Ultrasonographic dating of pregnancy causes significant errors in Down syndrome risk assessment that may be minimized by use of biparietal diameter-based means.

OBJECTIVES

Gestational dates assessed by ultrasonographic measurement of fetal dimensions are usually quoted in terms of complete weeks because the uncertainty of ultrasonographic measurement is approximately +/- 7 days. This study examines the effect of ultrasonographic dating on Down syndrome risk assessment.

STUDY

DESIGN

The effect of small changes in measured biparietal diameter resulting in a change in estimated gestational week and the benefits of a more precise measure of fetal gestational age (raw biparietal diameter) are examined mathematically.

RESULTS

If maternal serum alpha-fetoprotein and human chorionic gonadotropin are used to assess Down syndrome risk, risks assessed with ultrasonographically determined dates may be less than or equal to 45% too high for fetuses with biparietal diameter at the lowest end of the size band and less than or equal to 22% too low with biparietal diameter at the top of the size band. If the results for maternal serum alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol are used, these figures become less than or equal to 150% and less than or equal to 60%, respectively.

CONCLUSION

If ultrasonography is used to assess gestational age, the raw biparietal diameter and not the estimated week of gestation must be used to derive means for calculation of multiples of the mean.