The length of a woman's cervix at mid-pregnancy may indicate her risk of needing a Caesarean birth, a new study suggests.

"Women having their first baby who have a long cervix around 23 weeks of pregnancy are more likely to be delivered by emergency Caesarean section during labour at term," the study's lead author, Dr Gordon Smith, head of obstetrics and gynaecology at Cambridge University in the UK, said in a prepared statement. "The nature of this finding is not such that women should now have this measurement performed to predict Caesarean section risk," he added. "The key issue in this analysis is understanding the processes that lead to normal and abnormal labour."

Several experts said that doctors won't be measuring your cervix halfway through your pregnancy, any time soon, to predict whether or not you'll have a natural birth.

"If the cervix hasn't prepped itself for labour, there's something going on," said Dr Miriam Greene, an obstetrician at New York University Medical Center. "The idea that a longer cervix leads to a greater risk of Caesarean makes sense. The question is, if I start checking at 23 weeks and find a longer cervix, what then?"

Dr Robert Lorenz, director of maternal-foetal medicine at Beaumont Hospital in Royal Oak, Michigan, echoed Greene's sentiment. "This is a very interesting study that gives us another perspective on what happens in labour, but I don't think it has any immediate clinical applications," he said. Even years from now, he added, he's not sure anyone would intervene at 23 weeks, because "anything you do to change the cervix at 23 weeks might bring on preterm birth."

But both Greene and Lorenz said the new study was impressive because of the large numbers of women included, and because it adds to scientists' understanding of what happens to a woman's body during pregnancy and delivery.

The study, published in the March 27 issue of the New England Journal of Medicine, included almost 25,000 women who had their cervical length measured at 23 weeks of gestation using transvaginal ultrasound.

Caesarean rates were highest among those women with the longest cervix measurements. The women whose cervical measurements were between 40mm and 67mm long had a 25.7% C-section rate, compared to a 16% rate in those with the shortest cervical measurements - 16mm to 30mm. C-section rates were 18% and 22% for the third and fourth quartile cervix measurements, respectively. That translates to an 80% increased chance of a Caesarean section for a woman with the longest cervical measurements, according to the study.

"Studies in a number of species have suggested that the uterus prepares for labour well in advance of term. However, this is the first major study to suggest that the same is true in women," Smith said.

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Originally posted April 2008.

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