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

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

Methods: 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.

Results: 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).

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