Underweight black women risk preterm delivery

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.

So the researchers set out to determine whether they could identify any relationship between BMI, preterm birth, and vaginal inflammation, and whether the relationship varied by racial group

They analyzed data for 11,392 women participating in a study of vaginal infections and prematurity, all of whom were enrolled between 23 and 28 weeks of pregnancy.

BMI is based on the relationship between body weight and height, with values between 20 and 25 being normal. People with lower values are considered underweight, while those with higher values are considered overweight or obese.

Overall, the researchers found, women with a low BMI before pregnancy were 40 to 90 percent more likely to have a preterm birth than women with a normal BMI. Moreover, the magnitude of the relationship was greater in both blacks and Hispanics than in whites.

As noted, the researchers also found that black women with a low BMI were also at increased risk for vaginal inflammation, a finding not seen in the other ethnic groups.

The study was too small to address how low BMI, inflammation and preterm birth weight interact, or whether these interactions varied by race, Simhan and Bodnar note.

“Still, we speculate that our findings of ethnic differences in the contributions of both BMI to preterm birth and BMI to the vaginal inflammatory milieu may explain, in part, the racial disparity in preterm birth,” they conclude.

SOURCE: American Journal of Epidemiology, March 1, 2006.

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