Intrauterine growth restriction (IUGR) refers to poor growth of a baby while in the mother's womb during pregnancy. The causes can be many, but most often involve poor maternal nutrition or lack of adequate oxygen supply to the fetus.

At least 60% of the 4 million neonatal deaths that occur worldwide every year are associated with low birth weight (LBW), caused by intrauterine growth restriction (IUGR), preterm delivery, and genetic/chromosomal abnormalities, demonstrating that under-nutrition is already a leading health problem at birth.

The term IUGR is not synonymous with Small for Gestational Age (SGA). SGA refers to a birth weight that is below the 10th percentile for gestational age. Not all fetuses with IUGR are classified as SGA, and vice versa. IUGR is used to describe a pattern of intrauterine fetal growth that deviates from expected norms, whereas SGA is a category assigned based on birth weight.

There are 2 major categories of IUGR: symmetrical and asymmetrical.

Asymmetrical IUGR is more common. In asymmetrical IUGR, there is restriction of weight followed by length. The head continues to grow at normal or near-normal rates (head sparing). This is a protective mechanism that may have evolved to promote brain development. This type of IUGR is most commonly caused by extrinsic factors that affect the fetus at later gestational ages.