Rhesus (Rh) factor is an inherited trait that refers to a specific protein found on the surface of red blood cells. If your blood has the protein, you're Rh positive — the most common Rh factor. If your blood lacks the protein, you're Rh negative.
Although Rh factor doesn't affect your health, it can affect pregnancy. Your pregnancy needs special care if you're Rh negative and your baby's father is Rh positive.
If you're pregnant, your health care provider will recommend an Rh factor test during your first prenatal visit. The Rh factor test is a basic blood test that indicates whether you're Rh positive or Rh negative.