What we know about prematurity
But what does the future hold for these babies? Many survivors grow up healthy; others aren't so lucky. Even the best of care cannot always spare a premature baby from lasting disabilities such as cerebral palsy; intellectual disabilities and learning problems; chronic lung disease; and vision and hearing problems. Half of all neurological disabilities in children are related to premature birth.
Although providers have made tremendous advances in caring for babies born too small and too soon, we need to find out how to prevent preterm birth from happening in the first place. Despite decades of research, scientists have not yet developed effective ways to help prevent premature birth.
In fact, the rate of premature birth increased by more than 20 percent between 1990 and 2006. This trend and the dynamics underlying it underscore the critical importance and timeliness of the March of Dimes Prematurity Campaign. The rate fell to 12.3 percent in 2008 from 12.7 in 2007, a small but statistically significant decrease.
Why do women give birth early?
In nearly 40 percent 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 spontaneous premature labor.
- Infections/Inflammation. 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.
- 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 birth.
- Bleeding. The uterus may bleed because of problems such as placental abruption (the placenta peels away, partially or almost completely, from the uterine wall before 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 the release of chemicals that stimulate uterine contractions.
These four routes are not the only things to consider. Other factors, such as multiple pregnancy, inductions and cesarean sections, can also play a role. But knowledge about these four routes may help scientists develop more effective interventions that can halt the various chemical cascades that lead to premature birth.
Who will give birth early?
It is very difficult to predict which women will deliver prematurely. Currently, tests are not considered helpful in identifying low-risk women. However, there are two tests that 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 treatments.
- 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 birth. This test is fairly accurate in determining which women are at lower risk of premature birth. A woman can ask her provider about having an ultrasound to check for short cervix.
- 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 give birth 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 give birth prematurely. Many of the new tests measure biological markers associated with the various routes that lead to premature birth, 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.
Who is at increased risk?
Preterm labor and birth can happen to any pregnant woman. But it happens more often to some women than to others. Researchers continue to study preterm labor and birth. They have identified some risk factors, but still cannot predict which women will give birth too early. Having a risk factor does not mean a woman will have preterm labor or preterm birth. Three groups of women are at greatest risk of preterm labor and birth:
- Women who have had a previous preterm birth
- Women who are pregnant with twins, triplets or more
- Women with certain uterine or cervical abnormalities
If a woman has any of these three risk factors, it's especially important for her to know the signs and symptoms of preterm labor and what to do if they occur.
Some studies have found that certain lifestyle factors may put a woman at greater risk of preterm labor. These factors include:
- Late or no prenatal care
- Drinking alcohol
- Using illegal drugs
- Exposure to the medication DES
- Domestic violence, including physical, sexual or emotional abuse
- Lack of social support
- Extremely high levels of stress
- Long working hours with long periods of standing
- Exposure to certain environmental pollutants
Certain medical conditions during pregnancy may increase the likelihood that a woman will have preterm labor. These conditions include:
- Infections (urinary, vaginal, sexually transmitted; possibly others)
- High blood pressure and preeclampsia
- Clotting disorders (thrombophilia)
- Bleeding from the vagina
- Certain birth defects in the baby
- Being pregnant with a single fetus after in vitro fertilization (IVF)
- Being underweight before pregnancy
- Short time period between pregnancies (less than 6 to 9 months between birth and the beginning of the next pregnancy)
Medical researchers also have identified certain groups of women who are at increased risk of having a premature baby. These groups include:
- African-American women
- Women younger than 17 and older than 35
- Women who have a low income
Experts do not fully understand why and how these factors increase the risk that a woman will have preterm labor or birth.
Can treatment prevent premature birth?
Over the years, providers have tried various strategies to help prevent premature birth, 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 already had a preterm birth. This group is at especially high risk of having another early birth.
The American College of Obstetricians and Gynecologists (ACOG) recommends that progesterone (sometimes called 17P) may be given to a woman if both of these requirements describe her:
- She's had a spontaneous preterm birth before, when she was pregnant with just one baby. Spontaneous preterm birth means labor began on its own, without drugs or other methods. Or the sac around the baby broke early, causing labor.
- She is currently pregnant with just one baby.
Medical experts agree that progesterone shots can help prevent preterm birth, but only for women who meet both requirements listed above.*
ACOG says that progesterone also may be given to women who have a short cervix.
A number of studies have looked at the effectiveness of antibiotic treatment in reducing the risk of preterm birth. Antibiotic treatment appears to help prolong pregnancy in women with premature rupture of the membranes (the bag of waters breaks before 37 weeks). This condition (also called PROM) often results in preterm birth.
Antibiotics have been given to women with vaginal infections, such as bacterial vaginosis (BV) and trichomoniasis. These women may have an increased risk of premature birth. But most studies have failed to show that antibiotics reduce the risk of early birth in most women with these genital infections.
Some studies suggest that a procedure called cerclage (the doctor puts a stitch in the cervix to help keep it closed) may help reduce the risk of preterm birth in some women who have had a previous preterm birth and who also have certain cervical abnormalities. The doctor removes the stitch at around 37 weeks of pregnancy.
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 the risk of infant deaths and serious complications of prematurity, including respiratory distress syndrome (breathing problems) and bleeding in the brain. Doctors recommend corticosteroids if a woman is likely to deliver before 34 weeks.
Are there complications in the newborn?
Some premature babies face serious complications, including:
- Respiratory distress syndrome, which is a serious breathing problem that affects mainly babies born before 34 weeks of pregnancy.
- Bleeding in the brain, called intraventricular hemorrhage (IVH), which is most common in babies born before 32 weeks of pregnancy. It can cause pressure in the brain and brain damage.
- Patent ductus arteriosus, which is a heart problem that is common in premature babies. Untreated, it can lead to heart failure.
- Necrotizing enterocolitis (NEC), which is a potentially dangerous intestinal problem.
- Retinopathy of prematurity (ROP), which is an eye problem that occurs mainly in babies born before 32 weeks of pregnancy. In severe cases, treatment is needed to help prevent vision loss.
We have made progress in learning about the routes that lead to preterm birth. But we have a long way to go in developing treatments to prevent it. Researchers agree that we need to develop better screening tests to identify women destined to give birth early, and treatments that can be used early on to interrupt the cascade of events leading to prematurity.
*American College of Obstetricians and Gynecologists (ACOG) Committee Opinion #419: Use of progesterone to reduce preterm birth, October, 2008.
Last reviewed April 2012