You might have these antibodies if you've gotten blood from a donor or given birth before. There's also a chance your body could make them when your and your baby's blood types don't match.

Some of these antibodies can be passed from you into your baby's bloodstream, where they could do harm. Antibody testing lets you and your doctor know if you have them so you can take steps to protect your growing baby.

Why You Get Tested?

Your immune system makes antibodies to fight things it sees as "not you." Most of the time, that's great because antibodies usually target germs. And when you're pregnant, your immune system takes care of your baby, too. But if your red blood cells are different from your baby's, that may cause problems.

By far, the most common one is related to the + or - part of your blood type, called the Rh factor. Many people are Rh-positive, which means they have the Rh protein on their red blood cells. Rh-negative people don't. So they'll make antibodies to attack any Rh-positive blood cells that get into their body.