Being underweight - as evidence by a low body mass index (BMI) -- before pregnancy raises the risk of preterm birth in Black and Hispanic women to a greater extent than in White women, new research shows.

Being underweight also increases the risk of vaginal inflammation in Black women.

“These data suggest that there are racial differences in how nutritional status, as represented by BMI, might influence...the risk of spontaneous preterm birth,” Drs. Hyagriv N. Simhan and Lisa M. Bodnar of the University of Pittsburgh write in the American Journal of Epidemiology.

Low BMI has been tied to increased risk of early delivery across ethnic groups, the researchers note, while Black women are twice as likely as Whites to deliver prematurely. It is “biologically plausible” that BMI could increase preterm birth risk via effects on immunity and inflammation, they add.