LAUSR

Abstract Introduction: Preterm birth is a major cause of infant mortality. It is unknown whether body mass index (BMI) influences the risk of preterm birth in women, who prenatally use antidepressants. Materials and methods: The study cohort (N = 6920) consists of all primiparous European born women without previously diagnosed diabetes from the city of Vantaa, Finland, who delivered a singleton child between 2009 and 2015. Data on births, pre-pregnancy BMI and purchases of antidepressants from 12 months before conception until delivery were obtained from Finnish National Registers. Results: Of the primiparous women, 9.9% used antidepressants. The overall prevalence of preterm birth was 5.2%. In women with a pre-pregnancy BMI <18.5 kg/m2, the Odds Ratio (OR) for preterm birth among antidepressant users compared with those who were non-users was 1.91 (95% confidence intervals [CI] 0.40 to 9.15, adjusted for age, smoking, education, use of fertility treatments and number of previous pregnancies) while in women with a pre-pregnancy BMI ≥30 kg/m2, the OR was 0.53 (95% CI 0.21–1.36), respectively. Discussion: Primiparous women using antidepressants, who were underweight before conception should be closely monitored and provided tailored care in a maternity clinic to minimize the risk of preterm birth. Key messages In primiparous women, one in ten used antidepressant medications before pregnancy and/or during pregnancy. In primiparous women, the prevalence of preterm birth was 5%. Underweight primiparous women using antidepressants should be closely monitored and provided tailored care in a maternity clinic.