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The Merck Manual of Medical Information--Home Edition
Section 22. Women's Health Issues
Chapter 244

High-Risk Pregnancy

A high-risk pregnancy is a pregnancy in which the risk of illness or death before or after delivery is greater than usual for the mother or baby.

To identify a high-risk pregnancy, a doctor evaluates a pregnant woman to determine if she has conditions or characteristics that make her or the fetus more likely to become ill or die during the pregnancy (risk factors). Risk factors can be assigned a score corresponding to the degree of risk. Identifying a high-risk pregnancy ensures that a woman who most needs medical care receives it.

A woman with a high-risk pregnancy may be referred to a perinatal care center; perinatal refers to events immediately before, during, or after delivery. Usually, these centers are linked with an obstetric service and newborn intensive care unit to provide the highest level of care for a pregnant woman and baby. A doctor often refers a woman to a perinatal care center before delivery because early attention greatly reduces the likelihood that the baby will become ill or die. A woman is also sent to a center during labor when unexpected problems occur. The most common reason for referral is the risk of preterm delivery (before 37 weeks), which often occurs when the fluid-filled membranes containing the fetus break before it is ready to be born (premature rupture of the membranes). Treatment at a perinatal care center may reduce the likelihood that the baby will be born prematurely. (see page 1178 in Chapter 249, Complications of Labor and Delivery)

A pregnant woman dies (maternal mortality) in 6 out of 100,000 births in the United States. The leading cause of death is motor vehicle accidents or other injuries. Next most common are several problems related to pregnancy and delivery: blood clots that travel to the lungs, anesthesia complications, bleeding, infection, and high blood pressure complications.

The baby dies before, during, or after birth (perinatal mortality) in 16 out of 1,000 deliveries in the United States. Slightly more than half of these deaths are stillbirths. The rest occur in babies up to 28 days old. The leading cause of these deaths is birth defects, followed by prematurity.

Some risk factors are present before the woman becomes pregnant. Others develop during pregnancy.

Risk Factors Before Pregnancy

Before a woman becomes pregnant, she may have characteristics or conditions that increase risk during pregnancy. In addition, when a woman has had a problem in a pregnancy, her risk of having the same problem in subsequent pregnancies is increased.

Scoring a High-Risk Pregnancy

A score of 10 or more indicates high risk.

Risk FactorsScore
Before Pregnancy
Characteristics of the mother
Age 35 and over or 15 and under5
Weight less than 100 or more than 200 pounds5
Events in a previous pregnancy
Stillbirth10
Newborn death10
Premature baby10
Small for gestational age baby (smaller than expected for number of weeks of pregnancy)10
Fetal blood transfusion for hemolytic disease10
Late delivery (beyond 42 weeks)10
Repeated miscarriages5
Large baby (more than 10 pounds)5
Six or more completed pregnancies5
History of eclampsia (seizures during pregnancy)5
Cesarean section5
Epilepsy or cerebral palsy in mother5
History of preeclampsia (high blood pressure, protein in the urine, and fluid accumulation during pregnancy)1
Previous baby with birth defects1
Structural defects
Double uterus10
Weak (incompetent) cervix10
Small pelvis5
Medical conditions
Long-standing (chronic) high blood pressure10
Moderate to severe kidney disease10
Severe heart disease10
Insulin-dependent diabetes10
Sickle cell disease10
Abnormal results of a Pap test10
Moderate heart disease5
Thyroid disease5
History of tuberculosis5
Lung disease, such as asthma5
Positive blood test results for syphilis or human immunodeficiency virus5
History of bladder infection1
Family history of diabetes1
During Pregnancy
Exposure to drugs and infections
Use of drugs or alcohol5
Viral illness, such as German measles5
Influenza (severe)5
Smoking1
Medical complications
Moderate to severe preeclampsia10
Mild preeclampsia5
Kidney infection5
Diet-controlled diabetes of pregnancy (gestational diabetes)5
Severe anemia5
Bladder infection1
Mild anemia1
Pregnancy complications
Mother:
Abnormal location of the placenta (placenta previa)10
Premature detachment of the placenta (abruptio placentae)10
Too much or too little amniotic fluid around fetus10
Infection of placenta10
Ruptured uterus10
Late delivery (beyond 42 weeks or more than 2 weeks late)10
Rh sensitization to the fetus' blood5
Vaginal spotting5
Preterm labor5
Membranes rupture (water breaks) more than 12 hours before delivery5
Cervix stops dilating5
Labor lasting more than 20 hours5
Pushing more than 2 hours5
Rapid labor (less than 3 hours)5
Cesarean section5
Induced labor for medical reasons5
Induced labor by choice1
Baby:
Meconium-stained amniotic fluid (dark green)10
Abnormal presentation (such as breech)10
Breech delivery, assisted throughout delivery10
Multiple pregnancy (particularly triplets or more)10
Slow or very fast heart rate10
Umbilical cord in front of fetus (prolapsed cord)10
Weight less than 5.5 pounds at birth10
Meconium-stained amniotic fluid (light green)5
Need to use forceps or vacuum extractor5
Breech delivery, partially or not assisted5
General anesthesia to mother during delivery5

Characteristics of the Mother

The woman's age affects pregnancy risk. Girls aged 15 and under are more likely to develop preeclampsia (a condition in which high blood pressure, protein in the urine, and fluid accumulation develop during pregnancy) (see page 1158 in Chapter 245, Complications of Pregnancy) and eclampsia (seizures resulting from preeclampsia). They are also more likely to deliver underweight or undernourished babies. Women aged 35 and older are more likely to develop high blood pressure, diabetes, or fibroids (noncancerous growths) in the uterus and to have problems during labor. The risk of having a baby with a chromosomal abnormality such as Down syndrome increases rapidly after age 35. (see page 1132 in Chapter 242, Tests for Genetic Disorders) If an older pregnant woman is concerned about the possibility of abnormalities, chorionic villus sampling or amniocentesis may be performed to assess the fetus' chromosomes. (see page 1135 in Chapter 242, Tests for Genetic Disorders)

A woman who weighs less than 100 pounds when not pregnant is more likely to have a baby who is smaller than expected for the number of weeks she has been pregnant (small for gestational age). If she gains less than 15 pounds during pregnancy, her risk of having such a baby increases to almost 30 percent. Conversely, an obese woman is more likely to have a very large baby; obesity also increases the risk of developing diabetes and high blood pressure during pregnancy.

A woman shorter than 5 feet is more likely to have a small pelvis. Her risk of having preterm labor and an abnormally small baby whose growth in the uterus has been stunted (growth-retarded) is also greater than usual.

Events in a Previous Pregnancy

A woman who has had three consecutive miscarriages in the first 3 months of pregnancy has about a 35 percent chance of having another miscarriage. Miscarriage is also more likely for a woman who has had a stillborn baby between the fourth and eighth months of pregnancy or preterm labor in a previous pregnancy. Before trying to become pregnant again, a woman who has had a miscarriage may want to be checked for chromosomal or hormonal abnormalities, structural defects in the uterus or cervix, connective tissue disorders such as lupus, or an immune reaction to the fetus, usually Rh incompatibility. If the cause of the miscarriage is found, the condition may be treatable.

A stillbirth or newborn death may result from chromosomal abnormalities in the baby or from diabetes, long-standing (chronic) kidney or blood vessel disease, high blood pressure, drug abuse, or a connective tissue disorder such as lupus in the mother.

The more preterm deliveries a woman has had, the greater the risk of preterm deliveries in subsequent pregnancies. A woman who has had a baby weighing less than 3 pounds has a 50 percent chance of preterm delivery of her next baby. A woman who has had a baby whose growth in the uterus was stunted (growth-retarded) may do so again. She is evaluated for conditions that can stunt the fetus' growth, such as high blood pressure, kidney disease, inadequate weight gain, infection, cigarette smoking, and alcohol abuse.

If a woman has had a baby weighing more than 10 pounds at birth, she may have diabetes. (see page 1163 in Chapter 246, Diseases That Can Complicate Pregnancy) The risk of miscarriage or death of the woman or baby is increased if the woman has diabetes during pregnancy. Pregnant women are checked for diabetes by measuring blood sugar (glucose) levels between 20 and 28 weeks of pregnancy.

A woman who has had six or more pregnancies is more likely to have weak contractions during labor and bleeding after delivery because of weakened uterine muscles. She may also have rapid labor, which can increase the risk of heavy vaginal bleeding. In addition, she is more likely to have placenta previa (a placenta abnormally located in the lower part of the uterus). This condition may cause bleeding, and because the placenta may block the cervix, a cesarean section is usually necessary.

If a woman has had a baby with hemolytic disease, (see box, page 1211) the next baby may be at risk for the same disease, and the severity of the disease in the previous baby predicts its severity in the next. This disease develops when a mother whose blood is Rh-negative has a fetus whose blood is Rh-positive (Rh incompatibility) and the mother produces antibodies against the fetus' blood (Rh sensitization); these antibodies destroy the fetus' red blood cells. In such cases, the blood of both parents is tested. If the father has two genes for Rh-positive blood, all babies will have Rh-positive blood; if he has only one gene for it, a baby has about a 50 percent chance of having Rh-positive blood. This information helps doctors care for the mother and fetus in subsequent pregnancies. Usually, no problems develop in the first pregnancy with a baby whose blood is Rh-positive, but contact between the mother's and baby's blood at delivery causes the mother to produce Rh antibodies. As a result, subsequent babies are at risk. However, after the delivery of a baby whose blood is Rh-positive to a mother whose blood is Rh-negative, the mother is usually given Rh0(D) immune globulin, which destroys Rh antibodies. Consequently, hemolytic disease in babies is rare.

A woman who has had preeclampsia or eclampsia is likely to have it again, particularly if she has chronic high blood pressure when she isn't pregnant.

If a woman has had a baby with genetic disorders or birth defects, genetic analysis of the baby--even if stillborn--and both parents is usually performed before another pregnancy is attempted. If the woman becomes pregnant again, tests such as ultrasound scanning, chorionic villus sampling, and amniocentesis are performed to help determine if the abnormalities are likely to recur. (see page 1129 in Chapter 242, Tests for Genetic Disorders)

Structural Defects

Structural defects in a woman's reproductive organs, such as a double uterus or a weak cervix that can't support the developing fetus (an incompetent cervix), increase the risk of a miscarriage. Diagnostic surgery, ultrasound scans, or x-rays may be necessary to detect these defects; if a woman is having repeated miscarriages, these tests are performed before she becomes pregnant again.

Fibroids (noncancerous growths) in the uterus, which are more common in older women, may increase the risk of preterm labor, problems during labor, abnormal presentation of the fetus, abnormal location of the placenta (placenta previa), and repeated miscarriages.

Medical Conditions

Certain medical conditions in a pregnant woman may endanger her and the fetus. The most important are chronic high blood pressure, kidney disease, diabetes, severe heart disease, sickle cell disease, thyroid disease, systemic lupus erythematosus (lupus), and blood clotting disorders. (see page 1160 in Chapter 246, Diseases That Can Complicate Pregnancy)

Family History

A history of mental retardation or other hereditary disorders in the mother's or father's family increases the likelihood that the baby will have such a disorder. The tendency to have twins also runs in families.

Risk Factors During Pregnancy

A pregnant woman at low risk may undergo a change that increases her risk. She may be exposed to teratogens (agents that can produce birth defects), such as radiation, certain chemicals, drugs, and infections, or she may develop a medical condition or a complication related to pregnancy.

Exposure to Drugs or Infections

Drugs known to produce birth defects when taken during pregnancy include alcohol, phenytoin, drugs that oppose the actions of folic acid (such as triamterene or trimethoprim), lithium, streptomycin, tetracycline, thalidomide, and warfarin. (see page 1167 in Chapter 247, Drug Use During Pregnancy) Infections that may cause birth defects include herpes simplex, viral hepatitis, influenza, mumps, German measles (rubella), chickenpox (varicella), syphilis, listeriosis, toxoplasmosis, and infections with coxsackievirus or cytomegalovirus. Early in pregnancy, a woman is asked if she has taken any of these drugs or has had any of these infections since becoming pregnant. Of particular concern is how smoking cigarettes, drinking alcohol, and abusing drugs during pregnancy affect the fetus' health and development.

Cigarette smoking is the most common addiction among pregnant women in the United States. Despite publicity regarding the health hazards of smoking, the percentage of adult women who smoke or live with someone who smokes has dropped only slightly in 20 years, and the percentage of women who smoke heavily has increased. Smoking among teenage girls has increased substantially and exceeds that among teenage boys.

Although smoking harms both mother and fetus, only about 20 percent of women who smoke quit during pregnancy. The most consistent effect of smoking on the baby during pregnancy is reduction in birth weight: The more a woman smokes during pregnancy, the less the baby is likely to weigh. This effect seems to be greater among older smokers, who are more likely to have babies who weigh less and are shorter. Pregnant smokers also are more likely to have placental complications, premature rupture of the membranes, preterm labor, and uterine infections. A pregnant woman who doesn't smoke should avoid exposure to secondhand smoke because it may similarly harm the fetus.

Birth defects of the heart, brain, and face are more common in babies of smokers than in those of nonsmokers. Smoking by the mother may increase the risk of sudden infant death syndrome. In addition, children of smoking mothers have slight but measurable deficiencies in physical growth, intellectual development, and behavior. These effects are thought to be caused by carbon monoxide, which may reduce the oxygen supply to the body's tissues, and nicotine, which stimulates the release of hormones that constrict the vessels supplying blood to the placenta and uterus.

Drinking alcohol during pregnancy is the leading known cause of birth defects. Fetal alcohol syndrome, one of the major consequences of drinking during pregnancy, is found in about 2.2 out of 1,000 live births. This condition includes growth retardation before or after birth; facial defects; a small head (microcephaly), probably caused by subnormal brain growth; and abnormal behavioral development. Mental retardation more often results from fetal alcohol syndrome than from any other known cause. (see page 1214 in Chapter 252, Problems in Newborns and Infants) In addition, alcohol can cause problems ranging from miscarriage to severe behavioral problems in the baby or developing child, such as antisocial behavior and attention deficit. These problems can occur even when the baby has no obvious physical birth defects.

Small Babies

  • A premature baby is one who is born before 37 weeks of pregnancy.
  • A low-birth-weight (underweight) baby is any baby weighing 5.5 pounds or less at birth.
  • A small for gestational age baby is one who is unusually small for the number of weeks of pregnancy. This term refers to weight, not length.
  • A growth-retarded baby is one whose growth in the uterus has been stunted. This term refers to weight and length. A baby may be growth-retarded or small for gestational age or both.

The risk of miscarriage almost doubles when a woman drinks alcohol in any form during pregnancy, especially if she drinks heavily. Often, the birth weight of babies born to women who drink during pregnancy is below normal. The average birth weight is about 4 pounds for babies exposed to alcohol, compared with 7 pounds for all babies.

Drug abuse and addiction are seen in more and more pregnant women. More than five million people in the United States, many of whom are women of childbearing age, regularly use marijuana or cocaine.

An inexpensive, sensitive laboratory test called chromatography can be used to check a woman's urine for heroin, morphine, amphetamines, barbiturates, codeine, cocaine, marijuana, methadone, or phenothiazines. Women who inject drugs are at greater risk for anemia, infection of the blood (bacteremia) or heart valves (endocarditis), skin abscess, hepatitis, phlebitis, pneumonia, tetanus, and sexually transmitted diseases, including AIDS. About 75 percent of babies who develop AIDS have mothers who injected drugs or were prostitutes. These babies are at risk for other sexually transmitted diseases, hepatitis, and infections. Also, their growth in the uterus is more likely to be retarded, and their birth premature.

About 14 percent of pregnant women use marijuana to some extent. Its main ingredient, tetrahydrocannabinol (THC), can cross the placenta and thus may affect the fetus. Although no specific evidence shows that marijuana causes birth defects or slows growth in the uterus, some studies suggest that heavy use is linked with behavioral abnormalities in babies.

Cocaine abuse during pregnancy causes serious problems for both mother and fetus, and many who use cocaine also abuse other drugs, compounding the problem. Cocaine stimulates the central nervous system, acts as a local anesthetic, and constricts blood vessels. Constricted blood vessels may reduce blood flow so that the fetus sometimes doesn't get enough oxygen. The reduced blood and oxygen supply to the fetus can affect the growth of various organs and commonly results in skeletal defects and abnormally narrow sections of intestine. Nervous system and behavioral problems in babies of cocaine users include hyperactivity, uncontrollable trembling, and substantial learning problems, which may continue through age 5 years or even longer. (see page 1214 in Chapter 252, Problems in Newborns and Infants)

If a pregnant woman suddenly develops severe high blood pressure or bleeding from premature detachment of the placenta (abruptio placentae) or if she has an unexplained stillbirth, her urine is usually tested for cocaine. Among women who use cocaine throughout pregnancy, about 31 percent have a preterm delivery, 19 percent have a growth-retarded baby, and 15 percent have premature detachment of the placenta. If a woman stops using cocaine after the first 3 months of pregnancy, the risks of preterm delivery and premature detachment of the placenta are still increased but the fetus' growth will probably be normal.

Medical Conditions

If high blood pressure is first diagnosed when a woman is pregnant, a doctor may have difficulty determining whether it was caused by the pregnancy or by another condition. Treatment of high blood pressure during pregnancy is a problem; benefits for the mother have to be weighed against potential risks to the fetus. However, late in pregnancy, high blood pressure may indicate a serious threat to the mother and fetus and must be treated promptly.

If a pregnant woman has had a bladder infection in the past, a urine sample is evaluated early in the pregnancy. If bacteria are detected, a doctor prescribes antibiotics (see page 1170 in Chapter 247, Drug Use During Pregnancy) to try to prevent a kidney infection, which is associated with preterm labor and premature rupture of the membranes.

Bacterial infections of the vagina during pregnancy also may lead to preterm labor or premature rupture of the membranes. Treating the infection with antibiotics reduces the likelihood of these problems.

An illness causing a fever (temperature greater than 103° F.) in the first 3 months of pregnancy increases the likelihood of miscarriage and nervous system defects in the baby. Fever in late pregnancy increases the likelihood of preterm labor.

Emergency surgery during pregnancy increases the risk of preterm labor. Many disorders, such as appendicitis, a gallbladder attack, and intestinal obstruction, are difficult to diagnose because of the normal changes in the abdomen during pregnancy. By the time such a disorder is diagnosed, it may be in an advanced stage, which increases the woman's risk of illness and death.

Pregnancy Complications

Rh Incompatibility: Mother and fetus may have incompatible blood types. The most common is Rh incompatibility, (see page 1155 in Chapter 245, Complications of Pregnancy) which can lead to hemolytic disease of the newborn. This disease can develop only when a mother whose blood is Rh-negative and a father whose blood is Rh-positive have a fetus whose blood is Rh-positive and the mother produces antibodies against the fetus' blood. If an expectant mother's blood is Rh-negative, it's checked for antibodies to the fetus' blood every 2 months. The risk of producing these antibodies is increased after any bleeding episode in which the mother's and fetus' blood may mix, after amniocentesis or chorionic villus sampling, and within the first 72 hours after delivery if the baby has Rh-positive blood. At these times and at the 28th week of pregnancy, the mother is given Rh0(D) immune globulin, which combines with and thereby destroys these antibodies.

Bleeding: The most common causes of bleeding in the last 3 months of pregnancy are abnormal location of the placenta, premature detachment of the placenta from the uterus, and a disease of the vagina or cervix, such as an infection. All women who bleed at this time are considered at risk of losing the baby, bleeding excessively (hemorrhaging), or dying during labor and delivery. Ultrasound scanning, inspection of the cervix, and a Papanicolaou (Pap) test help determine the cause of the bleeding.

Problems With Amniotic Fluid: Too much amniotic fluid in the membranes surrounding the fetus stretches the uterus and puts pressure on the mother's diaphragm. This complication can lead to severe breathing problems in the mother or preterm labor. Too much fluid tends to develop if the mother has uncontrolled diabetes, if more than one fetus is present (multiple pregnancy), if mother and fetus have incompatible blood types, or if the baby has birth defects, especially a blocked esophagus or nervous system defects. About half the time, the cause is unknown. There tends to be too little amniotic fluid when the baby has birth defects in the urinary tract, is growth retarded, or dies.

Preterm Labor: Labor is more likely to be early if the mother has structural defects in the uterus or cervix, bleeding, mental or physical stress, or a multiple pregnancy or if she has had previous uterine surgery. Preterm labor often occurs when the fetus is in an abnormal position such as breech presentation, when the placenta detaches from the uterus prematurely, when the mother has high blood pressure, or when too much amniotic fluid surrounds the fetus. Pneumonia, kidney infection, and appendicitis can also cause preterm labor. About 30 percent of women who have preterm labor have infections of the uterus even though the membranes haven't ruptured. Whether antibiotics help these women isn't known.

Multiple Pregnancy: Having more than one fetus in the uterus also increases the likelihood of birth defects and problems with labor and delivery.

Postterm Pregnancy: In a pregnancy that continues beyond 42 weeks (postterm), death of the baby is three times more likely than that in a normal term pregnancy. Electronic heart monitoring and ultrasound scanning are used to monitor the fetus.

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