Will a pregnancy between a B positive (Rh positive) husband and an A negative (Rh negative) wife pose any risks due to Rh incompatibility?

Rh Incompatibility Risk in B Positive Husband and A Negative Wife

Yes, there is a significant risk of Rh disease in this pregnancy because the wife is Rh negative and the husband is Rh positive, meaning the fetus may inherit the father's Rh positive status and trigger maternal antibody formation that can harm this or future pregnancies. 1

Understanding the Core Problem

The critical issue is not the ABO blood type difference (B versus A), but rather the Rh status incompatibility. When an Rh negative mother carries an Rh positive fetus (inherited from the father), fetal red blood cells can enter maternal circulation and trigger the mother's immune system to produce anti-D antibodies. 2 This process, called Rh alloimmunization, occurs because:

• The RhD antigen develops on fetal red blood cells as early as 6 weeks of gestation 3, 1
• Fetomaternal hemorrhage occurs in approximately 50% of all deliveries, allowing fetal cells to enter maternal circulation 2
• As little as 0.1 mL of Rh-positive fetal blood can cause maternal sensitization 2
• Without prophylaxis, approximately 17% of Rh-negative women become immunized after pregnancy with an Rh-positive infant 2

Consequences Without Prevention

If the mother becomes sensitized, the anti-D antibodies can cross the placenta in subsequent pregnancies and destroy fetal red blood cells, leading to:

• Hemolytic disease of the fetus and newborn 1
• Fetal anemia requiring intrauterine transfusion 4
• Fetal hydrops 1
• Stillbirth 1
• Preterm delivery 1

Prevention Protocol: The Solution

The wife must receive RhD immune globulin (RhIg) prophylaxis according to this specific schedule: 1

Routine Prophylaxis

• At 28 weeks gestation: Administer 300 μg RhIg 1
• Within 72 hours after delivery: Administer 300 μg RhIg if the infant is confirmed Rh positive 1
• This two-dose protocol reduces alloimmunization rates from 1.8% to between 0.1% and 0.2% 1

Additional RhIg Indications During Pregnancy

The wife will also need RhIg (50 μg before 12 weeks, 300 μg after 12 weeks) within 72 hours if any of these events occur: 1

• Miscarriage or abortion (spontaneous or induced) - even in first trimester, as fetal RBCs display RhD antigens from 6 weeks onward 1
• Ectopic pregnancy 3
• Vaginal bleeding at any gestational age, particularly if heavy, associated with abdominal pain, or occurring near 12 weeks 1
• Amniocentesis or chorionic villous sampling 3
• Abdominal trauma - even minor trauma causes fetomaternal hemorrhage in 28% of cases 1
• External cephalic version 3

Critical Pitfalls to Avoid

Do not assume early pregnancy events are safe to ignore. Fetomaternal hemorrhage occurs in 48% of threatened abortions, 36% of complete abortions, and 22% of incomplete abortions. 1 Even minimal bleeding can cause sensitization, as bleeding severity does not reliably predict hemorrhage volume. 1

Do not miss the 72-hour window. While RhIg may provide some benefit up to 28 days after an event, protection decreases significantly with delayed administration. 1 Preferably administer within 72 hours. 1

Do not skip postpartum RhIg if antenatal dose was given. Both doses are necessary - the 28-week dose does not eliminate the need for postpartum prophylaxis. 1

Effectiveness of Prevention

When properly administered, RhIg prophylaxis is highly effective:

• Postpartum RhIg alone reduces alloimmunization from 12-13% to 1-2% 4
• Adding the antenatal dose at 28 weeks further reduces rates from 1.8% to 0.1-0.2% 1
• This prevention strategy reduced fetal mortality from Rh hemolytic disease from 120 per 100,000 live births to 1.5 per 100,000 by 1989 2

Special Consideration for This Couple

Since the husband is homozygous or heterozygous Rh positive, there is a 50-100% chance each pregnancy will result in an Rh positive fetus. 5 This makes consistent RhIg prophylaxis essential for every pregnancy, as the risk persists with each conception. The ABO incompatibility (B versus A) between parents actually provides no protective benefit in this scenario, unlike situations where ABO incompatibility might offer some protection against Rh sensitization. 6