A clinician contrasts ABO haemolytic disease with Rhesus disease. Which single statement about ABO haemolytic disease is correct?

Answer: C. It can occur in the first pregnancy because sensitisation can pre-exist

ABO haemolytic disease of the newborn

ABO incompatibility (typically a group O mother with a group A or B baby) differs from Rhesus disease in key ways.

Key features

• Because A and B substances occur in food and bacteria, the mother may already have anti-A/anti-B antibodies before pregnancy.
• Therefore ABO haemolytic disease can occur in the first pregnancy, without prior pregnancy sensitisation.
• Most natural ABO antibodies are IgM (which do not cross the placenta); the IgG fraction that does cross causes a usually milder haemolytic disease than Rh disease.

Clinical course

• Usually presents as neonatal jaundice (often mild), managed with phototherapy; severe disease and hydrops are uncommon.