Intrauterine growth restriction is a condition that is defined by the lack of expected growth in an unborn baby. The causes of this condition are varied and can include placental problems, chronic conditions in the mother, malnutrition, substance abuse, and others. Diagnosis can be difficult, but growth restriction can best be managed with early detection through routine testing in order to minimize potential complications for the baby.
What Is Intrauterine Growth Restriction?
Intrauterine growth restriction (sometimes called intrauterine growth retardation, or simply IUGR) is a blanket term used to describe babies that experience poor growth in the uterus and are in the bottom tenth percentile for fetal weight—meaning that 90 percent of babies of the same gestational age weigh more than that baby does. The weight here refers to the estimated fetal weight, normally determined by ultrasound measurements.
After the baby is born, if its weight is in the tenth percentile or less than expected for its gestational age at delivery, the baby is then referred to as SGA (small for gestational age). Thus IUGR is the intrauterine diagnosis, and SGA (if confirmed) is the diagnosis used after the baby is born. Ultrasound testing may give doctors reason to suspect IUGR, but a diagnosis of SGA cannot be confirmed until after the baby is born and its true weight is determined.
Another measure that may be used to identify intrauterine growth restriction is the abdominal circumference. An abdominal circumference in the tenth percentile or less for that particular gestational age may also be used to make a diagnosis.
The weight and overall size of a baby is determined by many factors, including genetics. Some babies may naturally be smaller—especially those whose mothers are small in stature—but these infants are not considered to have IUGR. Babies who have experienced intrauterine growth restriction are often born with other symptoms, and so can typically be distinguished easily from a baby that is naturally small.
How Common Is Intrauterine Growth Restriction?
In the United States, approximately 10 percent of babies are born with intrauterine growth restriction, and 80 percent of these babies are born at or close to full term.
Causes Of Intrauterine Growth Restriction
IUGR has many causes—genetic, environmental, and infectious. Most causes are not preventable, but some contributing factors, such as smoking, alcohol abuse, or other substance use, can be avoided. The most common cause is a placental abnormality, which causes the fetus to become malnourished.
Some other potential causes include:
- Congenital or chromosomal abnormalities
- Maternal heart disease
- Severe maternal diabetes
- Frequent exposure to high altitudes
- Hemoglobinopathies or severe anemia (low blood count)
- High blood pressure (hypertension)
- Infections, such as rubella, cytomegalovirus, toxoplasmosis, and syphilis
- Malformed uterus
- Severe maternal malnutrition
- Thrombophilias (blood clotting disorders)
- Twins or multiples
- Placental abnormalities (placenta tumor or placenta too small)
- Preeclampsia or eclampsia
- Umbilical cord abnormalities (thin cord, two-vessel cord)
- Chronic kidney disease
Types of Intrauterine Growth Restriction
Intrauterine growth restriction is classified as symmetric or asymmetric.
- Asymmetric IUGR: With asymmetric intrauterine growth restriction, the abdominal circumference is smaller than expected when measured during an ultrasound, but the head is of average size. The fetus may not be receiving enough nourishment through the placenta, so it adapts by sending most nutrients to the brain and the heart, leaving less available for the development of the liver, muscles, and fat. The limbs may also be smaller, the skin thinner, and the baby may appear very thin because of the lack of fat. This type of IUGR occurs during the third trimester.
Symmetric IUGR: the term symmetric intrauterine growth restriction refers to a baby who is small, yet still proportionate. There can be many causes for this, but fetal malnourishment is usually not the explanation.
Diagnosing Intrauterine Growth Restriction
There is no one test that can accurately diagnose IUGR. A healthcare professional will use several screening methods throughout pregnancy to identify babies that are at risk.
- Fundal height: Beginning at anywhere from 20 weeks or later, an obstetrician usually will measure the fundal height. The fundal height is a measurement taken between the top of the uterus and the pubic bone. During a pregnancy, the fundal height should increase appropriately. It is a simple, low-cost, low-risk way to tell if the uterus has increased in size between doctor visits.
A fundal height that is growing too quickly, or is not growing enough, can be an indication that more testing needs to be done. The other advantage of fundal height measurement is that it can be done by different doctors at every prenatal visit, so even if a pregnant woman sees different physicians, the measurement can be taken and the results compared to previous visits. The drawback of fundal height measurement is that it is not very specific, and varies greatly from woman to woman, and so will not always catch growth restriction or other problems.
- Ultrasound: Ultrasounds are often used to determine the health of the baby and to screen for any growth problems. Most pregnant women undergo an ultrasound between 18 and 20 weeks (mainly to check the baby’s anatomy and development) and again between 28 and 32 weeks (usually to check the baby’s size and position).
It may be a sign of growth restriction if the baby is smaller than expected, the abdomen is smaller than expected, or there is less than the expected amount of amniotic fluid. Ultrasounds may also be repeated at other points in the pregnancy for women who are suspected to be at risk for complications, or in the case of a fundal height that has not been progressing as expected.
- Weight: A pregnant woman’s weight is measured at every prenatal visit. If a pregnant woman is not gaining weight as expected, this could indicate that the baby is not growing appropriately.
- Doppler flow: A Doppler flow is a specialized type of ultrasound that measures the blood flow from the baby through the cord to the placenta (this is also known as an umbilical artery Doppler). In some cases, the results of this test may be the first indication that there is a problem with the baby’s growth, even before there are visible signs in the baby’s growth pattern.
- Amniocentesis: During an amniocentesis, amniotic fluid is taken with a needle that goes through the uterus. Amniocentesis carries risks, and is not a routine test. It is typically offered to women who are considered to be at high risk for pregnancy complications. The amniotic fluid can be tested for the presence of abnormalities that could lead to growth problems. This is rarely done, but if a chromosomal or genetic abnormality is suspected, it may be worthwhile.
Complications of Intrauterine Growth Restriction
Growth restriction in a baby can lead to a variety of complications, both during pregnancy and after birth. In some cases, problems may continue long after the baby is born. Babies with intrauterine growth restriction are at risk for:
- Premature birth
- Fetal distress during delivery
- High red blood cell count
- Inability to maintain body temperature
- Inhaling meconium (stool the baby passed while in the uterus)
- Low Apgar scores
- Low birth weight
- Low blood sugar at birth
- Low oxygen levels that can cause permanent damage to the baby
Treatment of IUGR
Specific treatments will depend on the underlying cause. Diagnosing intrauterine growth restriction early and managing any complications will allow the best chance for a good outcome. While growth restriction can’t be reversed, its effects can be lessened throughout the remainder of the pregnancy.
At the very least, the pregnancy should be closely monitored by a healthcare professional in order to keep close tabs on the extent of the growth restriction. Other possible treatments include improved nutrition for the mother, bed rest, or early delivery.
Fetal monitoring, sometimes two to three times per week, is an important part of the management of asymmetrical IUGR. Monitoring can provide reassurance that the baby is getting enough oxygen and nutrients. Serial ultrasounds (to measure fetal growth) and umbilical artery Doppler can help to delay delivery until the baby is fully developed, and can also be used to determine whether the baby needs to be delivered urgently.
Pregnant women diagnosed with intrauterine growth restriction should receive proper nutrition in order to provide the best nourishment for the baby. A change in diet nay be necessary if a problem is determined to have been caused by malnutrition in the mother.
In some cases, the mother may be put on bed rest or hospitalized in order to improve the blood flow to the baby. A change in delivery plans may also be necessary, and a baby may be delivered early if it is determined that the baby might be better treated and nourished outside the uterus.
Prevention of IUGR
Some cases of intrauterine growth restriction may not be preventable. However, there are several steps pregnant women can take to reduce their risk:
- Get prenatal care: With proper care by a qualified health care professional, problems with a pregnancy can be diagnosed and treated early.
- Eat a healthful diet: An appropriate diet during pregnancy can help avoid several complications in the mother and baby, including growth restriction.
- Don’t smoke: Smoking not only increases the risk of many different conditions for the mother, but also for the baby. As smoking can contribute to intrauterine growth restriction, pregnant women should not smoke.
- Don’t drink or use drugs: No amount of recreational alcohol or drug use is safe during pregnancy. Drinking and using drugs can be a risk factor for intrauterine growth restriction, as well as other conditions such as fetal alcohol syndrome.
- Manage chronic conditions: Because intrauterine growth restriction is associated with chronic conditions such as heart disease, diabetes, and hypertension, managing these conditions is important. If a chronic condition is managed properly before and during a pregnancy, there is less chance that it can have adverse effects on the growing baby and the mother.
- Pay attention to the baby: A baby that starts moving less could be experiencing distress, which could be a sign of growth restriction. Any variation in the baby’s movements, especially when regular movements seem to have stopped, should be reported to a healthcare professional immediately.