High risk of extremely premature birth if high BMI (low vitamin D) – June 2013

Maternal Obesity and Risk of Preterm Delivery BMJ June 2013

Sven Cnattingius, MD, PhD; Eduardo Villamor, MD, DrPH; Stefan Johansson, MD, PhD; Anna-Karin Edstedt Bonamy, MD, PhD; Martina Persson, MD, PhD; Anna-Karin Wikström, MD, PhD; Fredrik Granath, PhD

Author Affiliations: Clinical Epidemiology Unit, Department of Medicine Solna, Karolinska University Hospital (Drs Cnattingius, Johansson, Edstedt Bonamy, Persson, Wikström, and Granath), and Department of Clinical Science, Intervention, and Technology (Dr Persson), Karolinska Institutet, Stockholm, Sweden; Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor (Dr Villamor); and Department of Women's and Children's Health, Uppsala University, Uppsala, Sweden (Dr Wikström).

Importance Preterm birth is a leading cause of infant mortality, morbidity, and long-term disability, and these risks increase with decreasing gestational age.
Obesity increases the risk of preterm delivery, but the associations between overweight and obesity and subtypes of preterm delivery are not clear.

Objective To study the associations between early pregnancy body mass index (BMI) and risk of preterm delivery by gestational age and by precursors of preterm delivery.

Design, Setting, and Participants Population-based cohort study of women with live singleton births in Sweden from 1992 through 2010.
Maternal and pregnancy characteristics were obtained from the nationwide Swedish Medical Birth Register.

Main Outcomes and Measures Risks of preterm deliveries (extremely, 22-27 weeks; very, 28-31 weeks; and moderately, 32-36 weeks). These outcomes were further characterized as spontaneous (related to preterm contractions or preterm premature rupture of membranes) and medically indicated preterm delivery (cesarean delivery before onset of labor or induced onset of labor). Risk estimates were adjusted for maternal age, parity, smoking, education, height, mother's country of birth, and year of delivery.

Results Among 1 599 551 deliveries with information on early pregnancy BMI, 3082 were extremely preterm, 6893 were very preterm, and 67 059 were moderately preterm. Risks of extremely, very, and moderately preterm deliveries increased with BMI and the overweight and obesity-related risks were highest for extremely preterm delivery. Among normal-weight women (BMI 18.5-<25), the rate of extremely preterm delivery was 0.17%. As compared with normal-weight women, rates (%) and adjusted odds ratios (ORs 95% CIs) of extremely preterm delivery were as follows: BMI 25 to less than 30 (0.21%; OR, 1.26; 95% CI, 1.15-1.37), BMI 30 to less than 35 (0.27%; OR, 1.58; 95% CI, 1.39-1.79), BMI 35 to less than 40 (0.35%; OR, 2.01; 95% CI, 1.66-2.45), and BMI of 40 or greater (0.52%; OR, 2.99; 95% CI, 2.28-3.92). Risk of spontaneous extremely preterm delivery increased with BMI among obese women (BMI≥30). Risks of medically indicated preterm deliveries increased with BMI among overweight and obese women.

Conclusions and Relevance In Sweden, maternal overweight and obesity during pregnancy were associated with increased risks of preterm delivery, especially extremely preterm delivery. These associations should be assessed in other populations.

First line of the study
Maternal overweight and obesity has, due to the high prevalence and associated risks, replaced smoking as the most important preventable risk factor for adverse pregnancy outcomes in many countries.1

Results
The Birth Register recorded information for 1 863 768 live singleton births between 1992 and 2010. After excluding 3942 births to women with an incomplete national registration number and 2004 women with missing data on gestational age, the study population included 1 857 822 deliveries of live singleton infants. Information about early pregnancy BMI was available in 86% (1 599 551 deliveries) of all deliveries (Table 1 and Table 2).

During the study period, there were 93 419 (5.03%) preterm (≤36 weeks) deliveries. Overall rates of extremely (22-27 weeks), very (28-31 weeks), and moderately (32-36 weeks) preterm deliveries were 0.23%, 0.47%, and 4.36%, respectively. Preterm delivery rates increased with BMI among overweight and obese women and were also increased among short women (<155 cm). Higher rates of preterm delivery were seen among teenage and older women (≥35 years), women having their first or at least their fourth childbirth, smokers, women with low education, and women born outside of Nordic countries. Maternal diseases, especially preeclampsia and pregestational hypertension, were associated with increased rates of preterm delivery (Table 1 and Table 2).

Rates of overweight and obesity were increased among short, older, multiparous (≥4), and lower-educated women. Rates of overweight and obesity were substantially increased in women with hypertensive or diabetic diseases. Women with missing information on smoking, education, or country of birth also had high rates of missing information on BMI. Women with obesity-related diseases did not have high rates of missing information on BMI (eTable 1).

BMI and Risks of Preterm Delivery by Gestational Age
Compared with women having normal BMI, risks of extremely, very, and moderately preterm deliveries increased with BMI (Figure). The overweight and obesity-related risks were highest for extremely preterm delivery and lowest for moderately preterm delivery. Among normal-weight women (BMI, 18.5-24.9), the rate of extremely preterm delivery was 0.17%. Compared with normal-weight women, rates and adjusted ORs (AORs) of extremely preterm delivery were as follows: BMI 25 to less than 30 (0.21%; AOR, 1.23; 95% CI, 1.13-1.35), BMI 30 to less than 35 (0.27%; AOR, 1.53; 95% CI, 1.35-1.74), BMI 35 to less than 40 (0.35%; AOR, 1.97; 95% CI, 1.62-2.40), and BMI of 40 or greater (0.52%; AOR, 2.91; 95% CI, 2.21-3.83). Compared with normal-weight women, underweight women (BMI<18.5) had slightly increased risks of very and moderately preterm deliveries. Supplemental analyses using multiple imputation of participants with missing values on BMI and covariates resulted in modest changes in risks, which were most pronounced for extremely preterm delivery; risks were slightly increased in underweight women and attenuated in obese women, especially in women with grade 3 obesity (Figure).

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