Obstetric care of the affected carrier of hemophilia B.

Hemophilia B (factor IX deficiency) is an X-linked recessive clotting disorder similar to hemophilia A. In both diseases, phenotypic expression of the disorder may occur in significant numbers of heterozygous carriers. This is presumably due to random inactivation of one of the two X chromosomes of the female carrier as described by the Lyon hypothesis. Described herein is the obstetric care of a severely affected heterozygous carrier of hemophilia B. Predelivery automated plasma exchange was used to raise factor IX levels successfully. Commercial factor concentrates were avoided. The Lyon hypothesis is discussed in detail, and recommendations are made for the care of the clotting factor-deficient gravid woman.