MINNEAPOLIS – Black women who conceive using in vitro fertilization are at a markedly increased risk of any type of preterm birth, compared with other races, a retrospective cohort study indicates.
Hispanic women had a significantly lower rate of preterm births than did black women, and similar or slightly higher rates than did white women.
Native American women were not at an increased risk of any type of preterm birth, compared with whites, whereas Asians were at a reduced risk of preterm twin births.
"There exist notable racial and ethnic disparities in preterm births in infants conceived by IVF, with the highest rate in black women," Dr. Xu Xiong of the school of public health and tropical medicine at Tulane University in New Orleans reported at the annual meeting of the Society for Pediatric and Perinatal Epidemiologic Research.
Racial disparities in preterm birth may reflect biological differences resulting from genetic background or from variations in social demographics, socioeconomic status (SES), environmental exposures, lifestyles, cultural factors, and access to care and treatment, he said. SES factors (such as income, education, and occupation) were not available in the data set, although all of the women were able to finance expensive and time-consuming IVF treatment.
"In this study of relatively higher income women [who] conceived by IVF, our findings of an even greater difference in preterm births between black and white women than in the general population indicate that SES may not be as solely important a determinant for the racial gaps of preterm birth than previously postulated," Dr. Xiong said in an interview.
The investigators conducted a retrospective cohort study of 56,465 singleton and 23,748 twin pregnancies resulting from fresh nondonor IVF cycles in 82,519 women, using 2006-2008 data from the SART CORS (Society for Assisted Reproductive Technology Clinic Outcome Reporting System).
Among singleton births, 3.3% of black women had a very early preterm birth at less than 28 weeks, compared with 0.7% of white women, 1% of Hispanics, 0.7% of Asians, and 1% of Native Americans (P less than .05).
The pattern was similar for early preterm birth at less than 32 weeks (5.8% vs. 1.6%, 2.2%, 1.4%, and 1.4%, respectively), and for preterm birth at less than 37 weeks (13.3% vs. 6.6%, 8%, 6%, and 8%, respectively) (both P less than .05), Dr. Xiong reported.
In multivariate analysis with adjustment for confounding variables including maternal age; gravida status; smoking status; day of embryo transfer; number of embryos transferred; number of gestational sacs with heart rate; body mass index; prior spontaneous abortions; and infertility diagnosis, black women remained at significantly increased risk of very early preterm birth (odds ratio, 4.5), early preterm birth (OR, 3.7) and preterm birth (OR, 2.0), compared with white women (all P less than .05).
As expected, twin pregnancies increased preterm births for all groups, but once again, blacks had the highest rate of very early preterm birth at 7.7%, compared with 2.5% of whites, 3.7% of Hispanics, 2.3% of Asians, and 3% of Native Americans (P less than .05), he said.
Rates were also significantly higher for black women for early preterm twin birth (13.6% vs. 6.6%, 7%, 5.8%, and 5.8%, respectively) and for preterm twin birth (46.5% vs. 37.6%, 37.5%, 33%, and 41%, respectively; P less than .05).
In the adjusted analysis, black women delivering twins had a significantly increased risk of very early preterm twin births (OR, 3.3), early preterm birth (OR, 2.2) and preterm birth (OR, 1.4), compared with whites (P less than .05), reported Dr. Xiong and his coauthors.
At baseline, black women were more likely to be at least 40 years of age than were whites, Hispanics, Asians, and Native Americans (10.3% vs. 7.4%, 6.3%, 6.7%, and 6.6%, respectively), more likely to be obese (14.6%, 7.5%, 7.5%, 3%, and 9.5%, respectively), and more likely to report a prior preterm birth (69% vs. 59%, 60%, 58%, and 55.4%, respectively).
Roughly 7% of blacks and Native Americans and 6% of Asians smoked, compared with nearly 10% of white and Hispanic mothers. The majority of all women (57%) had two fresh embryos transferred, and 63.5% reported no prior use of IVF/assisted reproductive technology, with rates comparable across racial groups.
Dr. Xiong and his coauthors reported no conflicts of interest.