Rh-Negative Mother, Rh-Positive Father: Risks and the Anti-D Injection

When does Rh incompatibility happen?

It happens when the mother is Rh negative and the baby is Rh positive. During pregnancy, red blood cells from the baby can cross into the mother’s blood through the placenta; her immune system treats them as foreign and makes antibodies against them. These antibodies can cross back to the baby and destroy its red blood cells.

It does not happen if the mother is Rh positive, whatever the father’s blood type.

When is the baby Rh positive?

If the father is Rh positive the baby may be positive; if both parents are Rh negative the baby is always negative. Work out the chances with the baby blood type calculator, which includes an Rh inheritance chart.

Is the first baby affected?

Usually not, because the mother’s body takes time to make antibodies, unless her immune system was sensitised before, for example by an earlier miscarriage. But every later Rh-positive baby may be affected if the mother does not get the preventive treatment.

What is the anti-D injection, and when is it given?

It is a special immune globulin injection (RhoGAM is one brand) that stops the mother’s body from making antibodies against Rh-positive blood. According to MedlinePlus:

• An Rh-negative mother is given an injection in the second trimester if the father is Rh positive or his blood type is not known.
• She is given a second injection within a few days after delivery if the baby is Rh positive.
• Rh-negative mothers need the injections in every pregnancy, after a miscarriage or abortion, after prenatal tests such as amniocentesis and chorionic villus sampling, and after any injury to the abdomen during pregnancy.

Your doctor decides the right timing for you.

What happens to the baby if it is not prevented?

Symptoms range from very mild to severe. The antibodies break down the baby’s red blood cells, bilirubin rises and the skin and whites of the eyes turn yellow. Mild cases are usually treated with phototherapy, intravenous immune globulin may be used, and severe cases may need an exchange transfusion. Full recovery is expected in mild cases.

Frequently asked questions

Sources

This guide is based on the following published sources:

1. Rh incompatibility MedlinePlus (US National Library of Medicine)
2. Blood typing MedlinePlus (US National Library of Medicine)

Content and links last reviewed: 17 October 2026. How we write the tools

This tool gives general guidance based on published rules, which differ by country and change over time. It does not replace a medical assessment. The doctor at the donation centre always makes the final decision. In an emergency, call your local emergency number.