Objective: To assess the association between gestational age at delivery and postpartum severe acute maternal morbidity (SAMM) in twin pregnancies. Design: Population-based, national, prospective, cohort study Setting: From 02/2014 to 03/2015 in France Population: Women with twin pregnancies who gave birth after 32 weeks of gestation. We excluded women with fetal death or medical termination of either twin, with antepartum SAMM, with antepartum conditions responsible for postpartum SAMM. Methods: Gestational age at delivery was studied as the number of completed weeks of gestation. We assessed the association between gestational age at delivery and postpartum SAMM by using multivariable multilevel modified Poisson regression modelling. Main Outcome Measures: Composite criteria of postpartum severe acute maternal morbidity Results: Among the 7,713 women included, 410 (5.3%) developed postpartum SAMM, mainly (88.5%) postpartum haemorrhage. Compared with the reference category of 37 weeks, the risk of postpartum SAMM was significantly lower for all categories of earlier gestational age at delivery (from aRR=0.34, 95% CI 0.17-0.68 at 32 weeks to aRR=0.71, 95% CI 0.54-0.94 at 36 weeks), and did not differ for later categories. Conclusion: In twin pregnancies, compared with delivery at 37 weeks, delivery at earlier gestational ages is associated with a lower risk of postpartum SAMM. Continuing pregnancy beyond 37 weeks is not associated with an increased risk of postpartum SAMM. Funding: Supported by a grant from the French Ministry of Health (Programme Hospitalier de Recherche Clinique, AOM2012) and a grant from Université Toulouse III. Keywords: severe acute maternal morbidity, twin pregnancy, timing of delivery

Objectives: To assess the incidence, causes, risk factors and adverse outcomes of antepartum severe maternal morbidity (SMM) Design: Population-based case–control study Setting: 119 Maternity hospitals, 6 French regions Population: All women with antepartum SMM (cases, N=601), a randomly selected control sample of women who gave birth without SMM in the same hospitals during the same period (controls, N=3650) Methods: Uni- and multivariable logistic regression with multiple imputation Main outcomes measure: Antepartum SMM, defined as a morbid event occurring from 22 weeks of gestation and before the onset of labour Results: Antepartum SMM complicated 0.33% of pregnancies (95%CI, 0.30-0.36). Rates of prematurity, neonatal mortality, and transfer to the neonatal intensive care unit were 10 times higher for babies whose mothers had antepartum SMM than for the control mothers. Similarly, emergency caesarean and general anaesthesia were more frequent in women with antepartum SMM. Risk factors for antepartum SMM were maternal age >35 (aOR 1.55; 95% CI, 1.22-1.97), increased body mass index (aOR for 5kg/m2 increase, 1.24; 95% CI, 1.14-1.36), maternal birth in sub-Saharan Africa (aOR, 1.80; 95% CI, 1.29-2.53), pre-existing medical condition (aOR, 2.56; 95% CI, 1.99-3.30), nulliparity (aOR, 2.26; 95% CI, 1.83-2.80), previous pregnancy-related hypertensive disorders (aOR, 4.94; 95% CI, 3.36-7.26), multiple pregnancy (aOR, 5.79; 95% CI, 3.75-7.26), irregular prenatal care (aOR, 1.86; 95% CI, 1.27-2.72). Conclusion: Antepartum SMM is rare but associated with a massively higher incidence of adverse delivery and neonatal outcomes.