In nearly half of premature births, the cause is unknown. However, researchers have made some progress in learning the causes of prematurity. Studies suggest that there may be four main routes leading to premature labor.
- Maternal or fetal stress. Chronic psychosocial stress in the mother or physical stress (such as insufficient blood flow from the placenta) in the fetus appears to result in production of a stress-related hormone called corticotropin-releasing hormone (CRH). CRH may stimulate production of a cascade of other hormones that trigger uterine contractions and premature delivery.
- Infections. Studies suggest that premature labor is often triggered by the body’s natural immune response to certain bacterial infections, such as those involving the genital and urinary tracts and fetal membranes. Even infections far away from the reproductive organs, such as periodontal disease, may contribute to premature delivery.
- Bleeding. The uterus may bleed, due to problems such as placental abruption (when the placenta peels away, partially or almost completely, from the uterine wall prior to delivery). Bleeding triggers the release of various proteins involved in blood clotting, which also appear to stimulate uterine contractions.
- Stretching. The uterus may become overstretched by the presence of two or more babies, excessive amounts of amniotic fluid, or uterine or placental abnormalities, leading to release of chemicals that stimulate uterine contractions.
The finding that there are several routes that can result in premature delivery may help explain why prematurity is so difficult to prevent. Now that scientists have a better handle on possible pathways to prematurity, they may be able to develop more effective interventions that can halt the various chemical cascades that lead to it. But first they must identify the women who need these treatments.
(”Copyright March of Dimes. Used by permission.)
Who Will Deliver Early?
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 malformations. 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.
Tests, such as cervical length measurements and fetal fibronectin, are most 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 treatments.
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.
(”Copyright March of Dimes. Used by permission.)
Can Treatment Prevent Preterm Delivery?
Over the years, doctors have tried various strategies to help prevent premature delivery, including bedrest, intensive prenatal care for high-risk women, and drug therapy to stop uterine contractions. None of these are routinely effective, though they may help some individuals.
However, in 2003, two encouraging studies found that treatment with the hormone progesterone reduced the incidence of premature birth in women who had a previous preterm birth, a group that is at especially high risk of recurrence. The American College of Obstetricians and Gynecologists (ACOG) recommends that progesterone treatment (sometimes called 17p) be restricted only to pregnant women who are at high risk of preterm birth. A woman is considered high risk if she has previously been pregnant with a single baby and if she delivered that baby prematurely.
Studies showed that women with vaginal infections such as bacterial vaginosis (BV) and trichomoniasis were at increased risk of premature delivery. A 1995 study provided some encouraging results: treating certain high-risk women with antibiotics for bacterial vaginosis appeared to reduce their risk of premature delivery. Encouraged, physicians moved on to the next step: testing women for asymptomatic BV and trichomoniasis and treating them with antibiotics to reduce their risk. These, as well as later studies of high-risk women with symptomatic BV, failed to show that antibiotics reduce the risk of prematurity in most women with these genital infections.
Today women who develop preterm labor before about 34 weeks of pregnancy are often treated with one of several drugs (called tocolytics). These drugs often delay delivery for about 48 hours—buying some extra time to treat the pregnant woman with corticosteroid drugs. Corticosteroids speed maturation of fetal lungs and other organs, reducing infant deaths and cutting the incidence of the most serious complications of prematurity, including respiratory distress syndrome and bleeding in the brain.
Corticosteroids are recommended if delivery will occur before 34 weeks. Antibiotics are also usually given to protect the newborn from infection.
(”Copyright March of Dimes”. Used with permission.)