Abstract

OBJECTIVE:

To determine the relation between maternal serum inhibin-A and free beta-hCG concentrations in chromosomally normal pregnancies and to compare the two biochemical markers for their sensitivity in identifying trisomy 21 pregnancies.

SAMPLE:

Inhibin-A and free beta-hCG were measured in maternal serum samples from 800 chromosomally normal singleton pregnancies at 10 to 14 weeks of gestation and 76 singleton pregnancies with fetal trisomy 21.

RESULTS:

In the normal group maternal serum inhibin-A was significantly associated with both maternal weight and gestational age (F = 11.2, P < 0.0001). In pregnancies with trisomy 21 the maternal serum inhibin-A and free beta-hCG concentrations were significantly increased (mean difference inhibin = 0.51 SD, F = 18, P < 0.0001 and mean difference free beta-hCG = 1.13 SD, F = 80, P < 0.0001). For a 5% false positive rate, the sensitivity of maternal serum free beta-hCG in identifying pregnancies with trisomy 21 was 28.9% compared with 12.8% for maternal serum inhibin-A. Delta inhibin-A was significantly associated with delta-free beta-hCG (r = 0.345, P < 0.01) and the deviation from the normal mean for free beta-hCG was significantly greater than the deviation for inhibin-A (t = 4.0, P < 0.0001). For a 5% false positive rate, the sensitivity achieved by combining information from delta inhibin-A and delta free beta-hCG was similar to the sensitivity of free beta-hCG alone (30.3% compared with 28.9%).

CONCLUSION:

At 10 to 14 weeks of gestation fetal trisomy 21 is associated with increased maternal serum inhibin-A and free beta-hCG levels. However, the degree of elevation of inhibin-A is less than that of free beta-hCG, and there is a significant association between levels of the two proteins. The sensitivity for trisomy 21 achieved with the combination of maternal serum inhibin-A and free beta-hCG is not significantly different from that achieved with maternal serum free beta-hCG alone.