Maternal serum inhibin‐A and free β‐hCG concentrations in trisomy 21 pregnancies at 10 to 14 weeks of gestation

Objective To determine the relation between maternal serum inhibin‐A and free β‐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 β‐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 β‐hCG concentrations were significantly increased (mean difference inhibin = 0.51 SD, F= 18, P < 0.0001 and mean difference free β‐hCG = 1.13 SD, F= 80, P < 0.0001). For a 5% false positive rate, the sensitivity of maternal serum free β‐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 β‐hCG (r= 0.345, P < 0.01) and the deviation from the normal mean for free β‐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 β‐hCG was similar to the sensitivity of free β‐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 β‐hCG levels. However, the degree of elevation of inhibin‐A is less than that of free β‐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 β‐hCG is not significantly different from that achieved with maternal serum free β‐hCG alone.