NEW YORK (Reuters Health) - Obese pregnant women are known to be at increased risk of an unplanned C-section, and now a new study suggests that weaker contractions of the uterus may be one reason.

The researchers aren't sure why obese women might have inadequate uterine contractions, but high cholesterol is one possibility, they speculate. If this is the case, weight loss or cholesterol treatment before pregnancy might prevent some emergency C-sections, study co-author Dr. Siobhan Quenby told Reuters Health.

Quenby of the Liverpool Women's Hospital in the UK and her colleagues report the findings in BJOG, the British Journal of Obstetrics and Gynecology.

The researchers reviewed the records of 3,913 women who gave birth at their hospital in 2002. They also collected samples of uterine muscle tissue from another 73 women who underwent elective C-sections. The samples were analyzed to see whether the muscle's ability to contract varied among women based on their weight.

Overall, Quenby's team found, both the overweight and the obese women had a higher rate of C-sections than normal-weight women. In addition, obese women were 3.5 times more likely than normal-weight women to need a C-section due to a slowly progressing first stage of labor.

When the researchers analyzed the uterine muscle samples, they found evidence that those taken from the obese women had a poorer ability to contract. "We suggest that these findings indicate that obesity may impair the ability of the uterus to contract in labour," Quenby and her colleagues conclude.

Though the reasons aren't certain, it's possible that higher cholesterol levels that are often seen in obese women play a role, according to the researchers. Cholesterol is a key component of body cells' outer membranes, and research suggests that it's important in controlling muscle contraction.

Studies have also found that cholesterol is elevated not only in obese pregnant women's blood, but their uterine muscle tissue as well, Quenby and her colleagues note.

More research is needed, they say, to confirm whether high cholesterol affects uterine contractions, and if weight loss and cholesterol lowering might lower obese women's risk of C-section.