Objective
The purpose of this study was to determine the influence of maternal prepregnancy body mass index on preterm delivery (PTD), controlling for health and lifestyle variables.
Study Design
Prospective data were from 83,544 pregnancies in the Norwegian Mother and Child Cohort Study. PTD was divided into early PTD (22 + 0 to 31 + 6 weeks’ gestation) and late PTD (32 + 0 to 36 + 6 weeks’ gestation).
Results
The overall prevalence of PTD was 5.1%. Increased body mass index was associated with an increased risk of PTD; adjusted odds ratio (aOR) ranged from 1.11 (95% confidence interval [CI], 1.03–1.20) for preobesity to 2.00 (95% CI, 1.48–2.71) for grade-III obesity in the group that included all PTD subgroups. Grade-III obese women had an increased risk of both early and late PTD: aOR, 3.24 (95% CI, 1.71–6.14) and 1.81 (95% CI, 1.29–2.54), respectively.
Conclusion
Prepregnancy maternal overweight increases the risk of both early and late PTD.
Preterm delivery (PTD) is defined by the World Health Organization as delivery before 37 completed weeks of gestation. It is of great importance because it is a leading cause of neonatal death and morbidity. PTD is responsible for 3 million deaths worldwide annually.
Maternal obesity is a growing public health problem worldwide. It is known that women with excess weight have a higher risk of several adverse maternal and perinatal outcomes that include preeclampsia, gestational diabetes mellitus, cesarean section delivery, and fetal death. Women with lower than normal body mass index (BMI; kg/m 2 ) also have an elevated risk of adverse prenatal outcomes, such as PTD and intrauterine growth restriction.
Obesity is a condition that reflects long-term imbalance between energy intake and energy expenditure, and is a result of a complex relationship between genetic and environmental influences.
A recent systematic review and metaanalysis of 39 studies (36 cohort studies and 3 case-control studies that have been published over the last 40 years) on the relation between prepregnancy maternal BMI and PTD have shown an increased risk of PTD in some groups of overweight women. The metaanalysis did not have sufficient power to examine the relation by subgroups of BMI at different gestational ages. The review therefore pointed out the need for an evaluation of the relationship between maternal BMI and subtypes of PTD at different gestational ages that have been adjusted for possible confounders.
Diet and physical activity are lifestyle behaviors that influence obesity both in the general population and in pregnant women. Previous observational studies have shown mixed results regarding favorable influences of dietary behavior and physical activity on BMI, weight loss during pregnancy, and perinatal outcomes. However, the influence of these 2 variables on the relationship between prepregnancy maternal BMI and PTD is still unclear.
The Norwegian Mother and Child Cohort Study (MoBa) is a large pregnancy cohort that facilitates the examination of the relationship between prepregnancy BMI and PTD, while taking into account a wide range of potential confounding variables.
The aim of this study was to determine the influence of maternal prepregnancy BMI on PTD, while controlling for sociodemographic, health, and lifestyle variables.
Materials and Methods
The dataset that was used in this study is part of MoBa, which is a prospective population-based pregnancy cohort study that was conducted by the Norwegian Institute of Public Health. Participants were recruited from all over Norway between 1999 and 2008; 38.5% of the invited women consented to participate. The cohort includes approximately 108,000 children and 90,700 mothers. Follow-up evaluation was conducted by questionnaires at regular intervals and by linkage to national health registries. The target population was all women who gave birth in Norway. Hospitals and maternity units that delivered >100 women per year were to be included; 50 of 52 maternity units participated. Women were invited to participate during their routine ultrasound scan at gestational week 17-18. The cohort database is linked to the Medical Birth Registry of Norway (MBRN) and other national health registries. The MoBa has been described in detail elsewhere. Informed consent was obtained from each participant on recruitment. The MoBa was approved by the Regional Committee for Medical and Health Research Ethics and the Data Inspectorate in Norway.
This study is based on version 5 of the quality-assured data files that were released for research in August 2010. We used data from 3 different questionnaires (Q) that were answered at approximately gestational weeks 15 (Q1), 22 (Q2), and 30 (Q3), respectively.
A total of 99,426 pregnancies were available for inclusion in this study. We excluded 15,882 pregnancies because of multiple gestations, intrauterine fetal deaths, pregnancies that resulted from in vitro fertilization, and pregnancies with missing information on weight or height; the final study sample was 83,544 pregnancies.
PTD
The main outcome was PTD, which was determined from gestational length as registered in the MBRN and was based on expected date of parturition according to ultrasound scanning. If no ultrasound dates were available, gestational length was determined from the last date of menstruation.
PTD was defined in this study as singleton live birth that occurred between 22 + 0 and 36 + 6 weeks. A reference population was chosen according to the same criteria, except for delivery at term (ie, gestational age 37 + 0 to 41 + 6 weeks). Postterm deliveries (≥42 gestational weeks) were not included in the study.
PTD is a general term and can be divided into spontaneous and iatrogenic PTD. Spontaneous PTD is the largest subgroup and includes delivery after preterm labor or after preterm prelabor rupture of membranes. Iatrogenic PTD includes both induced and elective cesarean deliveries. The data were retrieved from the MBRN.
Covariates
Maternal BMI was calculated from the self-reported data for prepregnancy weight and height in Q1. The lower cutoff point for height was set at 140 cm. The lower and upper cutoff points for weight were 35 and 200 kg, respectively.
The women were divided into 6 BMI groups (underweight, <18.5 kg/m 2 ; normal weight, 18.5-24.9 kg/m 2 ; preobese, 25.0-29.9 kg/m 2 ; grade-I obese, 30.0-34.9 kg/m 2 ; grade-II obese, 35.0-39.9 kg/m 2 ; and grade-III obese, ≥40 kg/m 2 ), according to the latest classification by the World Health Organization.
Maternal age at delivery was retreived from the MBRN and used as a continous variable. Marital status was obtained from Q1 and categorized as either “living alone” or “cohabiting.” Parity was used as a dichotomous variable that denoted either “nulliparity” or “multiparity,” based on information from Q1. Women were defined as smokers during pregnancy if they reported either occasional or daily smoking in Q1 or Q3. Educational level was combined into a dichotomous variable (ie, “≤12 years of school” or “>12 years of school”), regardless of the kind of education. Previous PTD was a dichotomous variable that denoted either “had ≥1 previous PTD” or “no previous PTD,” based on information from MBRN.
Leisure physical activity during pregnancy was based on the respondents’ reports on participation in 13 different types of recreational exercise during the first trimester (Q1) and divided into 4 categories (no exercise, less than once weekly, 1-2 times weekly, and ≥3 times weekly). The self-reported activities have shown moderate correlations with motion sensor measurements.
Intake of dietary fiber and total calorie intake were included as potential confounding variables. Because of increasing underreporting of energy intake with increasing BMI, we adjusted the dietary fiber intake for total caloric intake and divided it into quartiles. This energy-adjusted fiber intake was regarded as the proxy for a healthy diet. Energy and fiber intakes were estimated on the basis of the validated MoBa food frequency questionnaire (Q2), which was answered by the mothers at approximately week 22 of gestation and the Norwegian food composition table. These 2 variables have been validated in another study in MoBa.
In MoBa, >99% of the participants are of white ethnicity, and ethnicity is not a relevant confounder.
Statistical methods
The Student t test and Mann-Whitney U test were performed to compare 2 groups, with respect to continuous variables.
Adjusted odds ratios (aORs) were obtained by binary logistic regression, with adjustment for maternal age, parity, smoking, education, marital status, and previous PTD. In the final analysis, we also included dietary fiber intake and leisure physical activity among the confounders. Women with missing data on any of the covariates were grouped in a “missing” category and included in the adjusted analyses. Exercise and dietary fiber had the largest proportions of missing data (approximately 12% in different subgroups of BMI). Fitting the models without the missing-data categories did not change the associations between BMI and PTD substantially but resulted in larger confidence intervals (CIs) because of a smaller number of subjects in the subanalysis.
Data analyses were performed with PASW software (version 19.0; SPSS, Inc, Chicago, IL).
Results
A total of 83,544 pregnancies were included in this study. The overall proportion of PTD was 5.1%. The lowest proportion of PTD (4.7%) was observed in normal-weight women, and the highest proportion (9.2%) was observed in grade-III obese women ( Table 1 ).
| Prepregnancy maternal body mass index | Total, n | Delivery, n (%) | ||
|---|---|---|---|---|
| 37 + 0 to 41 + 6 wk (n = 79,243) | 32 + 0 to 36 + 6 wk (n = 3735) | 22 + 0 to 31 + 6 wk (n = 566) | ||
| Underweight (<18.5 kg/m 2 ) | 2719 | 2527 (92.9) | 170 (6.3) | 22 (0.8) |
| Normal weight (18.5-24.9 kg/m 2 ) | 54,891 | 52,309 (95.3) | 2261 (4.1) | 321 (0.6) |
| Preobese (25.0-29.9 kg/m 2 ) | 18,062 | 17,109 (94.7) | 825 (4.6) | 128 (0.7) |
| Obese, grade I (30.0-34.9 kg/m 2 ) | 5757 | 5347 (92.9) | 342 (5.9) | 68 (1.2) |
| Obese, grade II (35.0-39.9 kg/m 2 ) | 1597 | 1481 (92.7) | 99 (6.2) | 17 (1.1) |
| Obese, grade III (≥40 kg/m 2 ) | 518 | 470 (90.8) | 38 (7.3) | 10 (1.9) |
Women with PTD had higher mean BMIs than did women who delivered at term (24.5 vs 24.0 kg/m 2 , respectively; P < .01).
Prepregnancy BMI increased with increasing age and parity and was higher in smokers, in women with low educational level, and in women with a history of spontaneous PTD, compared with the respective reference groups. Furthermore, prepregnancy BMI decreased with increasing dietary fiber intake and leisure physical activity ( Table 2 ).
| Maternal characteristics | n (%) | Prepregnancy body mass index a | P value |
|---|---|---|---|
| All | 83,544 | 24.0 (4.3) | |
| Age at pregnancy, y | < .001 b | ||
| <20 | 872 (1.0) | 22.7 (4.3) | |
| 20-29 | 36,719 (44.0) | 23.9 (4.3) | |
| 30-39 | 44,321 (53.0) | 24.1 (4.2) | |
| ≥40 | 1632 (2.0) | 24.7 (4.5) | |
| Parity | < .001 c | ||
| Nulliparous | 36,366 (43.5) | 23.6 (4.1) | |
| Multiparous | 47,178 (56.5) | 24.3 (4.4) | |
| Maternal education, y | < .001 c | ||
| ≤12 | 26,649 (32.6) | 24.7 (4.8) | |
| >12 | 55,187 (67.4) | 23.6 (3.9) | |
| Missing/other | 1708 | ||
| Marital status | .083 c | ||
| Married/cohabiting | 80,322 (96.6) | 24.0 (4.3) | |
| Single | 2821 (3.4) | 23.8 (4.7) | |
| Missing | 401 | ||
| Smoking prior to pregnancy | < .001 c | ||
| Non-smokers | 72,470 (86.7) | 23.9 (4.2) | |
| Daily smokers | 11,074 (13.3) | 24.4 (4.8) | |
| Previous spontaneous preterm delivery | < .001 c | ||
| Yes | 2192 (2.6) | 24.5 (4.5) | |
| No | 81,304 (97.4) | 24.0 (4.3) | |
| Missing | 48 | ||
| Energy-adjusted dietary fiber intake (g/kcal/day) | < .001 b | ||
| Quartile 1 | 17,924 (25.4) | 24.1 (4.5) | |
| Quartile 2 | 17,641 (25.0) | 24.0 (4.2) | |
| Quartile 3 | 17,437 (24.7) | 24.0 (4.2) | |
| Quartile 4 | 17,566 (24.9) | 23.9 (4.2) | |
| Missing | 12,976 | ||
| Leisure physical activity | < .001 b | ||
| None | 12,060 (16.3) | 24.8 (4.8) | |
| Less than weekly | 16,033 (21.6) | 24.4 (4.4) | |
| 1-2 times weekly | 23,921 (32.3) | 23.9 (4.1) | |
| >3 times weekly | 22,132 (29.8) | 23.3 (3.8) | |
| Missing | 9398 |
a Data are given as mean ± SD;
b Kruskal-Wallis test/Mann-Whitney U test;
The mean BMI for women with late PTD and early PTD was 24.4 and 25.0 kg/m 2 , respectively ( P < .01).
Women who were underweight were at increased risk of PTD (aOR, 1.47; 95% CI, 1.26–1.72) and late PTD (aOR, 1.49; 95% CI, 1.27–1.76), but not early PTD ( Table 3 ).
| Prepregnancy maternal body mass index | Preterm delivery (n = 4301) a | Late preterm delivery (n = 3735) b | Early preterm delivery (n = 566) c | ||||||
|---|---|---|---|---|---|---|---|---|---|
| n (%) | Crude OR (95% CI); P value | Adjusted OR (95% CI); P value | n (%) | Crude OR (95% CI); P value | Adjusted OR (95% CI); P value | n (%) | Crude OR (95% CI); P value | Adjusted OR (95% CI); P value | |
| Underweight: <18.5 kg/m 2 | 192 (7.1) | 1.54 (1.32–1.80); < .001 | 1.47 (1.26–1.72); < .001 | 170 (6.3) | 1.56 (1.33–1.83); < .001 | 1.49 (1.27–1.76); < .001 | 22 (0.8) | 1.42 (0.92–2.19); .114 | 1.33 (0.86–2.06); .201 |
| Normal-weight: 18.5-24.9 kg/m 2 | 2582 (4.7) | Reference | Reference | 2261 (4.1) | Reference | Reference | 321 (0.6) | Reference | Reference |
| Preobese: 25-29.9 kg/m 2 | 953 (5.3) | 1.13 (1.05–1.22); .002 | 1.11 (1.03–1.20); .008 | 825 (4.6) | 1.12 (1.03–1.20); .009 | 1.10 (1.01–1.20); .023 | 128 (0.7) | 1.22 (0.99–1.50); .059 | 1.19 (0.97–1.46); .103 |
| Obese, grade I: 30-34.9 kg/m 2 | 410 (7.1) | 1.55 (1.40–1.73); < .001 | 1.50 (1.35–1.68); < .001 | 342 (6.0) | 1.48 (1.32–1.66); < .001 | 1.43 (1.27–1.62); < .001 | 68 (1.1) | 2.07 (1.59–2.70); < .001 | 1.98 (1.51–2.58); < .001 |
| Obese, grade II: 35-39.9 kg/m 2 | 116 (7.3) | 1.59 (1.31–1.92); < .001 | 1.51 (1.24–1.84); < .001 | 99 (6.2) | 1.55 (1.26–1.90); < .001 | 1.48 (1.20–1.82); < .001 | 17 (1.1) | 1.87 (1.15–3.06); .012 | 1.75 (1.07–2.86); .027 |
| Obese, grade III: ≥40 kg/m 2 | 48 (9.3) | 2.07 (1.53–2.79); < .001 | 2.00 (1.48–2.71); < .001 | 38 (7.3) | 1.87 (1.34–2.61); < .001 | 1.81 (1.29–2.54); .001 | 10 (2.0) | 3.47 (1.84–6.55); < .001 | 3.24 (1.71–6.14); < .001 |
a 22 + 0 to 36 + 6 weeks’ gestation;
b 32 + 0 to 36 + 6 weeks’ gestation;
In the total PTD group, increasing BMI from preobese to grade-III obesity was associated with an increased risk of PTD; the aOR increased from 1.11 (95% CI, 1.03–1.20) to 2.00 (95% CI, 1.48–2.71), respectively. A parallel trend was evident in the late PTD subgroup ( Table 3 ). Women with increasing BMI from grade-I to grade-III obesity also had an increased risk of early PTD ( Table 3 ).
The mean BMI for women with spontaneous PTD and iatrogenic PTD was 23.7 and 25.2 kg/m 2 , respectively ( P < .01).
Women in all obesity categories had an increased risk of spontaneous PTD; however, no associations were found for the preobese group or for iatrogenic PTD ( Table 4 ).
| Prepregnancy maternal BMI | Spontaneous preterm delivery (n = 2537) | Iatrogenic preterm delivery (n = 1764) | ||||
|---|---|---|---|---|---|---|
| n (%) | Crude OR (95% CI); P value | Adjusted OR (95% CI); P value | n (%) | Crude OR (95% CI); P value | Adjusted OR (95% CI); P value | |
| Underweight: <18.5 kg/m 2 | 119 (4.4) | 1.52 (1.25–1.84); < .001 | 1.43 (1.17–1.75); < .001 | 73 (2.7) | 1.63 (1.25–2.12); < .001 | 1.55 (1.19–2.02); .001 |
| Normal-weight: 18.5-24.9 kg/m 2 | 1612 (2.9) | Reference | Reference | 970 (1.8) | Reference | Reference |
| Preobese: 25-29.9 kg/m 2 | 518 (2.9) | 1.06 (0.95–1.17); .293 | 1.04 (0.94–1.15); .430 | 435 (2.4) | 0.95 (0.84–1.07); .431 | 0.95 (0.84–1.07); .399 |
| Obese, grade I: 30-34.9 kg/m 2 | 214 (3.7) | 1.51 (1.31–1.75); < .001 | 1.46 (1.26–1.70); < .001 | 196 (3.4) | 1.04 (0.89–1.23); .605 | 1.04 (0.88–1.23); .634 |
| Obese, grade II: 35-39.9 kg/m 2 | 56 (3.5) | 1.51 (1.15–1.98); .003 | 1.42 (1.08–1.88); .013 | 60 (3.8) | 1.00 (0.76–1.32); .990 | 0.99 (0.75–1.31); .973 |
| Obese, grade III: ≥40 kg/m 2 | 18 (3.5) | 1.70 (1.05–2.74); .030 | 1.66 (1.02–2.69); .041 | 30 (5.8) | 1.27 (0.86–1.88); .228 | 1.30 (0.87–1.94); .192 |
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