![]() In addition, there is evidence that the association of BMI with PTB may vary by parity, with some suggestion of a stronger association of obesity with SPTB among nulliparous women and of different associations of underweight with SPTB and MPTB among nulliparous and parous women, although the numbers of women with underweight in these studies have been small. While the detrimental effects of MPTB are a trade off with detrimental effects of continued pregnancy in the presence of such conditions, SPTB is a major concern obstetrically because of its unpredictable nature.Įvidence from systematic reviews suggests an increased risk of PTB with both maternal overweight/obesity and underweight, with some studies indicating that underweight might be a greater factor than obesity in SPTB. Known risk factors for SPTB include infection and inflammation, genetic factors, and some lifestyle factors such as stress, smoking and alcohol intake, although the cause is often unknown. MPTB is driven by obstetric interventions (induction of labour or planned caesarean section) related to pregnancy complications such as pre-eclampsia or gestational diabetes and thus may be higher in women with overweight or obesity. PTB can be medically indicated (MPTB) or spontaneous (SPTB). Recent increases in PTB may be, in part, related to the obesity epidemic, with other possible factors including increased maternal age at pregnancy and changes in obstetric practice resulting in increased rates of preterm caesarean delivery It is the leading cause of perinatal mortality and morbidity, and of childhood death up to 5 years. ![]() Preterm birth (PTB birth before 37 completed weeks gestation) affects around 10% of pregnancies worldwide. ConclusionsĬonsistency of findings across different populations, despite differences between them in terms of the time period covered, the BMI distribution, missing data and control for key confounders, suggests that severe under- and overweight may play a role in PTB risk. In the meta-analysed data, the lowest risk of any PTB was at a BMI of 22.5 kg/m 2 (95% CI 21.5, 23.5) among nulliparous women and 25.9 kg/m 2 (95% CI 24.1, 31.7) among multiparous women, with values of 20.4 kg/m 2 (20.0, 21.1) and 22.2 kg/m 2 (21.1, 24.3), respectively, for MPTB for SPTB, the risk remained roughly largely constant above a BMI of around 25–30 kg/m 2 regardless of parity. The adjusted risk of any PTB and MPTB was elevated at both low and high BMIs, whereas the risk of SPTB was increased at lower levels of BMI but remained low or increased only slightly with higher BMI. We found non-linear associations between BMI and all three outcomes, across all datasets. ![]() The estimated BMI at which risk was lowest was calculated via differentiation and a 95% confidence interval (CI) obtained using bootstrapping. The results were combined using a random effects meta-analysis. Fractional polynomial multivariable logistic regression was used to examine the relationship of maternal BMI with any PTB, SPTB and MPTB, among nulliparous and parous women separately. We used three UK datasets, two USA datasets and one each from South Australia, Norway and Denmark, together including just under 29 million pregnancies resulting in a live birth or stillbirth after 24 completed weeks gestation. Our aim was to compare associations of maternal pre-pregnancy BMI with any PTB, SPTB and medically indicated PTB (MPTB) among nulliparous and parous women across populations with differing characteristics, and to identify the optimal BMI with lowest risk for these outcomes. Previous studies have largely explored established body mass index (BMI) categories. Evidence suggests an increased risk with both maternal underweight and obesity, with some studies suggesting underweight might be a greater factor in spontaneous PTB (SPTB) and that the relationship might vary by parity. Preterm birth (PTB) is a leading cause of child morbidity and mortality.
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