Tiny and healthy: how small for gestational age is different from intrauterine growth restriction

Intrauterine growth restriction (IUGR) and small for gestational age (SGA) are often confused. Small for gestational age is a clear-cut definition of weight, and simply means that the baby’s weight is less than 90%, or 9 out of every 10 babies born at the same gestational age. Some of the babies that are diagnosed with small for gestational age are completely healthy. These babies may be small because their parents are small, and we expect the baby to be small. These small for gestational age babies may have some of the complications that growth restricted babies have after birth, such as low blood sugar, but most of these healthy SGA babies do not have the complications associated with growth restriction later in life.

On the other hand, some small for gestational age babies are also growth restricted. This is the case when a baby is born to tall parents, but the baby is small. The most challenging cases of growth restriction for physicians to identify is when the baby’s weight is greater than the weight cut-off to be diagnosed as small for gestational age. These babies may have growth restriction even though they are not small for gestational age. For example, if a baby born to two very tall parents weighs more than 20%, or 2 out of every 10 babies born at the same gestational age, this baby is not small for gestational age. However, this baby may have growth restriction because we would expect this baby to weigh more than just 2 out of every 10 babies.

Diagnosing IUGR is more complicated than just a weight cut-off. The technical definition of intrauterine growth restriction is the inability for a fetus to meet her growth potential. Obstetricians and midwives will follow a pregnant mother closely to evaluate the mother for concerns for growth restriction. A fetus whose growth has slowed down toward the end of pregnancy, or who is small throughout pregnancy, is more likely to have growth restriction. Further testing may be done to determine if a baby has growth restriction, and if so, what caused the growth restriction.