ObjectiveTo measure the levels of digoxin-like immunoreactive substance and digitalis-like factor bioactivity as manifested by sodium-potassium-adenosine triphosphatase (ATPase) inhibition throughout pregnancy.MethodsSerum samples were collected from primigravidas in early (15 ± 1.8 weeks), mid (26 ± 1.2 weeks), and late (36 ± 1.1 weeks) gestation, as well as at 6 ± 1.1 weeks postpartum (mean ± standard error). Digoxin-like immunoreactive substance levels were determined by radioimmuno-assay and digitalis-like factor bioactivity was determined by inhibition of ATPase. Data were analyzed by means of repeated measures analysis of variance.ResultsIn 41 women with normal pregnancy outcomes, levels of digoxin-like immunoreactive substance rose progressively and significantly (P< .001) throughout pregnancy and returned to normal levels postpartum. Inhibition of ATPase activity also rose significantly (P< .004), but not as dramatically, during pregnancy and remained elevated 6 weeks postpartum.ConclusionAlthough digoxin-like immunoreactive substance levels rise in pregnancy, functional digitalis-like factor activity, as manifested by inhibition of ATPase, does not parallel this rise strictly, implying that digoxin-like immunoreactive substance receptors may be reset during normal pregnancy. The enhanced cardiac performance that occurs in normal pregnancy may be mediated in part by increased digitalis-like factor activity.