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Ultrasound Predicts Preemie Risk

Women with short cervixes likelier to deliver early

TUESDAY, Sept. 18, 2001 (HealthDayNews) -- Vaginal ultrasound to measure a pregnant woman's cervix is a good predictor of her risk of premature delivery, new research says.

A study of women with a history of premature deliveries shows that those with a cervical length of less than 25 millimeters on ultrasound tests in the second trimester may be as much as 4.5 times likelier than those with longer cervixes to go into labor at least five weeks early.

Scientists say it's impossible to set a cutoff length beneath which women will definitely suffer pregnancy problems. And they say doctors can do nothing to prevent preemie delivery. The findings appear in the Sept. 19 issue of the Journal of the American Medical Association.

"Preterm delivery is such a huge problem, and this is one really nice handle that we now have to predict who is going to deliver early," says Dr. Catherine Spong, chief of the Pregnancy and Perinatology Branch at the National Institute of Child Health and Development. "This study is likely to be the basis of future interventional trials to try to identify methods" of preventing preemie births in the future, she says.

Although no methods are known to work, some doctors perform a procedure called "cerclage" on women with a history of second-trimester spontaneous miscarriages that are believed to be related to an "incompetent" cervix. The operation involves sewing up the neck of the cervix (called the os) nearest the fetus, hoping that will help it bear the pressure of the swelling womb. "We like to think that it works, but we have no studies to show that it does," Spong says.

An estimated 11 percent of all babies in this country are preemies, putting them at increased risk of health complications and death. Prematurity also has been linked to developmental difficulties later in life.

The cervix is the portal at the base of the uterus, and it shortens as pregnancy progresses to help keep the growing fetus in the womb. But some women experience less cervical shrinkage than others, and previous studies have suggested that those with a shorter cervix may be more prone to premature delivery.

In the latest study, Dr. John Owen, an obstetrician, and his colleagues at the University of Alabama in Birmingham and at eight other university medical centers followed 183 women with a high risk of premature delivery, measuring their cervixes with several vaginal ultrasound scans from week 16 through week 23 of gestation.

Forty-eight, or 26 percent, delivered before week 35, or five or more weeks early, the researchers say. Those with a cervical length below 25 millimeters on the first reading -- about 40 percent shorter than normal -- were 3.3 times more likely to deliver early, while those who had subsequent readings in that range had up to a 4.5 times greater risk of preemie labor.

"Not everybody who shortened their cervix ended up delivering preterm," Owen says. But "the take-home message is that shortened cervical length increases the likelihood of preterm birth."

"Our goal in performing this study was to determine, number one, if these changes are visible early in pregnancy, and number two, is the predictive value high enough to warrant" studying ways to correct the problem? I think the answer is yes to both [questions]," Owen says.

Contrary to early evidence, the study did not find that reshaping of the internal os of the cervix raised the likelihood of premature delivery.

What To Do

Owen and Spong both stress that the results apply only to women at high risk of spontaneous premature delivery, defined as those with a history of labor before 32 weeks of gestation, or at least eight weeks early. For the average woman without such problems, the value, if any, of measuring cervical length during the second trimester isn't clear, they say.

To learn more about preterm deliveries, try the Association of Women's Health, Obstetric and Neonatal Nurses. You can also get more pregnancy information from Childbirth.org.

SOURCES: Interviews with John Owen, M.D., associate professor of obstetrics and gynecology, University of Alabama at Birmingham, and Catherine Spong, M.D., chief, Pregnancy and Perinatology Branch, National Institute of Child Health and Development, Bethesda, Md.; Sept. 19, 2001 Journal of the American Medical Association
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