Abstract

Background: Ula, part of the Kell blood group system, is a low-frequency antigen found predominantly in the Finnish population. Anti-Ula is a rare antibody to cause hemolytic disease of the fetus and newborn (HDFN).

Methods and Findings: We report one woman with two pregnancies in which anti-Ula caused severe HDFN requiring intrauterine (IU) transfusions. In the first affected pregnancy, two IU transfusions were performed, but the cause of fetal anemia was unknown at the time and an immune-based mechanism was not suspected. In the following pregnancy, anti-Ula with a titer of 1-2 was identified, and as the fetus was anemic, six IU transfusions were performed. The detection of anti-Ula is difficult, because it is not usually included in the antibody screening cells.

Conclusions: The possibility of a rare antibody as the cause of severe HDFN should be kept in mind in cases in which the antibody screening has been negative, and further antibody detection studies should be performed as part of fetal anemia investigations. If anti-Ula is identified in a pregnant woman, close ultrasound monitoring of the fetus is important because HDFN may develop even when the antibody titers are low.