Premature Labor and its Causes

Sep 27 2008

Unfortunately, we know very little about the cause of most instances of preterm labor. Occasionally it is due to an abnormality of the uterine body or cervix. As these abnormalities also may cause spontaneous abortion or miscarriage. Miscarriage is different from preterm birth in that a miscarried fetus cannot survive. Today, 24 weeks gestation is the usual cutoff used to distinguish preterm from miscarried babies.

One of the most likely causes of preterm labor is an infection in the uterus. This is called chorioamnionitis-an inflammation of the chorion and amnion, or the two membranes surrounding the fetus. Infection also may be a cause of rupture of the membranes-the bag of waters breaking. Preterm rupture of the membranes often leads to preterm labor.

Although infection is thought to be an important cause of preterm rupture of the membranes and preterm labor, the specific infectious organisms involved have not been identified conclusively. Bacterial vaginosis, a rather common condition in which the usual “flora” of the vagina is disturbed, is a possible cause. This may involve the organism Mycoplasma hominis, frequently found with premature rupture of the membranes. Chlamydia trachomatis, the most common sexually transmitted organism in the United States today, also may be involved in preterm birth and preterm rupture of the membranes, but studies do not prove this association.

Group B streptococcus is an organism possibly associated with preterm labor. It is also a major cause of severe newborn infection. It does not always cause any unusual symptoms in a woman, although it may cause urinary tract or uterine infection during pregnancy or after birth. The Centers for Disease Control and Prevention, in collaboration with the American College of Obstetricians and Gynecologists and the American Academy of Pediatrics, have developed specific guidelines for testing and treatment for Group B “strep” in pregnancy.

Treatment of Group B strep(GBS) in labor is intended to prevent the consequences of GBS disease in newborns. As CBS is one possible cause of preterm labor, treatment is recommended for any woman in labor or with ruptured membranes before 37 weeks gestation, unless a culture from her vagina and rectum taken at 35 to 36 weeks gestation is avaIlable and shows no GBS. If a prior culture is unavailable, then a culture can be taken at the time the woman sees her care provider or comes to the hospital. Antibiotics(usually penicillin) are started. If the culture is negative, the medication can be stopped. If the culture is positive, antibiotics are continued until and through labor. If delivery does not occur within 4 weeks, the culture should be repeated.

A urinary tract infection might lead to preterm labor if untreated. Any condition that causes the uterus to be overdistended(unusually large) may result in preterm labor. These conditions include hydramnios and multiple pregnancy. Among multiples, quadruplets deliver earlier than triplets, triplets deliver earlier than twins. Placental problems.

A pregnancy that occurs with an intrauterine device(IUD) in place has an increased chance of ending in preterm labor. If you have an IUD and become pregnant, you should have the IUD removed if you want to keep the pregnancy. Sometimes, however, the IUD strings have moved up into the cervical canal and removing the device would be dangerous to the pregnancy. While a retained IUD increases the chances of preterm birth, it does not cause anomalies in the fetus.

A recent review of studies on work and pregnancy found a number of work-related risks for preterm labor. These include heavy and/or repetitive lifting or carrying loads; manual labor; physical exertion; prolonged standing; shift work; and work fatigue.

Substance abuse, including cigarettes, drugs, or alcohol, may lead to preterm labor. Because more than 10 percent of pregnant women smoke, tobacco use is responsible for a large number of preterm births. Young adolescent mothers are more likely to deliver preterm and women living in poverty have a greater chance of having a preterm birth. Poor weight gain during pregnancy and some severe diseases of the mother can contribute to preterm labor.

Women who have had a preterm birth have an increased chance for another preterm birth. This does not explain the cause for preterm birth, but is a major risk factor.