Prenatal screening results for Down syndrome and fetal open neural tube defects are subject to substantial variation depending on the gestational age of the fetus at the time screening tests are performed. For open neural tube identification, ultrasonographic evaluation of gestational age by biparietal diameter measurement alone should be advantageous. Use of ultrasonographic dating for Down syndrome screening should increase the detection rate. Benn and associates conducted a study to determine whether gestational age should be based on ultrasonographic evaluation or on the last menstrual period in the interpretation of second-trimester maternal serum screening for Down syndrome and open neural tube defects.

The study included 24,000 pregnant women who underwent triple testing (maternal serum alpha-fetoprotein, human chorionic gonadotropin and unconjugated estriol) at 15.0 to 21.9 weeks of gestation. When dating by ultrasonographic evaluation and dating by last menstrual period were available before testing, the ultrasonographic data were used for gestational age assessment and test interpretation. Sixty percent of the test interpretations were based on ultrasonographic evaluation of gestational age. In 24 Down syndrome pregnancies, dating by ultrasonography and dating by last menstrual period were both available at the time of the screening. The proportion of patients who initially screened positive for Down syndrome was 5.02 percent for women with gestational age determined by ultrasound and 9.33 percent for women with gestational age determined by the last menstrual period. This represented a significant difference.

After age adjustment, the screen-positive rate for the ultrasonography group was still significantly different from the rate for the last menstrual period group. The detection rate for Down syndrome was higher when ultrasonographic dating was used than when last menstrual period was used (76 versus 60 percent). Fourteen of the 24 cases of Down syndrome had a higher risk for Down syndrome when dating from the last menstrual period was used. Dating using the last menstrual period gave a gestational age 5.3 days older than the gestational age obtained by ultrasound. Using these data, the odds of having a fetus with Down syndrome given a positive triple test was one in 63 when dating was based on the last menstrual period and one in 37 when dating was based on ultrasound.

The rate of women who initially screened positive for open neural tube defects and also screened positive on revised testing was significantly higher when gestational age was based on ultrasonographic dating than when dating was based on the last menstrual period.

The authors conclude that screening for Down syndrome appears to be almost twofold more efficient using ultrasonographic dating. Imprecision associated with dating from the last menstrual period resulted in a substantial number of post-test ultrasonographic evaluations, recalculations and retestings. The number of anmiocenteses recommended for the ultrasonography group was significantly less than that for the group dated by the last menstrual period. Although the authors do not recommend routine ultrasonographic examination for women undergoing triple testing, they do suggest that when ultrasonographic information is available, it should be preferentially used over data from the last menstrual period because it significantly improves the effectiveness of triple testing.