Some risk factors are present before women become pregnant. These risk factors include
- Certain physical characteristics, such as age, and social characteristics of women
- Problems in a previous pregnancy
- Certain disorders present before pregnancy
- Exposures that can harm the fetus
Other problems that increase risk can develop during pregnancy or during labor and delivery.
Needing to have surgery, particularly abdominal surgery, during pregnancy increases the risk of preterm labor and miscarriages, especially early in pregnancy. Thus, surgery is usually delayed if possible. However, if necessary, surgery should proceed without delay and is still usually reasonably safe.
The following characteristics of women affect risk during pregnancy.
About 13% of all pregnancies occur in adolescents. These girls are at increased risk of having the following:
Adolescent girls can have babies who are born underweight (small-for-gestational age).
Part of the reason for these risks is that adolescents are less likely to get medical care during pregnancy. Thus, they may not understand what activities and behaviors (such as smoking, drinking alcohol, and having sex without using a condom) can put their pregnancy at risk. Many adolescents smoke. They also have a higher risk of getting a sexually transmitted disease. Using condoms can help prevent sexually transmitted diseases.
Women aged 35 and older are at increased risk of having the following:
- A preexisting disorder that increases risk during pregnancy, such as high blood pressure or diabetes
- Problems related to the pregnancy, such as preeclampsia, gestational diabetes (diabetes that develops during pregnancy), chromosomal abnormalities in the fetus, and stillbirth
- Complications during labor, such as difficult labor or a placenta that detaches too soon (placental abruption) or is mislocated (placenta previa)
As women age, genetic evaluation for chromosomal abnormalities becomes more important.
Women who are very thin—with a body mass index (BMI) of less than 19.8 (see table Determining Body Mass Index)—or weigh less than 100 pounds before becoming pregnant are more likely to have
- Small, underweight babies
Overweight women (with a BMI of 25 to 29.9 before pregnancy) and obese women (with a BMI of more than 30) are more likely to have the following problems:
- Very large babies (large-for-gestational age), which may be difficult to deliver
- Babies who are born underweight (small-for-gestational age)
- Babies with birth defects
- Miscarriages and stillbirth
- Gestational diabetes
- Gestational hypertension (high blood pressure that first develops after 20 weeks of pregnancy)
- Preeclampsia (gestational hypertension accompanied by protein in the urine)
- A pregnancy that lasts 42 weeks or longer (postterm pregnancy)
- Need for a cesarean delivery
Doctors encourage overweight and obese women to exercise at least 3 times a week for a total of 150 minutes a week. These women should talk to their doctor about what exercises are appropriate for them. Changes to a healthier diet may be recommended.
Women shorter than 5 feet are more likely to have a small pelvis, which may make movement of the fetus through the pelvis and vagina (birth canal) difficult during labor. For example, the fetus's shoulder is more likely to lodge against the pubic bone. This complication is called shoulder dystocia. Also, short women are more likely to have preterm labor and a baby who is born underweight (small-for-gestational age).
Structural abnormalities in the uterus or cervix increase the risk of the following:
- A difficult labor
- A miscarriage during the 2nd trimester and preterm labor
- A fetus in an abnormal position
- Preterm labor or a premature baby
- Need for a cesarean delivery
Structural abnormalities include a double uterus, fibroids in the uterus, and a weak (incompetent) cervix (cervical insufficiency) that tends to open (dilate) as the fetus grows. Fibroids occasionally cause the placenta to be mislocated (called placenta previa), labor to begin too early (preterm labor), and miscarriages to occur. Cervical insufficiency increases the risk that a baby will be delivered too soon (preterm delivery).
Being unmarried or in a lower socioeconomic group increases the risk of problems during pregnancy. The reason these characteristics increase risk is unclear but is probably related to other characteristics that are more common among these women. For example, these women may be more likely to smoke, less likely to consume a healthy diet, more likely to have unprotected sexual intercourse, and less likely to obtain appropriate medical care.
Problems in a Previous Pregnancy
When women have had a problem in one pregnancy, they are more likely to have a problem, often the same one, in subsequent pregnancies. Such problems include having had any of the following:
- A premature baby
- An underweight baby (small-for-gestational age)
- A baby that weighed more than 10 pounds (large-for-gestational age)
- A baby with birth defects
- A previous miscarriage
- A late (postterm) delivery (after 42 weeks of pregnancy)
- Rh incompatibility that required a blood transfusion to the fetus
- Labor that required a cesarean delivery
- A baby who died shortly before or after birth (stillbirth)
- Too much amniotic fluid in the uterus (polyhydramnios)
- Too little amniotic fluid in the uterus (oligohydramnios)
- A fetus in an abnormal position, such as buttocks first (breech)
- A baby whose shoulder gets caught in the birth canal (shoulder dystocia)
- A baby with an injury that stretched the nerves in the baby's shoulder (brachial plexus injury) during delivery
- A previous pregnancy with more than one fetus (multiple births)
- A seizure disorder
- A baby with cerebral palsy
Women may have a condition that tends to make the same problem recur. For example, women with diabetes are more likely to have babies that weigh more than 10 pounds at birth.
Women who had a baby with a genetic disorder or birth defect are more likely to have another baby with a similar problem. Genetic testing of the baby, even if stillborn, and of both parents may be appropriate before another pregnancy is attempted. If these women become pregnant again, tests such as high-resolution ultrasonography, chorionic villus sampling, and amniocentesis may help determine whether the fetus has a genetic disorder or birth defect. These women may be referred to a specialist.
Having had five or more pregnancies increases the risk of very rapid labor and excessive bleeding after delivery.
Having had twins or more fetuses in one pregnancy increases the risk of the following:
Disorders Present Before Pregnancy
Before becoming pregnant, women may have a disorder that can increase the risk of problems during pregnancy. These disorders include
- High blood pressure
- Kidney disorders
- Kidney infections
- Heart failure
- Sickle cell disease
- Sexually transmitted diseases
Women who have one of these disorders should talk with a doctor and try to get in the best physical condition possible before they become pregnant. After they become pregnant, they may need special care, often from an interdisciplinary team. The team may include an obstetrician (who may also be a specialist in the disorder), a specialist in the disorder, and other health care practitioners (such as nutritionists).
Disorders During Pregnancy
During pregnancy, a problem may occur or a disorder may develop to make the pregnancy high risk.
Some disorders that occur during pregnancy are related to (are complications of) pregnancy. Other disorders are not directly related to pregnancy (see Pregnancy Complicated by Disease). Certain disorders are more likely to occur during pregnancy because of the many changes pregnancy causes in a woman's body.
Pregnancy complications are problems that occur during pregnancy. They may affect the woman, the fetus, or both and may occur at different times during the pregnancy. For example, complications such as a mislocated placenta (placenta previa) or premature detachment of the placenta from the uterus (placental abruption) can cause bleeding from the vagina during pregnancy. Women who have heavy bleeding are at risk of losing the baby or of going into shock and, if not promptly treated, of dying during labor and delivery.
Other pregnancy complications include
- Problems with amniotic fluid (the fluid that surrounds the fetus in the uterus)
- A weak cervix (cervical insufficiency) that tends to open (dilate) as the fetus grows
- A mislocated pregnancy (ectopic pregnancy)
- Severe nausea and vomiting during pregnancy (hyperemesis gravidarum)
- An infection of the tissues around the fetus, such as the amniotic fluid (intra-amniotic infection)
- A previous miscarriage or stillbirth
- Preeclampsia (a type of high blood pressure that develops during pregnancy)
- Rh incompatibility (when a pregnant woman has Rh-negative blood and the fetus has Rh-positive blood)
Exposures During Pregnancy
During pregnancy, being exposed to the following can increase the risk of having a baby with a birth defect:
- Certain infections
- Certain drugs
- Radiation and certain chemicals
These substances and conditions are called teratogens.
Birth defects are most likely to result if women are exposed to a teratogen 2 to 8 weeks after they become pregnant (4 to 10 weeks after their last menstrual period) because the fetus's organs are forming during this time. The risk of having a miscarriage is also increased.
Infections that are particularly dangerous during pregnancy include
- Herpes simplex
- Rubella (German measles)
- Infections with cytomegalovirus or coxsackievirus
- Zika virus infection
Drugs that may increase the risk of birth defects include
- Some prescription drugs (see table Some Drugs That Can Cause Problems During Pregnancy)
Exposure to high temperatures (for example, in a sauna) during the 1st trimester has been linked to the development of spina bifida.
Mercury in seafood
Consuming too much mercury in seafood may harm the fetus. However, seafood contains nutrients that are important for growth and development of the fetus and breastfed infants. Thus, the Food and Drug Administration (FDA) recommends the following for women who are pregnant, who may become pregnant, or who are breastfeeding:
- Do not eat tilefish from the Gulf of Mexico, shark, swordfish, big-eye tuna, marlin, orange roughy, and king mackerel.
- Limit the amount of albacore tuna eaten to 4 ounces (one average meal) a week.
- Before eating fish caught in local lakes, rivers, and coastal areas, check local advisories about the safety of such fish, and if mercury levels in the fish are not known to be low or if no advice is available, limit the amount eaten to 4 ounces (one average meal) a week and do not eat other high-mercury seafood during that week.
- Each week, eat 8 to 12 ounces (2 or 3 average meals) of a variety of seafood that is lower in mercury.
Seafood that is lower in mercury includes flounder, shrimp, canned light tuna, salmon, pollock, tilapia, cod, and catfish (see Advice About Eating Fish: For Women Who Are or Might Become Pregnant, Breastfeeding Mothers, and Young Children). Some authorities (Consumer Reports: Choose the Right Fish To Lower Mercury Risk Exposure) advise against eating any tuna during pregnancy.
The following are some English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.
- Centers for Disease Control and Prevention (CDC): Sexually Transmitted Diseases During Pregnancy: This web site provides links to a fact sheet about pregnancy and sexually transmitted diseases (STDs), statistics (including links to the prevalence of different STDs and their effect on pregnant women and their infants), and links to treatment of different STDs. It also provides links to general information about pregnancy and STDs.
- U.S. Food and Drug Administration (FDA): Advice About Eating Fish: For Women Who Are or Might Become Pregnant, Breastfeeding Mothers, and Young Children: This web site provides information that can help pregnant women (as well as other women and parents of young children) choose fish that are nutritious and safe to eat.
- Consumer Reports: Choose the Right Fish To Lower Mercury Risk Exposure: This web site identifies which fish are low in mercury and which are not. It also discusses how much mercury is too much.