Increased erythrocyte osmotic fragility in pregnancy.

An unexpected increase in erythrocyte osmotic fragility during pregnancy in two healthy women prompted a study of the effects of pregnancy on osmotic fragility. The incubated glycerol lysis time, a rapid, sensitive measure of osmotic fragility, was determined in 100 pregnant women and 50 nonpregnant control subjects. Twenty-two of the pregnant women (22%) showed abnormal results when compared to normal nonpregnant women (p less than 0.0005). Increased erythrocyte fragility was observed primarily in the last trimester of pregnancy. Twenty-one of 65 women in the last trimester (32.3%) had abnormal incubated glycerol lysis time values, but only one of 34 (2.9%) showed increased fragility during early pregnancy. Physiologic shifts in erythrocyte osmotic fragility may create a problem in the diagnosis of hereditary spherocytosis during the last trimester of pregnancy.