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 is called intrauterine growth restriction, or IUGR.
IUGR is when 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. For example, some are healthy babies who are just born smaller than average because their parents are small in stature.
The two types of IUGR are:
Symmetrical IUGR, in which a baby's body is proportionally small (meaning all parts of the baby's body are similarly small in size).
Asymmetrical IUGR, which is when 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:
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 some types of heart, lung, blood, or autoimmune disease, and anemia also can 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. This starts with correctly determining the baby's gestational age by accurately dating the pregnancy.
At first, gestational age is 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 watch 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 watch the baby's growth and may do more tests to see whether the baby has IUGR.
Watching 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 top of the uterus.
Another way is to use 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 exact, they help health care providers track the baby's growth and see if there's a problem. Ultrasounds also can help identify other issues, such as problems with the placenta or low amniotic fluid levels.
Health care providers also might do other tests if they believe a baby has IUGR, such as:
fetal monitoring to track the baby's heart rate and movements
screenings for infections
amniocentesis to help find the cause of IUGR (and sometimes to help determine lung maturity and whether the baby is likely to be able to breathe on his or her own)
When IUGR is diagnosed, treatment is decided based on the baby's condition and the woman's month of pregnancy. The baby will be closely watched (usually with frequent prenatal visits and ultrasounds) to keep track of growth and watch for other potential problems.
Treatment also might include 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 placed on bed rest to try to improve blood flow to the baby.
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. Although early delivery might be necessary, the goal is to keep the baby safe in the womb for as long as possible.
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 also might need special care after birth, such as help breathing or medicine to prevent infections.
Other problems that can be related to IUGR include:
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 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 sleep; and avoiding alcohol, drugs, and tobacco.