Fetal growth restriction

Fetal growth restriction is what happens when a baby, still inside the uterus, can't grow as much as it could. Every baby carries a growth potential in its genes, roughly the size it's meant to reach, and here a health problem holds the baby back before it gets there. It's a common complication of pregnancy, and most often the trouble lies in the placenta, the organ that connects the baby to the mother inside the uterus.

Here's what goes on inside. In placental insufficiency, which is when the placenta doesn't work the way it should, the spiral arteries, blood vessels in the uterus, aren't remodeled properly: they never fully make the change they're supposed to make. As a result, the placenta does less than it should, and the baby no longer grows the way it was meant to.

The restriction is also most often asymmetric, meaning uneven: the head and brain hold up fairly well, but the abdominal circumference, the measurement around the belly, comes up short. At the same time, the baby redirects its blood toward the brain at the expense of the rest of the body. Think of a house running low on water that shuts off the garden hose so the kitchen never runs dry.