Preterm Labor

The leading cause of infant death and medical complications is preterm birth. A full term pregnancy is considered to be between 37 and 42 weeks long. Babies born before 37 weeks are considered preterm. Advances in neonatal care have significantly improved the outcome of these infants but the rate of preterm deliveries has not changed despite increasing knowledge of what causes preterm labor and ultimately preterm birth. In 2000 11.6% of babies born in the United States were born prematurely, almost 2% were born before 32 weeks. Half of all perinatal deaths are in babies born before 32 weeks gestation. 85% of complications in newborn infants are attributable to preterm birth.

Causes of preterm birth can generally be divided into 4 categories. Some babies are delivered preterm because of medical complications in the mother or fetus. These would include problems such as a fetus that isn’t growing, problems with the placenta, or medical problems in the mother such as hypertension or diabetes. So called iatrogenic prematurity accounts for 20-30% of preterm births. The remainder of preterm births are due to preterm labor either due to an infection of the uterus, premature rupture of the membranes or preterm labor of unknown cause. Close to half of all mothers who have true preterm labor have no identifiable cause. As it is the fetus who initiates labor through a complicated cascade of hormonal changes some preterm labor may reflect attempts by the fetus to escape from a hostile intrauterine environment.

There are many risk factors for preterm labor but these factors will only identify half of all women destined to deliver preterm and most women with some risk factors will deliver full term babies. Some risk factors can be modified before or during pregnancy. Maternal age less than 18 or over 40, anemia, poor nutrition, cigarette smoking, being underweight during pregnancy, urinary tract and genital infections, lack of prenatal care and possibly strenuous work and high personal stress levels are potentially modifiable factors. Other risk factors for preterm labor include a prior preterm birth, low socioeconomic status, a cervical or uterine anomaly, multiple gestation, excessive uterine contractions and a prematurely dilated cervix. Despite the recognition of these risk factors there are no proven interventions to decrease risks of preterm labor in women designated "high risk."

Many pregnant women will experience preterm contractions. This is sometimes difficult to distinguish from true preterm labor. Braxton-Hicks contractions are used to describe painless uterine contractions that do not cause the cervix to change. True preterm labor is diagnosed when regular uterine contractions occur with a cervix that is dilated and/or effaced (e.g. getting shorter). Uterine contractions may or may not be painful. Other symptoms of preterm labor include menstrual type cramping, dull backache, pelvic pressure, vaginal bleeding and changes in vaginal discharge. Premature rupture of the membranes may occur with or without contractions. A watery vaginal discharge is the most common symptom of premature rupture of the amniotic sac.

Once a diagnosis of preterm labor has been made treatment is usually started. The most important and effective intervention is to administer steroids to the mother. Steroids do not alter the course of preterm labor but significantly decrease the incidence of respiratory distress syndrome, intraventricular hemorrhage (bleeding into the brain) and overall morbidity in infants who are born preterm. There are several medications used to halt preterm labor, these are effective in stopping labor long enough for the steroids to benefit the fetus. Medications to stop labor are usually not used when there is preterm rupture of the membranes but antibiotics have been shown to help prolong these pregnancies. Sometimes labor stops without any intervention. Bed rest is used by most Obstetricians in women in or at very high risk of preterm labor. Almost 20% of pregnancies have bed rest recommended (at a yearly cost of 250 million dollars) but there is no study that shows bed rest to be of significant benefit in prevention of preterm labor.

Decreasing the incidence and morbidity from premature delivery is going to require ongoing research, education and collaboration amongst women, health care providers and research scientists. Perhaps our biggest weapon against preterm labor at this time is to educate women about both risk factors for and symptoms of preterm labor. Recognition of risk factors before pregnancy may allow some intervention such as treatment of infections and cessation of cigarette smoking. During pregnancy early diagnosis of preterm labor allows timely administration of medications to help prolong the pregnancy and prevent some of the complications in very premature infants. Perhaps our hardest task as Obstetricians is identifying those women who will benefit from interventions to decrease the risk of preterm labor without over treating all pregnant women who may have a risk factor for preterm labor.