Last updated: 02/17/2009

Approximately 10% of pregnancies end in preterm delivery, defined as a delivery that occurs before week 37 of pregnancy (the average pregnancy lasts 40 weeks). Premature birth is a major cause of serious health problems in neonates (newborn babies),...

including respiratory distress, difficulty regulating body temperature, and infection. More than 85% of long-term disabilities in otherwise healthy babies and 75% of deaths among newborns occur as a result of preterm delivery.

A woman with an incompetent cervix is 3.3 times more likely to deliver prematurely. The cervix is the neck-shaped opening at the lower part of the uterus and is normally closed tight during pregnancy until the baby is ready to be delivered, at which point it expands (dilates) to roughly 4 in (10 cm) in diameter. An incompetent cervix is prone to dilating and/or effacing (shortening) prematurely during the second trimester. The growing fetus subsequently places too great a strain on the cervix, leading to miscarriage (loss before week 20 of pregnancy) or premature delivery (loss after week 20). Approximately 1% of women will be diagnosed with an incompetent cervix (one in 500–2,000 pregnancies). It is the cause of 25% of losses during the second trimester.

A doctor might recommend a cerclage be performed if a woman has one or more of the following risk factors:

• a previous preterm delivery
• previous trauma or surgery to the cervix
• early rupture of membranes ("breaking water")
• hormonal influences
• abnormalities of the uterus or cervix
• exposure as a fetus to diethylstilbestrol (DES), a synthetic hormone that was used in the mid-twentieth century to treat recurrent miscarriages