: In France, 7% of women have a BMI under 18,5kg/m² in early pregnancy. Adverse pregnancy outcomes associated with underweight are rarely described in French literature. This study evaluates obstetric and neonatal outcomes of underweight women compared with women with a normal BMI.

: We conducted a retrospective monocentric study within University hospital of Bicetre. This study included 285 women with singleton pregnancy and a BMI <18.5 kg /m² who gave birth after 15 weeks of gestation between January 2017 and February 2019. Their socio-demographic characteristics, obstetrical complications, neonatal and postpartum outcomes were compared with those of 285 women with normal BMI.

: The underweight women are more socially vulnerable and have a significantly higher risk of anemia (p = 0,045) and having small for gestational age infants (p < 0,01). There was no significant difference regarding the mode of delivery and in the early health status of the newborns. Inadequate pregnancy weight gain appeared to be an independent risk factor of low birth weight.

: Our study suggests that a lower-than-normal BMI in early pregnancy is associated with more social difficulties and exposes women to specific morbidity. This is an important element in the initial assessment of obstetrical risk, which justifies an adapted pregnancy follow-up.