( BBC ) - Pregnant women who have very low levels of cholesterol may be at a greater risk of giving birth prematurely, US research suggests.
The National Human Genome Research Institute study confirmed previous findings linking high cholesterol to a raised risk of premature birth.
But the researchers were surprised to find low cholesterol levels also seemed to raise the risk.
The study is published in the journal Pediatrics.
Prematurity is associated to a raised risk of health problems, including lung and heart conditions, blindness, deafness and learning difficulties.
The researchers examined more than 1,000 women from South Carolina and their newborn babies.
They found 5% of women with moderate cholesterol levels gave birth prematurely.
In contrast, the prematurity rate among women with the highest cholesterol levels - more than 261milligrams per decilitre (mg/dl) - was about 12%.
And among white women with the lowest cholesterol levels - less than 159mg/dl - it was 21%.
However, no increase was observed among African American women in the low-cholesterol category.
The researchers found no difference in the rate of birth defects - but women with very low cholesterol were more likely to give birth to babies with a smaller head size.
Lead researcher Dr Max Muenke said more research was needed to confirm the findings.
"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."
"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."
Low maternal cholesterol levels can be related to a woman's genetic make-up, but also factors such as diet.
Professor Andrew Shennan, an obstetrics expert and spokesman for the charity Tommy's, said it had been observed that underweight women had a tendency to give birth early.
"We know that the uterus will contact less efficiently in women who have a lot of blood lipids (fats) floating around," he said.
"It is possible that the converse could be true: that the uterus is more excitable and more likely to deliver spontaneously if blood lipid levels are low."