What factors are related to recurrent preterm birth among underweight women?

Our objective was to identify factors associated with recurrent preterm birth among underweight women.

Maternally linked hospital and birth certificate records of deliveries in California between 2007 and 2010 were used. Consecutive singleton pregnancies of women with underweight body mass index (BMI <18.5 kg/m²) in the first pregnancy were analyzed. Pregnancies were categorized based on outcome of the first and second birth as: term-term; term-preterm; preterm-term and preterm-preterm.

We analyzed 4971 women with underweight BMI in the first pregnancy. Of these, 670 had at least one preterm birth. Among these 670, 86 (21.8%) women experienced a recurrent preterm birth. Odds for first term - second preterm birth were decreased for increases in maternal age (aOR: 0.90, 95%CI: 0.95-0.99) whereas inter-pregnancy interval <6 months was related to both first term - second preterm birth (aOR:1.66, 95%CI: 1.21-2.28) and first preterm birth - second term birth (aOR: 1.43, 95%CI: 1.04-1.96). Factors associated with recurrent preterm birth were: negative or no change in pre-pregnancy weight between pregnancies (aOR: 1.67, 95%CI: 1.07-2.60), inter-pregnancy interval <6 months (aOR: 2.14, 95%CI: 1.29-3.56), and maternal age in the first pregnancy (aOR: 0.93, 95%CI: 0.90-0.97).

Recurrent preterm birth among underweight women was associated with younger age, short inter-pregnancy interval, and negative or no weight change between pregnancies.