Stillbirth or neonatal death before 45 post-menstrual weeks in relation to gestational duration in pregnancies at 39 weeks of gestation or beyond: the impact of parity and body mass index. A national cohort study

Objective: To investigate the risk of stillbirth or neonatal death before 45 post-menstrual weeks in relation to gestational duration, stratified by body mass index (BMI) and parity. Design: Retrospective study. Setting: Data from the Swedish Medical Birth Register. Population: Singleton, cephalic births at between 39⁺⁰ and 42⁺² weeks of gestation, 2005–2016 (n = 892 339). Methods: Relative risk ratios for mortality in relation to gestational duration were stratified by parity and BMI, and were adjusted for maternal age, smoking, country of birth and educational level. Main outcome measures: Primary outcome: stillbirth or neonatal death before 45 post-menstrual weeks. Secondary outcome: stillbirth. Results: Among children of primiparous women, children born at 41⁺³ weeks of gestation, or later, were at increased risk of stillbirth or neonatal death before 45 post-menstrual weeks compared with children born between 39⁺⁰ and 40⁺² weeks of gestation (aRR 1.29, 95% CI 1.10–1.52). For primiparous women with BMIs of <25, 25–29.9 and (Formula presented.) 30 kg/m², the corresponding aRRs were: 1.04 (95% CI 0.81–1.34), 1.25 (95% CI 0.94–1.66) and 1.52 (95% CI 1.10–2.10), respectively. No significant increase in risk with gestational age was detected for multiparous women, regardless of BMI class. Among primipara, the risk of stillbirth increased with gestational duration in all BMI classes, with the highest risk increase for BMI ≥ 30 kg/m², from 0.8/1000 at 40⁺³–40⁺⁶ weeks of gestation to 4.0/1000 at 42⁺⁰–42⁺² weeks of gestation. Conclusions: At 41⁺³–42⁺² weeks of gestation, pregnancy duration was associated with an increased risk for stillbirth or neonatal death before 45 post-menstrual weeks among primiparous women, especially among women who were obese. For multiparous women, no significant association between gestational duration and mortality was found. Tweetable abstract: In term pregnancies the risk for stillbirth and neonatal death is affected by gestational age, parity and BMI.