Newborns are little wonders. We all marvel how something of such a small size can be a living, breathing person. When a baby is too small, however, doctors become concerned. They want to know the reason for the newborn’s small size. They also know that complications may occur in babies (and fetuses) that do not appear to be growing as well as expected. Such infants and infants-to-be are said to be showing signs of intrauterine growth retardation (IUGR).
In order to understand what we mean by IUGR, it’s helpful to understand growth percentiles. The same principles that relate to percentiles as they apply to children apply to fetuses as well. Obstetricians use physical examination and ultrasound techniques to estimate the size of the fetus and compare it with its expected size based on the estimated gestational age. If that estimated size comes out as being under the 10th percentile—that is, if 90 percent of fetuses of that age are larger—it may have IUGR.
More commonly, IUGR is diagnosed after the baby is born. The “hands-on” examination by the pediatrician contains items that can usually more accurately estimate the gestational age. At this stage, providers also look for clues to IUGR such as a wrinkled appearance and a lack of prominent muscle tissue.
But All Babies Are Small. What’s the Problem?
Like so much in medicine, the answer to this question has a few parts. First of all, providers following your pregnancy and, subsequently, your newborn want to find out what caused your baby to be small in the first place. Unfortunately, no specific cause is found in about 40 percent of babies with IUGR. Among those where a cause is found, about one third will have a genetic disease: one that is inherited from the parents’ DNA, or that shows up when the DNA-containing chromosomes don’t behave normally. These babies tend to have what’s known as symmetric IUGR, where the head size and length are decreased in addition to the weight. And many babies who suffer from a genetic disease have other problems. For example, the heart or brain may not be formed properly.
The remaining two thirds of infants with IUGR for whom a cause can be found will have an environmental cause. This has nothing to do with the landfill down the street; it actually refers to the environment that the fetus experiences inside Mom’s uterus. One important environmental cause is infection with certain viruses, bacteria, and parasites. Like genetic syndromes, these infections can cause other abnormalities in the baby.