First, there was morning sickness, then there was weight gain and breast tenderness, and now, as your pregnancy creeps into the third trimester, you start to notice your shoes just don't fit the way they used to.

No, you're not imagining things, and no, it's not all attributable to pregnancy-related weight gain. You are, in fact, swelling, and generally speaking, it's another lovely and "normal" side effect of the pregnancy process. Fantastic, right?

That said, some swelling can be a sign of a more serious health issue, so it's important to know what to be on the lookout for and when to call your doctor.

Why third trimester swelling happens

It's no secret your body is undergoing significant changes during pregnancy to support the needs of your growing baby. But it's not just your belly and your boobs that are suddenly taking up more space—your blood and fluid content is growing astronomically as well.

"The composition of blood changes during pregnancy and women tend to hold onto more fluids," explained Betsy Greenleaf, D.O., FACOG, an advisor with