The study, by the National Human Genome Research Institute (NHGRI), part of the National Institutes of Health (NIH), confirms findings from previous studies that very high levels of maternal cholesterol can increase the risk of premature birth.
However, in a surprising new twist, the researchers found that low maternal cholesterol levels, which may be related to a woman's genetic makeup, diet or other health factors, also may lead to adverse birth outcomes, including premature birth and low birth weight.
"Based on our initial findings, it appears that too little cholesterol may be as bad as too much cholesterol during pregnancy, but it is too early to extrapolate these results to the general population. More research is needed to replicate this outcome and to extend it to other groups," said Dr. Muenke, the study's senior author and chief of the Medical Genetics Branch in NHGRI's Division of Intramural Research.
The study involved pregnant women between the ages of 21 and 34 who were referred to South Carolina clinics for routine prenatal care between 1996 and 2001. According to their medical records, they were all nonsmokers without diabetes who were carrying just one child. It looked at cholesterol levels from their second trimester of pregnancy. Premature birth was defined as delivery before 37 weeks of gestation.
The researchers examined the effects of maternal cholesterol levels on rates of premature delivery, impaired fetal growth and birth defects. In addition, they analyzed measurements of newborn weight, length and head circumference. After examination, no differences were seen in the rate of birth defects, but researchers did detect a trend towards smaller head sizes among babies born to women with very low cholesterol.
"The right amount of cholesterol is fundamental for good health, both before and after birth. During pregnancy, cholesterol is critical for both the placenta and the developing baby, including the brain," explained Dr. Muenke.
The researchers found about 5 percent of the women with cholesterol levels in the moderate range of 159-261 milligrams per deciliter (mg/dl) gave birth prematurely.
In contrast, white women with the lowest cholesterol levels - less than 159 mg/dl - had a 21 percent incidence of premature births. Interestingly, no increase in premature births was observed among African American women in the low-cholesterol category.
However, full-term babies born to both white and African Americans with low cholesterol weighed 5 ounces less on average than full-term babies born to women with moderate cholesterol.
"This study sheds important light on the intricate biological mechanisms at work during human gestation. In light of these findings, researchers have a renewed impetus to establish the genetic and environmental causes of low cholesterol levels because of its relevance to pregnancy," said NHGRI Scientific Director Eric Green, M.D., Ph.D.
The study is published in the October issue of the journal Pediatrics.