Babies come in all sizes. Some are just naturally larger or smaller than others.
But in certain cases, babies in the womb are smaller than they should be. When this happens, it could be due to a condition known as intrauterine growth restriction, or IUGR.
The term IUGR refers to a condition in which a baby in the womb fails to grow at the expected rate during the pregnancy. In other words, at any point in the pregnancy, the baby is not as big as would be expected for how far along the mother is in her pregnancy (this timing is referred to as an unborn baby's "gestational age").
Babies who have IUGR often have a low weight at birth. If the weight is below the 10th percentile for a baby's gestational age (meaning that 90% of babies that age weigh more) the baby is also referred to as "small for gestational age," or SGA.
It's important to note that not all babies who are small for gestational age had IUGR while in the womb. Some are healthy babies who are just born smaller than average because their parents are small in stature.
The two types of IUGR are:
- In symmetrical IUGR, a baby's body is proportionally small. That means that all parts of the baby's body are similarly small in size.
- In asymmetrical IUGR, the baby has a normal-size head and brain but the rest of the body is small.
In many cases, IUGR is the result of a problem that prevents a baby from getting enough oxygen and nutrients. This lack of nourishment slows the baby's growth. It can happen for a number of reasons. A common cause is placental insufficiency, in which the tissue that delivers oxygen and nutrients to the baby is not attached properly or isn't working correctly.
Other possible causes during a woman's pregnancy include:
- certain behaviors, such as smoking, drinking alcohol, or abusing drugs
- exposure to infections (passed from the mother), such as cytomegalovirus, German measles (rubella), toxoplasmosis, and syphilis
- taking certain medications
- high blood pressure
- genetic disorders or birth defects
- living in high altitudes
IUGR is more likely to occur in women who are carrying more than one baby or who had a previous baby who was SGA or had IUGR. Certain medical conditions, such as heart, lung, blood, or autoimmune disease, and high blood pressure or anemia can also increase a woman's risk of developing IUGR. So can eating poorly or being underweight before or during pregnancy.
Since not all babies who are small have IUGR, an accurate diagnosis is important. Initially, this diagnosis relies on correctly determining the baby's age by accurately dating the pregnancy.
Gestational age is initially estimated using the first day of a woman's last menstrual period. Later in the pregnancy (usually between weeks 8 and 13), it is confirmed through an ultrasound. Once a baby's gestational age is known, doctors use it to monitor the baby's growth and compare it with the expected growth rate. If the baby is growing more slowly than expected (sometimes referred to as "small for dates"), doctors will continue to monitor the baby's growth and may perform more tests to determine whether the baby has IUGR.
Monitoring of growth is done in several ways. A measurement called the uterine fundal height helps estimate a baby's size by measuring a mother's belly from the top of the pubic bone to the uterus. Another way to monitor a baby's growth is by using ultrasounds. In fact, IUGR is usually diagnosed through an ultrasound examination.
During an ultrasound, a technician coats the woman's belly with a gel and then moves a probe (wand-like instrument) over it. High-frequency sound waves "echo" off the body and create pictures of the baby on a computer screen. These pictures can be used to estimate the baby's size and weight.
Although these estimates might not be exactly correct, they help health care providers track the baby's growth and determine whether a problem exists. Ultrasounds also can help identify other potential troubles, such as problems with the placenta or low amniotic fluid levels.
Health care providers might also perform some additional tests to diagnose IUGR, such as:
- fetal monitoring to track the baby's heart rate and movements
- screenings for infections
- amniocentesis to help determine the cause of IUGR
If a Baby has IUGR
Once IUGR has been diagnosed, treatment is individualized for each patient depending on the baby's condition and the woman's month of pregnancy.
The baby will be closely monitored, usually with frequent prenatal visits and ultrasounds, to keep track of growth and watch for other potential problems. In some cases, health care providers will recommend inducing labor and delivery early if monitoring shows that a baby has stopped growing or has other problems.
A cesarean section (C-section) might be done if the stress of a vaginal delivery is considered too risky for the baby. Although early delivery might be necessary, the goal is to keep the baby safe in the womb for as long as possible. Sometimes amniocentesis can help determine lung maturity and whether the baby is likely to be able to breathe on his or her own.
Treatment for IUGR also includes addressing the possible causes, including managing any maternal illness and ensuring that the mother eats a healthy and nutritious diet and gains the appropriate amount of weight. Some women are told to avoid aerobic exercise and some are placed on bed rest to improve blood flow to the baby.
Unfortunately, babies with IUGR are more likely to have health problems both before and after birth. Those who are born prematurely or are very small at birth are more likely to have problems that result in longer hospital stays. They might also need to receive special care after birth, such as help breathing or medicine to prevent infections.
Other problems that can be associated with IUGR include:
- decreased tolerance for birth, resulting in increased likelihood of C-section delivery
- problems with breathing and feeding
- trouble maintaining body temperature
- abnormal blood cell counts
- low blood sugar level (hypoglycemia)
- decreased ability to fight infection
- increased likelihood of neurological problems
- increased likelihood of stillbirth (dying in the womb before birth)
The long-term effects of IUGR may depend on the condition that caused the problem in the first place.
When a woman learns that her baby has or might have IUGR, the best thing she can do is to keep all of her prenatal visits and testing appointments and follow her health care provider's recommendations. She also should take care of herself by eating a healthy diet, getting enough rest, and avoiding alcohol, drugs, and tobacco.
Reviewed by: Larissa Hirsch, MD
Date reviewed: July 2011