Rh sensitization can occur when a person with Rh-negative blood is exposed to Rh-positive blood. During pregnancy, an Rh-negative woman can become sensitized if she is carrying an Rh-positive fetus.

Factors that increase the risk of blood mixing and sensitization during pregnancy include:³

• Delivery.
• Abdominal trauma, such as from a car accident.
• Abdominal surgery, such as a cesarean section.
• Placenta abruptio or placenta previa, both of which can cause placental bleeding.
• External cephalic version for a breech fetus.
• Obstetric procedures such as amniocentesis, fetal blood sampling, or chorionic villus sampling (CVS).
• Miscarriage (spontaneous abortion), ectopic pregnancy, or elective abortion (medical or surgical abortion) after 8 weeks of fetal age (when fetal blood cell production begins).
• Partial molar pregnancy involving fetal growth beyond 8 weeks.

Although rare, Rh sensitization has been known to occur after needle sharing between intravenous drug users. Transfusing Rh-positive blood in an Rh-negative person can also trigger sensitization; however, this is extremely rare because blood is always tested prior to transfusion.