It is very difficult to predict which women will deliver prematurely. Doctors know that certain women are at high risk of premature delivery, including women who have had a previous premature birth, those with multiple gestations (twins, triplets or more), and women with certain uterine or cervical abnormalities.

But tests are not accurate in determining which of these women will actually deliver early. Available tests are even less helpful in identifying low-risk women who will have a premature delivery, and are generally not recommended for these women.

The two tests described below are useful in determining which high-risk women or women having contractions are unlikely to deliver within the next two weeks. These tests can relieve worries and spare women unnecessary treaments.

• Cervical length. The length of a woman's cervix is measured using vaginal ultrasound. Women with a shorter-than-average cervix and those whose cervix shortens on subsequent exams are at increased risk of premature delivery. This test is fairly accurate in determining which women are at lower risk of premature delivery.
• Fetal fibronectin. Fibronectin is a biological glue that helps attach the fetal sac to the uterine lining. It is normally seen in vaginal secretions up to 22 weeks of pregnancy, then not until 1 to 3 weeks before delivery. A swab is used to take a sample of vaginal secretions between 22 and 34 weeks of pregnancy. If fibronectin is seen, a woman appears to be at increased risk of premature labor. This test shows moderate success in predicting who will not deliver prematurely. In some cases, this test may be combined with a measurement of cervical length to increase accuracy.

Researchers continue to develop new tests for identifying women who will deliver prematurely. Many of the new tests measure biological markers associated with the various routes that lead to premature delivery, such as the stress-related hormone CRH or various immune and clotting factors. To date, tests that measure only one of these biological markers have not proven successful, but tests that measure a number of markers are showing some promise.

Medical experts are also looking for variant forms of genes that may increase the risk of preterm labor. This research may lead to improved screening tests.

July 2009