A normal pregnancy lasts approximately 40 weeks. While most pregnant women go into labor at the 40-week mark, some women go into labor a little earlier. Preterm labor is characterized by contractions that begin to open the cervix before the 37th week of pregnancy.
Preterm labor can lead to a premature birth, which poses many risks to the baby. Premature babies often need additional care after birth and sometimes have long-term health problems that can affect them throughout their entire lives. The earlier in pregnancy a baby is born, the more likely the baby will have physical or mental disabilities.
Preterm labor occurs in approximately 12 percent of pregnancies. The cause of preterm labor isn’t always known, but there are certain risk factors that can increase the likelihood of going into labor early.
Any pregnant woman can have preterm labor and premature birth, even if she’s done everything right during pregnancy. However, certain factors can make some women more likely than others to go into labor and give birth early. These risk factors include:
- multiple gestation (more than one baby in the womb)
- history of premature birth
- vaginal bleeding in the middle of pregnancy
- polyhydramnios (an excessive amount of amniotic fluid surrounding the baby)
- problems with the cervix
- problems with the uterus
- certain genetic conditions
- drug and alcohol use
- limited access to prenatal care
It’s important to remember that most women with these risk factors will carry their pregnancy to full term. However, it’s helpful to be aware of your risk so you can be thoroughly evaluated and closely monitored by your doctor.
Multiple gestation puts a pregnant woman at risk simply because the uterus must stretch more when it’s holding two or more babies. The uterus, just like any other muscle in the body, tends to contract when it’s stretched beyond a certain point. In a multiple gestation pregnancy, the uterus may be stretched to an extent where contractions begin before the babies are fully developed.
The risk for preterm delivery increases with each additional baby in the womb:
|Number of babies in the womb||Average gestational age at birth*|
*Gestational age refers to the number of weeks a woman is pregnant. It is usually calculated from the first day of the last known menstrual period.
Multiple gestation also places an expectant mother and her babies at an increased risk for other complications. The mother has a higher risk of developing preeclampsia and gestational diabetes, while the babies have a greater risk of getting severe anemia. They are also more likely to have low birth weights and birth defects. All these complications are problems in themselves, but they can also make preterm labor more difficult to manage and treat. It is likely that you will need the care of a high-risk obstetrical specialist if you have a multiple gestation pregnancy, to help prevent any adverse outcomes.
A woman who has delivered a premature baby in the past is much more likely to experience preterm labor and delivery in subsequent pregnancies. The likelihood depends on the number of previous premature births and how early they occurred. The earlier a previous premature birth took place, the more likely it is that the next birth occurs as early or even earlier.
It should be noted, however, that these risks primarily apply to women who have spontaneous premature births, not just preterm labor. A woman who has delivered a baby at full term has a very low chance of delivering a subsequent baby early. In addition, the more pregnancies a woman has delivered at full term, the less likely that subsequent births will be premature. Even when a woman has had one premature birth in the past, her chances of having another are reduced when she’s had at least one full-term pregnancy in between.
History of Abortion
Some researchers believe a history of abortion can increase a woman’s likelihood of giving birth early. Women who have had more than one abortion seem to be more likely to have a premature birth later in life. It's unclear why an abortion may cause preterm labor in a later pregnancy. One possibility is that the cervix may become damaged during abortion procedures. A woman may also have an incompetent cervix, which means that the cervix will abnormally open early in the pregnancy and result in preterm abortion. This can affect each subsequent pregnancy unless it is addressed by a doctor, typically by surgical means. Another possibility is that women who have had several abortions tend to have less access to healthcare and other resources than those who have never had an unplanned pregnancy. Both of these circumstances can increase risk for preterm labor and premature birth in future pregnancies.
Women who experience vaginal bleeding between the 12th and 24th weeks of pregnancy have a greater risk of experiencing preterm labor and delivery. The severity of the risk depends on the cause of the bleeding.
Placenta previa and placental abruption are two main causes of vaginal bleeding during pregnancy. Placenta previa occurs when the placenta partially or completely covers the opening of the cervix. Placental abruption happens when the placenta separates too early from the uterine walls. Both conditions are clearly linked with early labor and delivery.
Women who experience vaginal bleeding at any time during pregnancy should see their doctor immediately for evaluation. While vaginal bleeding doesn’t always signify a problem, it’s critical to pinpoint the cause of bleeding so any problems can be resolved quickly.
The presence of a bacterial or viral infection during pregnancy can increase risk for preterm labor and premature birth. An infection can develop in any part of a woman’s reproductive or urinary tract, including the vagina, cervix, uterus, urethra, bladder, or kidneys.
An infection can also occur in the bloodstream. In some pregnant women, the body’s response to the infection can trigger early labor and delivery.
To cause labor, the infection must reach the uterus, where it stimulates a chemical reaction that encourages the uterus to contract. Not all bacteria and viruses that reach the uterus trigger contractions. However, if they cross the two membranes surrounding the baby and enter the amniotic cavity, labor is much more likely to occur.
Some infections that are associated with preterm labor and premature birth include gonorrhea, chlamydia, trichomoniasis, and bacterial vaginosis.
Polyhydramnios refers to an excess amount of amniotic fluid, the fluid that surrounds the baby in the uterus. An increased amount of amniotic fluid causes the uterus to stretch more than usual. When the uterus is stretched beyond a certain point, it may begin contracting early and lead to premature birth.
Symptoms that may indicate polyhydramnios include unusually large abdomen for gestational age, difficulty breathing, decreased urine output, and increased swelling in the legs and feet.
To confirm the diagnosis, your doctor may order an ultrasound to determine the amount of amniotic fluid in the uterus. If polyhydramnios is diagnosed, your doctor may remove some of the excess fluid by performing an amniocentesis. During this procedure, an ultrasound is used to help guide a long needle through your abdomen and into the amniotic sac to extract the excess fluid.
An ultrasound can also be used to help determine the cause of polyhydramnios. The same needle that is inserted to remove excess fluid can also be used to take tissue samples, or biopsies. The results of this procedure can show whether something has gone wrong with the mother, the placenta, or the baby. The most common maternal causes for polyhydramnios are diabetes and red blood cell incompatibilities (for example, the Rh factors in the blood of the mother and the baby are incompatible). Placental causes are rare but include chorioangioma, which is a benign tumor of the blood vessels in the placenta. Fetal causes are more common and include multiple gestation, infection, birth defects that impair the growing fetus’ ability to swallow, and nonimmune hydrops, a condition in which the baby is swollen with fluid.
It’s important to determine the cause of polyhydramnios when possible, as the risk of preterm labor is largely related to the cause rather than to the severity of the condition. For example, women are more likely to experience preterm labor when a birth defect in the baby causes polyhydramnios.
The cervix, which forms the lower part of the uterus, normally remains closed throughout pregnancy to hold the baby securely inside the womb. Once labor begins, contractions cause the cervix to soften and shorten so that it can open for delivery. Sometimes, however, the cervix begins to dilate before it should. When this occurs, the condition is known as cervical insufficiency, or incompetent cervix. Women with cervical insufficiency are more likely to go into early labor and have a premature birth.
Cervical insufficiency may be caused by injury, surgery, or medication. The following factors can increase risk for cervical insufficiency:
- History of trauma to the cervix. If a woman's cervix tears during a delivery, for example, her cervix may be weak in future pregnancies.
- Previous operations on the cervix. Certain cervical operations, such as a cone biopsy, may be performed after a woman has an abnormal Pap smear. During these operations, a portion of the cervix is removed to examine for cancerous or precancerous cervical changes. This procedure is associated with increased risk for cervical insufficiency.
If you have cervical insufficiency, your doctor will monitor you closely throughout your pregnancy. You may also need a procedure known as cervical cerclage, which your obstetrician can perform. It can strengthen a weak cervix and allow for a full-term pregnancy.
A woman may have abnormalities of the uterus that have been present since birth. Some of the most common abnormalities include:
- presence of a second, completely formed uterus
- presence of a wall (septum) inside the uterus that divides it in two
- an irregularly shaped uterus
The risk for preterm labor depends on the type of uterine abnormality present. Women with an abnormally shaped uterus have the highest risk of complications, while those with a septum inside the uterus have the least risk.
Aside from medical conditions, certain outside influences can affect risk for preterm labor and premature birth.
Genetics and Race
Certain inherited traits can increase a woman’s risk for early childbirth. In the United States, African-American women are more likely to experience preterm labor than other ethnicities, even when social and economic factors are taken into account. The risk tends to be greatest in the earlier weeks of pregnancy.
Researchers aren’t exactly sure why African-American women have a higher risk of going into labor early. However, African-American women tend to have higher rates of infection affecting the reproductive and urinary tracts, which increases the risk for preterm labor.
Low-income women are more likely to deliver prematurely because they often lack sufficient food, shelter, and prenatal care. Without adequate nutrition, a woman is likely to begin pregnancy well below her ideal weight. This is an additional risk factor for preterm labor.
Premature births are also more likely to occur when the father or mother of the baby is unemployed or doesn’t have health insurance. This can affect the mother’s ability to receive quality prenatal care. The stresses associated with low income or unemployment may also contribute to premature birth.
Numerous social factors determine a woman’s risk for preterm labor. These include:
- being under age 16 or over age 40
- being single
- being physically or emotionally abused
- drinking alcohol, using recreational drugs, or smoking during pregnancy
- having a lack of support from family, friends, or community members
- being frequently exposed to chemicals and pollutants
- working long hours
Having a risk factor doesn’t necessarily mean you’ll experience preterm labor and give birth early. However, it does increase your chances. Therefore, it is very important that you speak with your doctor early in your pregnancy about what you can do to lower your risk.
You asked, we answered
- What are the warning signs of preterm labor?
Signs of preterm labor almost always include lower abdominal contractions and/or back pain that may be accompanied by a loss of fluid, vaginal discharge, vaginal bleeding, and pelvic fullness or pressure.- Tyler Walker, MD