outcome, we estimated the stillbirth risk run by older (> or =35 years) mothers who delivered in 1994, with respect to their younger counterparts. The differences between the areas are reflected in the higher risk in southern compared to northern Italy: the maximum value occurred in Sicily (odds ratio 2.02, 95% confidence interval, 1.51-2.70) and the minimum value, even lower than in the north but not statistically significant, was found in Sardinia (odds ratio 1.25, 95% confidence interval, 0.81-1.91), known to be characterized by peculiar cultural and biological features.

Birthweight by gestational age in preterm babies according to a Gaussian mixture model.

OBJECTIVE: To provide a statistically sound criterion for identifying implausibly large birthweights for gestational age. DESIGN: Review of ISTAT 1990-1994 national newborn records. SETTING: Italy POPULATION: Forty-two thousand and twenty-nine single first and second liveborn preterm babies. METHODS: Two-component Gaussian mixture models are used to describe the birthweight distributions stratified by gestational age. Implausibly large babies are identified through model-based probabilistic clustering. MAIN OUTCOME MEASURES: Gestational age misclassification and weight-for-gestational age centile curves RESULTS: Gestational age appears under-estimated by about six weeks in 12.3% of the cases. Large babies are equally present in males and females, but are more frequent in second-borns than in first-borns, even when parity-specific models are fitted. CONCLUSIONS: The approach allows for a quantification of the gestational age under-estimate error and for data correction through model-based clustering. Correct birthweight distributions and growth curves are also provided.