Mette Juhl

The Danish National Birth Cohort - its background, structure and aim

Background: It is well known that the time from conception to early childhood has importance for health conditions that reach into later stages of life. Recent research supports this view, and diseases such as cardiovascular morbidity, cancer, mental illnesses, asthma, and allergy may all have component causes that act early in life. Exposures in this period, which infl uence fetal growth, cell divisions, and organ functioning, may have long-lasting impact on health and disease susceptibility. Methods: To investigate these issues the Danish National Birth Cohort (Better health for mother and child) was established. A large cohort of pregnant women with long-term follow-up of the offspring was the obvious choice because many of the exposures of interest cannot be reconstructed with sufficient validity back in time. The study needs to be large, and it is aimed to recruit 100,000 women early in pregnancy, and to continue follow-up for decades. The Nordic countries are better suited for this kind of research than most other countries because of their population-based registers on diseases, demography and social conditions, linkable at the individual level by means of the unique ID-number given to all citizens. Exposure information is mainly collected by computer-assisted telephone interviews with the women twice during pregnancy and when their children are six and 18 months old. Participants are also asked to fill in a self-administered food frequency questionnaire in mid-pregnancy. Furthermore, a biological bank has been set up with blood taken from the mother twice during pregnancy and blood from the umbilical cord taken shortly after birth. Data collection started in 1996 and the project covered all regions in Denmark in 1999. By August 2000, a total of 60,000 pregnant women had been recruited to the study. It is expected that a large number of gene-environmental hypotheses need to be based on case-control analyses within a cohort like this.

Leisure time physical exercise during pregnancy and the risk of miscarriage: a study within the Danish National Birth Cohort

Objective  To examine the association between leisure time physical exercise during pregnancy and the risk of miscarriage.

Does leisure time physical activity in early pregnancy protect against pre‐eclampsia? Prospective cohort in Danish women

Objective  To examine the association between physical activity in early pregnancy and risk of pre‐eclampsia.

Is Swimming During Pregnancy a Safe Exercise

Background: Exercise in pregnancy is recommended in many countries, and swimming is considered by many to be an ideal activity for pregnant women. Disinfection by-products in swimming pool water may, however, be associated with adverse effects on various reproductive outcomes. We examined the association between swimming in pregnancy and preterm and postterm birth, fetal growth measures, small-for-gestational-age, and congenital malformations. Methods: We used self-reported exercise data (swimming, bicycling, or no exercise) that were prospectively collected twice during pregnancy for 74,486 singleton pregnancies. Recruitment to The Danish National Birth Cohort took place 1996–2002. Using Cox, linear and logistic regression analyses, depending on the outcome, we compared swimmers with physically inactive pregnant women; to separate a possible swimming effect from an effect of exercise, bicyclists were included as an additional comparison group. Results: Risk estimates were similar for swimmers and bicyclists, including those who swam throughout pregnancy and those who swam more than 1.5 hours per week. Compared with nonexercisers, women who swam in early/mid-pregnancy had a slightly reduced risk of giving birth preterm (hazard ratio = 0.80 [95% confidence interval = 0.72–0.88]) or giving birth to a child with congenital malformations (odds ratio = 0.89 [0.80–0.98]). Conclusions: These data do not indicate that swimming in pool water is associated with adverse reproductive outcomes.

Psychosocial and physical work environment, and risk of pelvic pain in pregnancy. A study within the Danish national birth cohort

Objective: The sparse knowledge of the aetiology of pelvic pain in pregnancy makes evidence based prevention a limited option. The aim of this study was to examine the relation between pelvic pain in pregnancy and physical and psychosocial working conditions. Methods: This study used self reported data on working conditions for 1219 cases and 1539 controls, sampled as a nested case-control study within the Danish national birth cohort. Cases and controls were selected on the basis of self reported pelvic pain intensity, pain localisation, and pain impact on daily living activities. Exposure data were collected prospectively; early in pregnancy and before the onset of pelvic pain. Main outcome measures were odds ratios for pelvic pain in pregnancy as a function of physical and psychosocial working conditions. Results: Pregnant women with fixed evening work and with rotating shifts (without night shift) had odds ratios for pelvic pain in pregnancy of 1.76 (95% confidence intervals 1.04 to 2.96) and 1.65 (1.22 to 2.24), respectively, compared with women with day work. Physically strenuous work was associated with an almost 50% increased risk of pelvic pain in pregnancy (1.47; 1.17 to 1.84). In women who were under high psychosocial strain at work odds ratio was 1.39 (1.12 to 1.74) compared with women with low job strain. Conclusion: Both physically and psychosocially demanding working conditions, measured by physically strenuous work, rotating shifts, and high job strain, are associated with an increased reporting of pelvic pain in pregnancy.

Pregnancy-related pelvic pain is more frequent in women with increased body mass index

Objective. To investigate the association between pre‐pregnancy overweight/obesity and pregnancy‐related pelvic pain. Design: Nested case‐control study. Setting and population. The Danish National Birth Cohort, a cohort of pregnant women and their children recruited 1996–2002. Methods. The women were interviewed twice during pregnancy and twice after childbirth. The first pregnancy interview provided information on self‐reported pre‐pregnancy body mass index (BMI) and possible confounders, while data on pregnancy‐related pelvic pain came from an interview six months postpartum. Cases (n=2 271) were selected on the basis of self‐reported pelvic pain, and controls were randomly selected among women who did not report pelvic pain (n=2 649). We used logistic regression analysis to calculate pregnancy‐related pelvic pain odds ratios (OR (95% confidence intervals)) according to pre‐pregnant BMI. Main outcome measure. Self‐reported pregnancy‐related pelvic pain. Results. In the total cohort, 18.5% of all pregnant women reported pregnancy‐related pelvic pain. In the nested case‐control study, the adjusted ORs for overall pelvic pain were 0.9 (0.7–1.2) in underweight women, 1.2 (1.1–1.4) in overweight women, 1.5 (1.2–2.0) in obese women Class 1 (30≤BMI<35), and 1.9 (1.3–2.8) in obese women Class 2 + 3 (BMI≥35), all relative to normal weight women. The correspondent ORs for severe pelvic pain were 0.8 (0.6–1.2), 1.4 (1.2–1.7), 1.7 (1.3–2.2), and 2.3 (1.6–3.4). The associations were stronger among women who had not given birth before. Conclusion. The risk of pregnancy‐related pelvic pain increased with pre‐pregnancy BMI in an exposure–response relation and potentially adds another maternal complication to obesity.

Distribution and predictors of exercise habits among pregnant women in the Danish National Birth Cohort

Physical activity is recommended during pregnancy, although strong evidence on reproductive health is lacking. We present exercise habits and predictors of exercise during pregnancy. From the Danish National Birth Cohort (1996–2002), 88 200 singleton pregnancies were analyzed in logistic regression. About one‐third of the women exercised in early/mid pregnancy and slightly less in late pregnancy. Bicycling, swimming, and low‐impact activities were most common. Exercising more than three times per week was strongly correlated with older age, being a student or out of work, eating disorders, moderate alcohol consumption, and a healthy diet. Multiparity, a normal or less good self‐rated health, smoking, and a less health conscious diet were the strongest predictors of not doing exercise. Women of 25 years or older, with metabolic or psychiatric disorders, or who had received subfecundity treatment were more likely to increase their activity level substantially from early to late pregnancy than comparison groups. In conclusion, exercising during pregnancy correlated with a number of maternal characteristics. The findings may be used to identify pregnant women not likely to exercise, to target activities that may fit their needs, and, for research purposes, to identify adjustment variables or guide sensitivity analyses when data on confounders are lacking.

Psychosocial job strain and risk of adverse birth outcomes: a study within the Danish national birth cohort

Objective A number of studies examined the effects of prenatal stress on birth outcomes with diverging and inconclusive results. We aimed to examine if working with high job strain during pregnancy measured in week 16 was associated with risk of giving birth to a child born preterm or small/large for gestational age (SGA/LGA), and second, if social support affected any associations. Design Study population was 48 890 pregnancies from the Danish National Birth Cohort. Multinomial logistic regression estimated ORs. Covariates included: maternal age, BMI, parity, exercise, smoking, alcohol and coffee consumption, manual work, serious maternal disease, parental height and gestational age at interview. In accordance with Good Epidemiological Practice, a protocol outlined the study design before analyses were initiated. Results High job strain was associated with significantly lower odds of being born LGA (OR=0.81, CI 0.70 to 0.92) when compared to low-strain jobs, but no associations between high strain and preterm birth or SGA were found. Stratification by social support showed a non-statistically significant tendency of higher odds of preterm birth when exposed to high strain and low social support. Conclusions In spite of the high statistical precision, we did not find any clear association between job strain and preterm birth nor between job strain and SGA.

Occupational lifting and pelvic pain during pregnancy: a study within the Danish National Birth Cohort

OBJECTIVES Pelvic pain during pregnancy is a common ailment, and the disease is a major cause of sickness absence during pregnancy. It is plausible that occupational lifting may be a risk factor of pelvic pain during pregnancy, but no previous studies have examined this specific exposure. The aim of this study was to examine the association between occupational lifting and pelvic pain during pregnancy. METHODS The study comprised 50 143 pregnant women, enrolled in the Danish National Birth Cohort in the period from 1996-2002. During pregnancy, the women provided information on occupational lifting (weight load and daily frequency), and six months post partum on pelvic pain. Adjusted odds ratios for pelvic pain during pregnancy according to occupational lifting were calculated by logistic regression. RESULTS Any self-reported occupational lifting (>1 time/day and loads weighing >10 kg) was associated with an increased risk of pelvic pain during pregnancy as compared to no such lifting. A confounder-adjusted exposure-response relation was observed between self-reported total loads lifted and pelvic pain during pregnancy. Daily lifting of both medium (11-20 kg) and heavy loads (>20 kg) were associated with increased risk, and the highest risk was observed among women who lifted heavy loads independent of exposure to medium loads. CONCLUSION Occupational lifting may increase the risk of pelvic pain during pregnancy.

Occupational lifting during pregnancy and risk of fetal death in a large national cohort study

OBJECTIVE The aim of the present study was to examine the association between occupational lifting and the risk of fetal death according to gestational age. METHODS We used data from the Danish National Birth Cohort (1996-2002). Among 71,500 occupationally active women, 2886 experienced a fetal death. Information on lifting and relevant covariates was collected in interviews around week 16 of pregnancy. The majority of fetal losses (N=2032) happened before the scheduled interview, and exposure data were collected retrospectively from these women. We analyzed early miscarriage (≤12 weeks), late miscarriage (13-21 weeks), and stillbirth (≥22 weeks), using Cox-regression models with gestational age as the underlying time variable. RESULTS The adjusted early miscarriage risk increased with frequency of daily lifts and total burden lifted per day. For example, the hazard ratio was 1.38 [95% confidence interval (95% CI) 1.10-1.74] for a total weight load per day of 101-200 kg and 2.02 (95% CI 1.23-3.33) for a daily load >1000 kg as compared to non-lifters (P for trend <0.0001). Late miscarriage was associated with total daily weight load (P for trend=0.0073) but not with number of lifts per day. There was no association between occupational lifting and stillbirth. CONCLUSIONS In the present study, the risk of miscarriage increased with the number of lifts and total burden lifted per day at work. There may be a case for advising pregnant women against heavy lifting in particular during early pregnancy.