Rhesus factor (Rh factor) incompatibility can be a relevant topic in marriage counseling, particularly for couples planning to have children, as it can impact pregnancy outcomes and require specific medical interventions. 

The Rh factor is a protein found on the surface of red blood cells, and individuals are either Rh positive (have the protein) or Rh negative (do not have the protein). 

Rh incompatibility occurs when a Rh-negative woman is carrying a Rh-positive fetus. The mother's immune system can develop antibodies against the Rh-positive fetal blood cells, which can lead to complications in future pregnancies.