Up to two-thirds of women who deliver their babies early have no risk factors.
The remaining one-third of women who deliver early have one of the following risk factors (listed in order of importance):
- multiple gestation (more than one baby in the womb);
- history of preterm 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; and
- genetic, economic, and social factors (for example, women who are African-American, who smoke, or who are significantly underweight when they get pregnant have higher risk for preterm labor.)
It is important to remember that most women with risk factors will deliver their babies at full term. However, it is still important to learn as much as you can about your risk and to be thoroughly evaluated and closely followed by yourcare provider. Each of these risks is explained below in more detail.
Multiple gestation puts a woman at risk simply because the uterus must stretch more when it is holding two or more babies. The uterus is a muscle and, like any muscle, it tends to contract when it gets stretched beyond a certain point. In a multiple pregnancy, the uterus may be stretched to a point where contractions-and thus labor-begin before the babies are ready to be born.
As the chart below indicates, the risk for preterm delivery increases with each additional baby in the womb:
|Number of babies in the womb||Average gestational age at birth*|
Did You Know?
In the U.S., reproductive technologies have led to an increase in multiple births. Often doctors advise women to terminate one or more of the fetuses (a process called selective reduction) in order to reduce the risk of preterm labor. Although reduction does help, these pregnancies still have a greater risk of preterm labor than those that started out with only one fetus. There is also a risk of losing the entire pregnancy when the procedure is performed. Further, studies have shown that twins from pregnancies that have been selectively reduced are born earlier and weigh less than twins from pregnancies that have two fetuses from the beginning. It is far better to start out with a smaller number of fetuses. If you undergo fertility treatment, talk to your doctor about how to minimize the risk of multiple births.
*Gestational age refers to the number of weeks a woman is pregnant. It is usually calculated from the first day of the last normal menstrual period.
Multiple gestation also places a woman and her babies at increased risk for other complications. A mother has increased risk of developing preeclampsia (high blood pressure during pregnancy, which occurs in 20 to 33% of all multiple pregnancies) and gestational diabetes. Babies born from multiple gestations have a greater risk of low birth weight: over 50% of twins and about 90% of triplets will have low birth weight compared to about 6% of babies born from single pregnancies. The baby also has an increased risk for birth defects and severe anemia. One fifth of triplet pregnancies and one half of quadruplet pregnancies result in at least one child with a major handicap. All these complications are problems in themselves, but they can also make preterm labor, if it begins, more difficult to manage and treat.
A woman who has delivered a preterm baby in the past has a 15 to 40% chance of preterm labor and delivery in subsequent pregnancies. The chances depend on the number of previous preterm births and how early they occurred. The earlier a preterm birth occurs, the more likely it is that the next birth, if preterm, will be as early or even earlier. For example, a woman who delivered during the second trimester in a previous pregnancy has a 26 to 40% chance of delivering earlier than 35 weeks in a following pregnancy. Alternatively, if a previous preterm baby was born in the third trimester, the chance of delivering earlier than 35 weeks is reduced to 15 to 25%.
Remember, though, that these risks apply to women who had spontaneous preterm births, not just preterm labor. A woman who has delivered a baby at full term and has never delivered preterm has a less than 1% chance of delivering a subsequent baby at 32 weeks or earlier and no more than a 4% chance of delivering preterm at all. In addition, the more pregnancies a woman has delivered at full term, the less likely that the next birth will be preterm. Even when a woman has had one preterm 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 studies have found an association between a history of abortion and subsequent preterm births among women who have had more than one abortion. It's unclear if the abortion procedure causes preterm labor in a later pregnancy. It may be that women who have multiple abortions have circumstances in their lives that make it difficult for them to carry a pregnancy to term. Usually, women who have had several abortions have less access to health care and other resources than those who have never had an unplanned pregnancy. (See Social and Economic factors, below).
Did You Know
Two out of five women have bleeding or spotting during the first twelve weeks of their pregnancies. This bleeding increases their risk of miscarriage. But if these women do not miscarry and the bleeding stops before the second trimester, there is no increased risk for preterm delivery.
Multiple studies have shown that a woman who begins bleeding between the 12th and 24th week of pregnancy has an increased risk of preterm delivery. The full extent of her risk depends on the cause of the bleeding.
Placenta previa (when the placenta partially or completely covers the opening of the cervix) and placental abruption (when the placenta separates too early from the uterine wall) are two important causes of vaginal bleeding during pregnancy, and they are clearly linked with early delivery.
If there is no obvious reason for the bleeding, the most likely cause is bleeding from the edge of the placenta. This is known as a "marginal bleed" because the bleeding is thought to come from the margin (or edge) of the placenta. Marginal bleeding can be considered a minor form of abruption because the bleeding is caused by a small separation between the edge of the placenta and the uterine wall. Since a marginal bleed (also called a chronic abruption) can be difficult to distinguish from a more complete abruption (and can lead to complete abruption), a woman with this kind of bleeding needs to see her doctor for careful evaluation.
Bacterial infection may cause up to 30% of all preterm labors. These infections can occur in any part of a pregnant woman's reproductive or urinary tract, including the vagina, cervix, uterus, urethra, bladder, and kidneys-as well as in the membranes and fluid surrounding the baby (amniotic cavity). It's thought that infections usually begin in the lower parts of the reproductive tract (in the vagina, cervix, and urethra) and travel up toward the uterus, bladder, and kidneys.
Infection can also occur in the mother's bloodstream. In some women, the body's response to the infection can trigger early labor.
To cause labor, infection must reach the uterus, where it can stimulate a chemical reaction causing the uterus to contract. Not all bacteria that reach the uterus stimulate contractions, but if the bacteria cross the two membranes surrounding the baby (the chorion and the amnion) and enter the amniotic cavity, labor almost always begins.
Some infections that are associated with preterm labor include gonorrhea, chlamydia, trichomoniasis, bacteriuria, and bacterial vaginosis (BV). The association between BV and preterm labor is the most clear and will be discussed below.
Bacterial vaginosis is a condition in which a woman has an overgrowth of certain bacteria in her vagina. The main bacteria involved are Gardnerella vaginalis and Bacteroides, Prevotella, and Mycoplasma species. Up to 50% of women who have BV do not have symptoms. Women with BV appear to be at increased risk for preterm labor.
Women of some ethnic groups, particularly African-American women, are more likely to get BV than Caucasian women. For more information about risk factors associated with race, see the section below entitled "Genetic, Economic, and Social Factors."
When a woman has polyhydramnios, she has an excess amount of amniotic fluid (more than 2000 milliliters) surrounding the baby in her uterus. This excess fluid puts a woman at risk for preterm labor because the uterus tends to contract when it gets stretched beyond a certain point. Up to 40% of women with polyhydramnios start labor early.
Some signs that may indicate polyhydramnios are:
- the uterus is larger than expected for the length of the pregnancy;
- it is difficult to feel the body parts of the fetus because of the excess fluid;
- the wall of the uterus is tense and tender; and
- the woman has difficulty breathing.
To confirm the diagnosis, your doctor may order an ultrasound to estimate the amount of fluid in the uterus. If the diagnosis is confirmed, your doctor may remove some of the excess fluid. An ultrasound is used to help guide a long needle through your abdomen and into the amniotic sac (a procedure called amniocentesis ); excess fluid is then extracted.
The ultrasound can also be used to help determine the cause of the excess fluid. While in about 60% of cases the cause is unknown (idiopathic), the ultrasound may help determine whether something has gone wrong with the mother, the placenta, or the baby-or a combination of the three. The most common maternal causes for polyhydramnios are diabetes and red blood cell incompatibilities (for example, when the Rh factors in the blood of the mother and the baby are incompatible). Placental causes are rare but include chorioangioma, a benign (non-cancerous) tumor of the blood vessels in the placenta. Fetal causes are more common and include multiple gestation (two or more babies in the womb), nonimmune hydrops (a condition in which the baby is swollen with fluid), infection, and birth defects, including defects in the nervous system and in blood circulation.
It is important to determine the cause of polyhydramnios when possible because the risk of preterm labor is related more to the cause of the excess fluid than to its degree. For example, when a birth defect in the baby causes polyhydramnios, about 39% of mothers have preterm labor; when the cause is diabetes in the mother, about 22% of women have preterm labor; and when the cause is unknown, the risk of preterm labor is even lower.
The cervix, which forms the lower part of a woman's uterus, should remain closed throughout pregnancy, holding the baby securely inside the womb. When a woman starts labor, contractions cause the cervix to soften and shorten, so that it can open for delivery. Sometimes, however, the cervix begins to soften and shorten before it should. This may be due to a weakness in the cervix called cervical insufficiency, or incompetent cervix.
Sometimes cervical insufficiency occurs because of congenital abnormalities (abnormalities present at birth). It can also be caused by injury, surgery, or medication. A woman may be at greater risk for cervical insufficiency, and thus for preterm labor and birth, if she:
- has 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.
- has had operations on the cervix. Two of the most common cervical operations-cone biopsy and loop electrosurgical-excision procedure-are performed after a woman has an abnormal pap smear. During these operations, a portion of the cervix is removed to examine for cancerous or pre-cancerous cervical changes. Cone biopsy is associated with risk for cervical insufficiency.
- was born to a mother who took DES (diethylstilbestrol) during 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-a complete septum divides the uterus into two separate parts and a partial septum divides only part of the uterus.
- an abnormally shaped uterus (for example, a bicornuate or unicornuate uterus, as shown in the diagram). These shapes can make it difficult for the fetus and placenta to fit inside the uterus and develop normally.
The risk for preterm labor depends on the type of uterine abnormality present. A review of 182 women with uterine abnormalities (some followed through several pregnancies) found that preterm labor occurred in about 25% of the 265 pregnancies that were studied. If a woman had a uterus with a complete septum, her baby had the best chance of surviving-an approximately 86% survival rate. If her uterus was bicornuate, the baby had a 50% chance of survival. And if her uterus was unicornuate, the survival rate of the baby dropped further to about 40%.
A woman often does not know that she has a uterine abnormality until some other problem arises. Thus, her doctor should always suspect such an abnormality whenever obstetrical problems arise-especially problems like cervical insufficiency, preterm labor, and pregnancy where the baby is turned sideways or with the head up in the womb (breech) during late third trimester. When a woman delivers preterm, her doctor should explore the inside of her uterus, either by cesarean section or through the vagina to see if abnormalities are present.
If the doctor finds a uterine abnormality, further tests should be done. This is because uterine abnormalities are often associated with other hidden birth defects in the mother, like problems in the urinary tract system (for example, in the kidneys).
All women have uterine contractions throughout pregnancy. These are usually painless, and a woman is often unaware that contractions are occurring. Some studies have shown that women who have increased uterine contractions during pregnancy have a higher risk of delivering a preterm baby. However, this increased risk is small. Therefore, it is difficult to predict whether a woman will deliver early based on uterine contractions alone.
Genetics and Race
Certain inherited traits can increase a woman's risk for preterm labor. These include being underweight for one's height. Having parents who are closely related to each other (inbreeding) also increases a woman's risk.
African-American women have an increased risk for preterm labor. Even when social and economic factors are taken into account, African-American women are twice as likely as Caucasian women to deliver an infant prematurely. This risk is greatest in the earlier weeks of pregnancy. For example, African-American women (in comparison to Caucasian women) are 1.5 times more likely to deliver a baby at 36 weeks but almost four times more likely to deliver a baby at less than 28 weeks. Interestingly, though, African-American infants who are born prematurely are more likely to survive than Caucasian infants who are born prematurely.
An African-American woman's increased risk for preterm labor is probably due less to race and more to certain conditions associated with race. According to a well-known American pathologist named Naeye, African-American women have higher rates of infection of the vagina, cervix, and bladder than do other ethnic groups-and these infections can increase a woman's risk for preterm labor. Untreated bacteriuria (the presence of bacteria in the urine) probably accounts for about 5% of the increased risk for preterm labor among African-American women. A second condition, BV is about 1.5 to 2.5 times more common in African-American women than Caucasian women and probably accounts for another 30% of the increased risk.
The reasons for higher rates of sexually transmitted and non-sexually transmitted infections among African-American women are poorly understood. Differences in reported sexual behaviors do not account for the higher rates.
In every society in the world, a poorer woman is more likely to deliver prematurely or to have a very small infant. This is because poor women often lack adequate food, shelter, and prenatal care. Without adequate nutrition, a woman is likely to begin pregnancy well below her ideal weight-an additional risk factor for preterm labor.
Preterm births are also more likely when the father of the baby or the mother's father is unemployed. These factors are associated not only with poverty but also with inadequate or no health insurance, which affects a woman's ability to get quality prenatal care. The stresses associated with poverty and unemployment may also contribute to preterm birth.
A number of social factors affect a woman's risk for preterm labor. Her risk increases if she:
- is less than 16 years old or more than 40 years old;
- is single;
- drinks alcohol, uses recreational drugs, or smokes (smoking can double the risk of premature birth);
- lacks social support, like a family or community network (this is especially problematic if a woman is caring for several children under six years old or if she lacks transportation for prenatal care); and
- suffers from physical or psychological stress (for example, living in a neighborhood with high crime rates).
Because some of the signs and symptoms and risks for preterm labor are not always obvious, it is important to receive consistent prenatal care. If you have any of the above signs, symptoms or risks, you should receive a thorough evaluation.