Heterogeneity in the risk of cardiovascular disease mortality after the hypertensive disorders of pregnancy across mothers' lifetime reproductive history.

BACKGROUND: Prior studies on maternal cardiovascular disease (CVD) mortality and hypertensive disorders of pregnancy (HDP) have focused only on a woman's first birth and have not accounted for successive affected pregnancies. OBJECTIVES: The objective of this study is to identify mothers' risk of CVD mortality considering lifetime reproductive history. METHODS: We used data from the Medical Birth Registry of Norway, the Norwegian Cause of Death Registry, and the Norwegian National Population Register to identify all mothers who gave birth from 1967 to 2020. Our outcome was mothers' CVD death before age 70. The primary exposure was the lifetime history of HDP. The secondary exposure was the order of HDP and gestational age at delivery of pregnancies with HDP. We used Cox regression models to estimate hazard ratio (HR) and 95% confidence interval (CI), adjusting for education, mother's age, and year of last birth. These models were stratified by the lifetime number of births. RESULTS: Among 987,378 mothers, 86,294 had HDP in at least one birth. The highest CVD mortality, relative to mothers without HDP, was among those with a pre-term HDP in their first two births, although this represented 1.0% of mothers with HDP (HR 5.12, 95% CI 2.66, 9.86). Multiparous mothers with term HDP in their first birth only had no increased risk of CVD relative to mothers without HDP (36.9% of all mothers with HDP; HR 1.12, 95% CI 0.95, 1.32). All other mothers with HDP had a 1.5- to 4-fold increased risk of CVD mortality. CONCLUSIONS: This study identified heterogeneity in the risk of CVD mortality among mothers with a history of HDP. A third of these mothers are not at higher risk compared to women without HDP, while some less common patterns of HDP history are associated with severe risk of CVD mortality.

Risk of adverse pregnancy outcomes in twin- and singleton-born women: An inter-generational cohort study.

OBJECTIVE: To compare the risk of adverse pregnancy outcomes between twin-born and singleton-born women. We also evaluated whether in utero exposure to pre-eclampsia or preterm delivery affected adverse pregnancy outcomes in women's own pregnancies. DESIGN: Population-based cohort study. SETTING: Medical Birth Registry of Norway 1967-2020. POPULATION: 9184 twin-born and 492 894 singleton-born women during 1967-2005, with their later pregnancies registered during 1981-2020. METHODS: Data from an individual's birth were linked to their later pregnancies. We used generalised linear models with log link binomial distribution to obtain exponentiated regression coefficients that estimated relative risks (RRs) with 95% confidence intervals (CIs) for associations between twin- or singleton-born women and later adverse pregnancy outcomes. MAIN OUTCOME MEASURES: Pre-eclampsia, preterm delivery or perinatal loss in twin-born compared with singleton-born women. RESULTS: There was no increased risk for adverse outcomes in twin-born compared with singleton-born women: adjusted RRs for pre-eclampsia were 1.00 (95% CI 0.93-1.09), for preterm delivery 0.96 (95% CI 0.90-1.02) and for perinatal loss 1.00 (95% CI 0.84-1.18). Compared with singleton-born women exposed to pre-eclampsia in utero, twin-born women exposed to pre-eclampsia had lower risk of adverse outcomes in their own pregnancies; the aRR for pre-eclampsia was 0.73 (95% CI 0.58-0.91) and for preterm delivery was 0.71 (95% CI 0.56-0.90). Compared with preterm singleton-born women, preterm twin-born women did not differ in terms of risk of pre-eclampsia (aRR 1.05, 95% CI 0.92-1.21) or perinatal loss (aRR 0.99, 95% CI 0.71-1.37) and had reduced risk of preterm delivery (RR 0.83, 95% CI 0.74-0.94). CONCLUSIONS: Twin-born women did not differ from singleton-born women in terms of risk of adverse pregnancy outcomes. Twin-born women exposed to pre-eclampsia in utero, had a lower risk of pre-eclampsia and preterm delivery compared with singleton-born women exposed to pre-eclampsia.

Maternal Fiber Intake During Pregnancy and Development of Attention-Deficit/Hyperactivity Disorder Symptoms Across Childhood: The Norwegian Mother, Father, and Child Cohort Study.

BACKGROUND: Epidemiological studies suggest that maternal diet quality during pregnancy may influence the risk of neurodevelopmental disorders in offspring. Here, we investigated associations between maternal intake of dietary fiber and attention-deficit/hyperactivity disorder (ADHD) symptoms in early childhood. METHODS: We used longitudinal data of up to 21,852 mother-father-child trios (49.2% female offspring) from MoBa (the Norwegian Mother, Father, and Child Cohort Study). The relationships between maternal fiber intake during pregnancy and offspring ADHD symptoms at ages 3, 5, and 8 years were examined using 1) multivariate regression (overall levels of ADHD symptoms), 2) latent class analysis (subclasses of ADHD symptoms by sex at each age), and 3) latent growth curves (longitudinal change in offspring ADHD symptoms). Covariates were ADHD polygenic scores in child and parents, total energy intake and energy-adjusted sugar intake, parental ages at birth of the child, and sociodemographic factors. RESULTS: Higher maternal prenatal fiber intake was associated with lower offspring ADHD symptom scores at all ages (Bage3 = -0.14 [95% CI, -0.18 to -0.10]; Bage5 = -0.14 [95% CI, -0.19 to -0.09]; Bage8 = -0.14 [95% CI, -0.20 to -0.09]). Of the derived low/middle/high subclasses of ADHD symptoms, fiber was associated with lower risk of belonging to the middle subclass for boys and girls and to the high subclass for girls only (middle: odds ratioboys 0.91 [95% CI, 0.86 to 0.97]/odds ratiogirls 0.86 [95% CI, 0.81 to 0.91]; high: odds ratiogirls 0.82 [95% CI, 0.72 to 0.94]). Maternal fiber intake and rate of change in child ADHD symptoms across ages were not associated. CONCLUSIONS: Low prenatal maternal fiber intake may increase symptom levels of ADHD in offspring during childhood, independently of genetic predisposition to ADHD, unhealthy dietary exposures, and sociodemographic factors.

The relationship between cesarean delivery and fecundability: a population-based cohort study.

BACKGROUND: Previous studies have found that women who undergo cesarean delivery have fewer pregnancies. Cesarean delivery is also more common among women with lower fecundability. The potential role of cesarean delivery in reduced fecundability is not known. OBJECTIVE: This study aimed to assess the bidirectional relationship between cesarean delivery and fecundability. STUDY DESIGN: This was a prospective cohort study based on data from the Norwegian Mother, Father, and Child Cohort study linked with the Medical Birth Registry of Norway. We estimated the fecundability ratio (per cycle probability of pregnancy) and relative risk of infertility (time to pregnancy ≥12 months) by mode of delivery in the previous delivery among 42,379 women. For the reverse association, we estimated the relative risk of having a cesarean delivery by fecundability (the number of cycles women needed to conceive) among 74,024 women. RESULTS: The proportion of women with infertility was 7.3% (2707/37,226) among women with a previous vaginal delivery and 9.9% (508/5153) among women with a previous cesarean delivery, yielding an adjusted relative risk of 1.21 (95% confidence interval, 1.10-1.33). Women with a previous cesarean delivery also had a lower fecundability ratio (0.90; 95% confidence interval, 0.88-0.93) than women with a previous vaginal delivery. When assessing the reverse association between fecundability and cesarean delivery, we found that women who did not conceive within 12 or more cycles had a higher risk for cesarean delivery (adjusted relative risk, 1.57; 95% confidence interval, 1.48-1.66) than women who conceived within the first 2 cycles. The associations remained after controlling for sociodemographic and clinical risk factors and were observed across parity groups. CONCLUSION: Among women with more than 1 child, those who had a previous cesarean delivery subsequently had a lower fecundability ratio and an increased infertility risk than those who had a vaginal delivery. However, women who needed a longer time to conceive were also more prone to be delivered by cesarean delivery, indicating a bidirectional relationship between cesarean delivery and fecundability. This could suggest a common underlying explanatory mechanism and that the surgical procedure itself may not or only partly directly influence fecundability.

Pregnancy complications in last pregnancy and mothers' long-term cardiovascular mortality: does the relation differ from that of complications in first pregnancy? A population-based study.

BACKGROUND: Women who experience complications in first pregnancy are at increased risk of cardiovascular disease (CVD) later in life. Little corresponding knowledge is available for complications in later pregnancies. Therefore, we assessed complications (preeclampsia, preterm birth, and offspring small for gestational age) in first and last pregnancies and the risk of long-term maternal CVD death, taking women´s complete reproduction into account. DATA AND METHODS: We linked data from the Medical Birth Registry of Norway to the national Cause of Death Registry. We followed women whose first birth took place during 1967-2013, from the date of their last birth until death, or December 31st 2020, whichever occurred first. We analysed risk of CVD death until 69 years of age according to any complications in last pregnancy. Using Cox regression analysis, we adjusted for maternal age at first birth and level of education. RESULTS: Women with any complications in their last or first pregnancy were at higher risk of CVD death than mothers with two-lifetime births and no pregnancy complications (reference). For example, the adjusted hazard ratio (aHR) for women with four births and any complications only in the last pregnancy was 2.85 (95% CI, 1.93-4.20). If a complication occurred in the first pregnancy only, the aHR was 1.74 (1.24-2.45). Corresponding hazard ratios for women with two births were 1.82 (CI, 1.59-2.08) and 1.41 (1.26-1.58), respectively. CONCLUSIONS: The risk for CVD death was higher among mothers with complications only in their last pregnancy compared to women with no complications, and also higher compared to mothers with a complication only in their first pregnancy.

Birth Weight in Consecutive Pregnancies and Maternal Cardiovascular Disease Mortality Among Spontaneous and Iatrogenic Term Births: A Population-Based Cohort Study.

Knowledge on the association between offspring birth weight and long-term risk of maternal cardiovascular disease (CVD) mortality is often based on firstborn infants without consideration of women's consecutive births. We studied long-term CVD mortality according to offspring birth weight patterns among women with spontaneous and iatrogenic term deliveries in Norway (1967-2020). We constructed birth weight quartiles (Qs) by combining standardized birth weight with gestational age in quartiles (Q1, Q2/Q3, and Q4) for the women's first 2 births. Mortality was estimated using Cox regression and expressed as hazard ratios (HRs) with 95% confidence intervals (CIs). Changes in offspring birth weight quartiles were associated with long-term maternal CVD mortality. Compared with women who had 2 term infants in Q2/Q3, women with a first offspring in Q2/Q3 and a second in Q1 had higher mortality risk (HR = 1.33, 95% CI: 1.18, 1.50), while risk was lower if the second offspring was in Q4 (HR = 0.78, 95% CI: 0.67, 0.91). The risk increase associated with having a first infant in Q1 was eliminated if the second offspring was in Q4 (HR = 0.99, 95% CI: 0.75, 1.31). These patterns were similar for women with iatrogenic and spontaneous deliveries. Inclusion of information from subsequent births revealed heterogeneity in maternal CVD mortality which was not captured when using only information based on the first offspring.

Long-term cardiovascular mortality in women with twin pregnancies by lifetime reproductive history.

BACKGROUND: Women with one lifetime singleton pregnancy have increased risk of cardiovascular disease (CVD) mortality compared with women who continue reproduction particularly if the pregnancy had complications. Women with twins have higher risk of pregnancy complications, but CVD mortality risk in women with twin pregnancies has not been fully described. OBJECTIVES: We estimated risk of long-term CVD mortality in women with naturally conceived twins compared to women with singleton pregnancies, accounting for lifetime number of pregnancies and pregnancy complications. METHODS: Using linked data from the Medical Birth Registry of Norway and the Norwegian Cause of Death Registry, we identified 974,892 women with first pregnancy registered between 1967 and 2013, followed to 2020. Adjusted hazard ratios (aHR) with 95% confidence intervals (CI) for maternal CVD mortality were estimated by Cox regression for various reproductive history (exposure categories): (1) Only one twin pregnancy, (2) Only one singleton pregnancy, (3) Only two singleton pregnancies, (4) A first twin pregnancy and continued reproduction, (5) A first singleton pregnancy and twins in later reproduction and (6) Three singleton pregnancies (the referent group). Exposure categories were also stratified by pregnancy complications (pre-eclampsia, preterm delivery or perinatal loss). RESULTS: Women with one lifetime pregnancy, twin or singleton, had increased risk of CVD mortality (adjusted hazard [HR] 1.72, 95% confidence interval [CI] 1.21, 2.43 and aHR 1.92, 95% CI 1.78, 2.07, respectively), compared with the referent of three singleton pregnancies. The hazard ratios for CVD mortality among women with one lifetime pregnancy with any complication were 2.36 (95% CI 1.49, 3.71) and 3.56 (95% CI 3.12, 4.06) for twins and singletons, respectively. CONCLUSIONS: Women with only one pregnancy, twin or singleton, had increased long-term CVD mortality, however highest in women with singletons. In addition, twin mothers who continued reproduction had similar CVD mortality compared to women with three singleton pregnancies.

Cesarean delivery in Norwegian nulliparous women with singleton cephalic term births, 1967-2020: a population-based study.

BACKGROUND: Nulliparous women contribute to increasing cesarean delivery in the Nordic countries and advanced maternal age has been suggested as responsible for rise in cesarean delivery rates in many developed countries. The aim was to describe changes in cesarean delivery rates among nulliparous women with singleton, cephalic, term births by change in sociodemographic factors across 50 years in Norway. METHODS: We used data from the Medical Birth Registry of Norway and included 1 067 356 women delivering their first, singleton, cephalic, term birth between 1967 and 2020. Cesarean delivery was described by maternal age (5-year groups), onset of labor (spontaneous, induced and pre-labor CD), and time periods: 1967-1982, 1983-1998 and 1999-2020. We combined women's age, onset of labor and time period into a compound variable, using women of 20-24 years, with spontaneous labor onset during 1967-1982 as reference. Multivariable regression models were used to estimate adjusted relative risk (ARR) of cesarean delivery with 95% confidence interval (CI). RESULTS: Overall cesarean delivery increased both in women with and without spontaneous onset of labor, with a slight decline in recent years. The increase was mainly found among women < 35 years while it was stable or decreased in women > = 35 years. In women with spontaneous onset of labor, the ARR of CD in women > = 40 years decreased from 14.2 (95% CI 12.4-16.3) in 1967-82 to 6.7 (95% CI 6.2-7.4) in 1999-2020 and from 7.0 (95% CI 6.4-7.8) to 5.0 (95% CI 4.7-5.2) in women aged 35-39 years, compared to the reference population. Despite the rise in induced onset of labor over time, the ARR of CD declined in induced women > = 40 years from 17.6 (95% CI 14.4-21.4) to 13.4 (95% CI 12.5-14.3) while it was stable in women 35-39 years. CONCLUSION: Despite growing number of Norwegian women having their first birth at a higher age, the increase in cesarean delivery was found among women < 35 years, while it was stable or decreased in older women. The increase in cesarean delivery cannot be solely explained by the shift to an older population of first-time mothers.

Association of sweetened carbonated beverage consumption during pregnancy and ADHD symptoms in the offspring: a study from the Norwegian Mother, Father and Child Cohort Study (MoBa).

PURPOSE: Intrauterine exposures influence offspring health and development. Here we investigated maternal intake of sweetened carbonated beverages (SCB) during pregnancy and its association with ADHD symptoms in the offspring. METHODS: This study was based on the Norwegian Mother, Father and Child Cohort Study (MoBa) and the Medical Birth Registry of Norway. Maternal diet mid-pregnancy was assessed using a food frequency questionnaire (FFQ). All mothers who responded to the FFQ and a questionnaire when their child was 8 years of age were included (n = 39,870). The exposure was defined as maternal intake (daily servings) of SCB, using no daily intake as reference. Outcome was offspring ADHD symptoms, evaluated as a continuous standardized ADHD score and as a binary outcome of six or more ADHD symptoms vs. five symptoms or less. Associations were analysed using log-binomial regression and linear mixed regression models with adjustment for covariates. RESULTS: The adjusted regression coefficients for the standardized ADHD offspring symptom score were 0.31 [95% confidence intervals (0.001, 0.62)] and 0.46 (0.15, 0.77) for maternal daily intake of ≥ 1 glasses of SCB, when the models included adjustments for total energy intake or energy intake from other sources than SCBs and sweet drinks, respectively. The corresponding adjusted relative risks were 1.16 (1.004, 1.34) and 1.21. (1.05, 1.39) for drinking ≥ 1 glasses daily. CONCLUSION: In a large pregnancy cohort with offspring followed until 8 years of age, we found an association between maternal daily intake of SCB and offspring ADHD symptoms. These results suggest a weak positive relationship between prenatal exposure to SCB and offspring ADHD.

Diet quality, stress and common mental health problems: A cohort study of 121,008 adults.

BACKGROUND & AIMS: Overall diet quality may partially mediate the detrimental effects of stress and neuroticism on common mental health problems: stressed and/or neurotic individuals may be more prone to unhealthy dietary habits, which in turn may contribute to depression and anxiety. Lifestyle interventions for depressed, anxious or at-risk individuals hinge on this idea, but evidence to support such pathway is missing. Here, we aim to prospectively evaluate the role of overall diet quality in common pathways to developing depression and anxiety. METHODS: At baseline, N = 121,008 individuals from the general population (age 18-93) completed an extensive food frequency questionnaire, based on which overall diet quality was estimated. Participants also reported on two established risk factors for mental health problems, i.e. past-year stress exposure (long-term difficulties, stressful life-events) and four neuroticism traits (anger-hostility, self-consciousness, impulsivity, vulnerability). Depression and anxiety were assessed at baseline and follow-up (n = 65,342, +3.6 years). Overall diet quality was modeled as a mediator in logistic regression models predicting the development of depression and anxiety from common risk factors. RESULTS: High stress and high neuroticism scores were - albeit weakly - associated with poorer diet quality. Poor diet quality, in turn, did not predict mental health problems. Overall diet quality did not mediate the relationship between stress/neuroticism and common mental health problems: effects of stress, neuroticism and stress-by-neuroticism interactions on mental health problems at follow-up consisted entirely of direct effects (98.6%-100%). CONCLUSIONS: Diet quality plays no mediating role in two established pathways to common mental health problems. As overall diet quality was reduced in stressed and neurotic individuals, these groups may benefit from dietary interventions. However, such interventions are unlikely to prevent the onset or recurrence of depression and anxiety.

Evaluation of incomplete maternal smoking data using machine learning algorithms: a study from the Medical Birth Registry of Norway.

BACKGROUND: The Medical Birth Registry of Norway (MBRN) provides national coverage of all births. While retrieval of most of the information in the birth records is mandatory, mothers may refrain to provide information on her smoking status. The proportion of women with unknown smoking status varied greatly over time, between hospitals, and by demographic groups. We investigated if incomplete data on smoking in the MBRN may have contributed to a biased smoking prevalence. METHODS: In a study population of all 904,982 viable and singleton births during 1999-2014, we investigated main predictor variables influencing the unknown smoking status of the mothers' using linear multivariable regression. Thereafter, we applied machine learning to predict annual smoking prevalence (95% CI) in the same group of unknown smoking status, assuming missing-not-at-random. RESULTS: Overall, the proportion of women with unknown smoking status was 14.4%. Compared to the Nordic country region of origin, women from Europe outside the Nordic region had 15% (95% CI 12-17%) increased adjusted risk to have unknown smoking status. Correspondingly, the increased risks for women from Asia was 17% (95% CI 15-19%) and Africa 26% (95% CI 23-29%). The most important machine learning prediction variables regarding maternal smoking were education, ethnic background, marital status and birth weight. We estimated a change from the annual observed smoking prevalence among the women with known smoking status in the range of - 5.5 to 1.1% when combining observed and predicted smoking prevalence. CONCLUSION: The predicted total smoking prevalence was only marginally modified compared to the observed prevalence in the group with known smoking status. This implies that MBRN-data may be trusted for health surveillance and research.

Attention-deficit/hyperactivity disorder and smoking habits in pregnant women.

BACKGROUND: Attention-deficit/hyperactivity disorder (ADHD) has been associated with an increased risk of tobacco smoking, and more difficulties with smoking cessation compared to non-ADHD individuals. Women with ADHD may therefore show elevated rates of smoking during pregnancy. AIMS: To examine the association between ADHD and smoking habits among pregnant women in Sweden and Norway. METHODS: Women pregnant for the first time were identified in Sweden (n = 622,037), and Norway (n = 293,383), of which 1.2% (n = 7,444), and 1.7% (n = 4,951) were defined as having ADHD, respectively. Data on smoking habits were collected early and late in pregnancy. RESULTS: In Sweden, ADHD was associated with an increased risk of smoking early in pregnancy, adjusted risk ratio (adjRR) 2.69 (95% confidence interval, 2.58-2.81), and late in pregnancy, adjRR 2.95 (2.80-3.10). Similar findings were observed in the Norwegian data, early in pregnancy, adjRR 2.31 (2.21-2.40), and late in pregnancy, adjRR 2.56 (2.42-2.70). Women with ADHD were more likely to continue smoking during pregnancy, compared to women without ADHD, both in Sweden adjRR 1.13 (1.10-1.17), and in Norway, adjRR 1.16 (1.12-1.20). Having a sibling diagnosed with ADHD was associated with an increased risk of smoking early and late in pregnancy, in both Sweden and Norway. CONCLUSIONS: Women with ADHD are considerably more likely to smoke early and late in (their first) pregnancy and are less likely to stop smoking between the two time points. Smoking, early and late in pregnancy, co-aggregates in families with ADHD. Smoking prevention and intervention programs should be targeted towards women with ADHD, specifically during their childbearing years, to ensure better mother and child outcomes.

Development of Social and Demographic Differences in Maternal Smoking Between 1999 and 2014 in Norway.

INTRODUCTION: The Medical Birth Registry of Norway provides national coverage of all births in Norway. In this study trends of smoking at the beginning of pregnancy and the variation of these trends across different sociodemographic groups are analyzed. This knowledge is important for planning interventions both in the general population and specifically among pregnant women. METHODS: All births registered in Medical Birth Registry of Norway 1999-2014 with information on mothers' smoking status were included in the overall analyses of the smoking trends (806 298). Records from 210 268 births in two time periods 1999-2000 and 2013-2014 were selected, and for the multivariate analyses we used general linear models to provide adjusted risk ratios. RESULTS: The prevalence of maternal smoking at the beginning of pregnancy decreased from 25% to 8% over the 15-year period. Mothers with low, medium, and high education, respectively, had a 46% (RR 0.54, CI 0.52-0.55), 62% (RR 0.38, CI 0.37-0.40), and 80% (RR 0.20, CI 0.19-0.22) reduction in maternal smoking between the two time periods. Similarly, the decline in smoking was greater for mothers who were married or living together (64%; RR 0.36, CI 0.35-0.37) than for single mothers (39%; RR 0.61, CI 0.58-0.64). Immigrants had a lower smoking prevalence than Norwegians in the entire period 1999-2014. CONCLUSION: The prevalence of smoking during pregnancy in Norway decreased in all population groups, but the relative differences between the educational groups increased from 1999-2000 to 2013-2014. This is a public health concern and a strong contributor to health disparities. IMPLICATIONS: Women with low and medium education level and single mothers had the highest smoking prevalence and the lowest decline over the two time periods. These groups constitute 45% of our study population, the relatively high maternal smoking in these groups continues to be of public health concern for maternal and child health.

Can 'early programming' be partly explained by smoking? Results from a prospective, population-based cohort study.

BACKGROUND: Numerous studies have focused the association between low birthweight and later disease. Our objective was to study the association between birthweight and later adult smoking and thereby explore a possible mechanism for the association between low birthweight and later adult disease. METHODS: We studied associations between birthweight of women (n=247704) born in 1967-1995 and smoking habits at the end of their pregnancy 13-42 years later in a prospective, population-based cohort study from The Medical Birth Registry of Norway. Similarly, the association between birthweight of men (n=194393) and smoking habits of their partners were assessed. Finally, we studied the relation between smoking habits of the participating women and the cause specific death of their mothers (n=222808). RESULTS: Twenty per cent of women with birthweight less than 2000 g were adult daily smokers compared with 11% with birthweight 4000-4499 g [relative risk=1.8, 95% confidence interval 1.4, 2.2]. Similarly, we found an association between men's birthweight and their partners smoking habits. Mothers of smoking women had doubled risk of dying from lung cancer and from cardiovascular disease compared with mothers of non-smoking women. CONCLUSIONS: Being born with low birthweight is associated with smoking in adulthood. Associations of adult smoking with partners' birthweight and mothers' smoking-related causes of death suggest a shared smoking environment, and may account for some of the established association between birthweight and later cardiovascular disease.

Smoking during pregnancy from 1999 to 2004: a study from the Medical Birth Registry of Norway.

BACKGROUND: The aims of the study were to describe changes in smoking habits and evaluate secular trends among all Norwegian pregnant women during the period 1999-2004. We wanted to investigate whether there was a general decline in smoking habits among pregnant women. We also wanted to identify population subgroups with diverging trends. METHODS: The Medical Birth Registry of Norway (MBR) has national coverage of all births of 16 or more gestational weeks. Since 1999, women have been asked about tobacco smoking at the beginning and at the end of pregnancy. We included records from 304,905 women giving birth in the period January 1999 through April 2004. Women born outside Norway were handled separately. The selection left a dataset containing 259,573 Norwegian-born women. RESULTS: We obtained information on smoking habits from 86% at the end of pregnancy. Among those, the daily smoking prevalence was reduced from 17.3% in 1999-2001 to 13.2% in 2002-2004. Higher smoking prevalence was found among multiparous (3+), teenage mothers, single women, and women with low educational level. CONCLUSIONS: From 1999 to 2004, a substantial decline in smoking prevalence among Norwegian pregnant women was identified in all subgroups. However, an increasing social polarisation with regard to education and smoking habits was observed in the study period. In order to reduce the smoking-related risks for unsuccessful pregnancy outcome, special attention should be paid to smoking habits among multiparous, teenage women, single women and women with low education.