The value of using gestational age by ultrasonography in Down syndrome and neural tube defects screening

Abstract

The optimal gestational age for prenatal screening for neural tube defects and Down syndrome is usually between 15 and 18 weeks. Elevated concentrations of maternal serum alpha-fetoprotein are a diagnostic indication of neural tubal defects where as low levels of alpha-fetoprotein, unconjugated estriol and elevated human chorionic gonadotropin are indication of Down syndrome. Accurate assessment of gestational age at the time of testing for these analytes is very important for the interpretation of the test results. The present report is designed to determine whether gestational age should be based on last menstrual period (LMP) data or ultrasonographic evaluation in the interpretation of maternal serum screening. The present study population consisted of 4,200 women at 15 to 19 weeks gestation. Seventy-seven percent of test interpretations were based on ultrasonographic evaluation of gestational ages. For 56 Down syndrome pregnancies both ultrasonography and LMP dates were available. Whenever the test results were positive for Down syndrome or neural tube defects, the serum samples were collected and again assayed for results. We found both initial and revised screen positive rates for Down syndrome were significantly lower and detection rates for Down syndrome seemed to be higher with ultrasonography (68% vs. 49% for LMP dates). The opposite trend was seen for neural tube defects. Serum-positive rates for neural tube defects were higher for women when gestational age was based on ultrasonographic examination than those women referred with LMP dates. The detection rates for neural tube defects by ultrasonography was slightly lower than those by LMP dates. Our data indicate that ultrasonography is preferred for the effectiveness of maternal serum screening. The ultrasonographic biometric measurement for estimating gestational age for both neural tube defects and Down syndrome screening by biparietal diameter measurement was disassumed.