Additionally, 12.7 percent of white women with low cholesterol levels gave birth prematurely, compared to just five percent of those with higher cholesterol levels. No such association was found in black women, however.
"To our surprise, we found that white women with very low cholesterol also have a significant risk of having babies born prematurely," said Dr. Max Muenke, chief of medical genetics at the U.S. National Human Genome Research Institute in Bethesda, Maryland.
Results of the study are in the October issue of Pediatrics.
"This study intuitively makes some sense," said Dr. Robert Welch, chair and program director for obstetrics and gynecology at St. John Health's Providence Hospital in Southfield, Michigan. "Cholesterol is a building block for membranes, hormones and proteins, so it makes sense that if you have low cholesterol, your baby won't have the substrate it needs to grow."
Each year, more than half a million babies are born prematurely in the United States. Medical advances have brought down the incidence rates for many diseases and conditions, but that's not been the case with premature or low birth weight babies.
In 1994, 11 percent of babies were born prematurely, and 7.3 percent were considered low birth weight. In 2004, those numbers were 12.5 percent and 8.1 percent, respectively. A preterm birth is one that occurs before 37 weeks, and a baby under five and a half pounds is considered to have a low birth weight.
According to Muenke, because previous studies have shown that high cholesterol levels -- over 300 mg/dL -- may lead to premature birth, the researchers wanted to know if very low levels could also have an effect.
The researchers recruited more than 1,000 women from prenatal clinics in South Carolina. The women were between 21 and 34 years old, didn't smoke, didn't have diabetes and were only pregnant with one baby. Cholesterol levels were measured at about the 17th or 18th week of pregnancy.
Overall, 118 of the women had low cholesterol levels mid-pregnancy and 940 women had levels higher than 159 mg/dL.
The study found that white women had five times higher odds of delivering prematurely if they had low cholesterol. There was no association between cholesterol levels and preterm delivery in black women, according to the study.
However, for both races, babies weighed an average of 150 grams less (about one-third of a pound) when born to mothers with low cholesterol. Additionally, babies born to mothers with low cholesterol levels were about twice as likely to have a small head circumference.
Muenke said the low cholesterol levels are likely caused by a combination of genetics and nutrition. According to the study authors, the low cholesterol levels may stem from poor diet and nutritional deficiencies. What isn't yet known is if raising cholesterol levels in these women would have a positive effect on the baby's health.
"I wouldn't recommend routine screening for cholesterol in pregnant women right now," cautioned Welch. "It would increase health care expenditures tremendously without a known benefit. This was a first study; others need to done. We also need to find some explanation of why it's taking place only in white women and not in black women."
"What this study does suggest is that diet in pregnancy is important, and that not paying attention to nutrition could be harmful," said Welch.
Muenke agreed, adding that, "At this point, the message is really to 'stay tuned' for a repeat study. In the meantime, talk with your obstetrician about a healthy lifestyle, including exercise and healthy nutrition."