Study finds most stillbirths not linked to known maternal risk factors
RESEARCH TRIANGLE PARK, N.C. — Half of all stillbirths result from pregnancy disorders and conditions affecting the placenta, according to results reported by a National Institutes of Health research network established to find the causes of stillbirth as well as ways to prevent or reduce its occurrence.
RTI International served as the data coordinating and analysis center for the network and co-authored reports for two network studies published in the Dec. 13 issue of Journal of the American Medical Association (Causes of Death Among Stillbirths & Association Between Stillbirth and Risk Factors Known at Pregnancy Confirmation).
The researchers also found that most stillbirths could not be accounted for by pregnancy history and other maternal characteristics at the time the women in the study learned they were pregnant. However, the researchers found that some characteristics were associated with an increase in risk for stillbirth. These include a previous stillbirth, being a first-time mother, a history of miscarriage in earlier pregnancies, gestational diabetes, AB blood type, drug addiction, smoking three months before getting pregnant and maternal overweight and obesity.
The studies were conducted by the Stillbirth Collaborative Research Network of the NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). The Network researchers enrolled women who delivered a stillbirth as well as a representative sample of women with live births for comparison. The women were enrolled between 2006 and 2008 in 59 community and research hospitals in certain counties in Georgia, Massachusetts, Rhode Island, Texas and Utah.
The Network researchers in the study conducted a comprehensive medical evaluation after each stillbirth to identify the cause of death. With such an evaluation, the researchers could identify a probable cause of death in 61 percent of cases and a probable or possible cause of death in 76 percent of cases. Earlier studies, which typically were limited to analyzing medical records, could identify a cause of death in only about 50 percent of cases.
A stillbirth is the death of a baby at or after the 20th week of pregnancy. Stillbirth occurs in 1 out of 160 pregnancies in the United States. Since 2003, the stillbirth rate has remained at about 26,000 stillbirths each year.
Clinical sites for the Network are Brown University, Emory University, University of Texas Medical Branch at Galveston, Health Sciences Center at San Antonio, and the University of Utah Health Sciences Center in Salt Lake City.
Of the stillbirths in the study, 500 had a complete evaluation. The evaluation included an autopsy of the fetus, examination of the placenta, a test to check for abnormalities in the baby's chromosomes (known as a karyotype), as well as other tests and a review of the medical records.
The study authors noted that fewer than 50 percent of stillbirths in the United States undergo an autopsy exam. They added that the decision to conduct such examinations depends on a number of factors, such as reluctance by the family and physician to conduct an autopsy and concerns about cost.
"Our study showed that a probable cause of death—more than 60 percent—could be found by a thorough medical evaluation," said one of the NICHD authors of the paper Uma M. Reddy, M.D., M.P.H., of the Pregnancy and Perinatology Branch. "Greater availability of medical evaluation of stillborn infants, particularly autopsy, placental exam and karyotype, would provide information to better understand the causes of stillbirth."
The researchers determined that pregnancy or birth-related complications contributed to the largest proportion of stillbirths (29 percent). Such complications can include preterm labor or premature rupture of membranes that hold the amniotic fluid. Another such complication is abruption of the placenta, in which the placenta separates from the wall of the uterus. Other causes the researchers identified were abnormalities of the placenta (24 percent of cases), genetic conditions or birth defects (14 percent), infection (13 percent), problems with the umbilical cord (10 percent), maternal high blood pressure (9 percent) and other medical conditions affecting the mother (8 percent).
Infection contributed to more cases of stillbirth among African-American women (25 percent) than among white (7 percent) or Hispanic (9 percent) women. Pregnancy or birth complications also were implicated in significantly more stillbirths to African-American women (44 percent) than to Hispanic (25 percent) or white women (22 percent). African-American women were much less likely (4 percent) to experience stillbirth as a result of an umbilical cord abnormality than were their white (13 percent) and Hispanic (13 percent) counterparts.
The researchers also compared 614 cases of stillbirth with 1,816 live births in an effort to identify factors present at the start of pregnancy that could indicate an increased risk for stillbirth. The researchers confirmed findings by other studies that African-American women were at greater risk for stillbirth compared with white women and Hispanic women, and showed that about 22 percent of the excess risk for African Americans was associated with maternal factors present at the time the women learned they were pregnant. The study also found that the stillbirth risk for African-American women was greatest for deliveries before the 24th week of pregnancy.
They noted that other studies had found that, among African-American women, the rate of preterm birth before 28 weeks of pregnancy is three times that for white women. Because stillbirth and preterm birth among African-American women are more likely to occur in this early part of pregnancy, it is possible that some underlying factor or combination of factors is contributing to both conditions, the study authors wrote. For this reason, studies of early pregnancy may yield information on how to reduce the disparity in stillbirth and preterm birth between African-American women and other groups.