The study also shows that African-American women have shorter intervals of birth spacing and higher preterm births overall.
Study co-author and maternal-fetal medicine specialist Emily DeFranco, DO, an assistant professor in the department of obstetrics and gynecology at the University of Cincinnati College of Medicine, said they need to place a particular focus on waiting at least 18 months before becoming pregnant again in order to minimize the potential risk for preterm births for all women.
The study, conducted by DeFranco and colleagues at the Center for Prevention of Preterm Birth within Cincinnati Children's Hospital Medical Center's Perinatal Institute, studied outcomes of 454,716 live births from women with two or more pregnancies over a six-year period.
The researchers, using birth records from the Ohio Department of Health, compared the pregnancy lengths of three groups of women: those who had waited 18 months from delivery to conception (the optimal time for birth spacing identified from other studies), women with spacing of 12 to 18 months from delivery to conception and those with under 12 months from delivery to conception.
The study results, according to DeFranco, show that mothers with shorter times between birth and subsequent conception were more likely to give birth prior to 39 weeks (53.3 percent compared to 37.5 percent with the optimal 18 months of birth spacing). To define it further, all women with birth spacing of less than 12 months were twice as likely to have a preterm birth-under 37 weeks-as those who waited the optimal 18 months.
The findings have been published in BJOG: An International Journal of Obstetrics and Gynaecology.