Because of this severe complication, the American College of Obstetricians and Gynecologists (ACOG) recommends that all RhD negative mothers, regardless of fetal blood type, receive RhIG at about 28 weeks gestation, and again shortly after delivery in the case of an RhD positive or RhD unknown baby. It should be given within 3 days of a potential exposure to Rh positive blood from the baby such as may occur during second or third trimester miscarriage, amniocentesis, cordocentesis, chorionic villus sampling, external cephalic version, trauma, or delivery. It should be considered after first trimester miscarriage. The '28 weeks' recommendation comes from the fact that 92% of women who develop an anti-D during pregnancy do so at or after 28 weeks gestation. It is given by intramuscular injection as part of modern routine antenatal care. Despite excellent results, the medication retains an FDA Pregnancy Category C.