Rh Factor & Pregnancy
Understanding Rh incompatibility
Rh incompatibility can occur when a pregnant person is Rh-negative and the fetus is Rh-positive. With routine screening and Rh immunoglobulin, it is largely preventable.
What is the Rh factor?
The Rh factor refers to the RhD antigen, a protein on the surface of red blood cells. If the protein is present, a person is Rh-positive; if it is absent, they are Rh-negative. This is separate from ABO blood type (A, B, AB, O) and is the plus or minus after your letter.
Rh-negative is the recessive trait: an Rh-negative person has two d alleles (dd), while an Rh-positive person has DD or Dd. A baby is Rh-positive if it inherits at least one D allele.
When can Rh incompatibility occur?
Rh incompatibility can occur when an Rh-negative pregnant person is carrying an Rh-positive fetus. This is only possible when the other biological parent is Rh-positive, since they must contribute a D allele. In most pregnancies there are no problems; your prenatal care team can explain when closer monitoring may be recommended.
Small amounts of fetal blood can cross into the parent's circulation during delivery, miscarriage, certain procedures such as amniocentesis, abdominal trauma, or bleeding. If the parent's immune system forms anti-D antibodies (sensitization), those antibodies can affect a later Rh-positive pregnancy. The baby blood type calculator shows the Rh probability for any pair of parental types.
How clinicians manage it
Standard prenatal care includes a blood type and antibody screen early in pregnancy. If the pregnant person is Rh-negative and has not formed antibodies, Rh immunoglobulin (often called RhoGAM in the United States) is typically offered. It clears any Rh-positive fetal cells before the immune system can respond.
ACOG describes Rh immunoglobulin being given around 28 weeks and again within 72 hours after the birth of an Rh-positive baby, as well as after events that could mix fetal and maternal blood. If antibodies have already formed, pregnancies are monitored more closely and treatments are available for the baby if needed. Your care team will personalize care based on your situation.
Questions to ask your prenatal care team
Being informed helps you feel confident and supported. Consider raising these at your next visit:
- What are my Rh type and my partner's Rh type?
- Do I need any additional testing during pregnancy?
- If I am Rh-negative, when would Rh immunoglobulin be offered?
- Should I receive it after bleeding, a procedure, or an injury?
- Will the baby's blood type be tested at birth, and what happens next?