Detection of trisomy 18 by double screening in a low-risk pregnant population

A combination of low concentrations of maternal serum alpha fetoprotein (MSAFP) and human chorionic gonadotropin (hCG) was used to screen for trisomy 18 in early 2nd-trimester pregnancies in a low-risk child-bearing population.

Women less than 37 years of age were offered screening between 15 and 20 weeks of gestation. Those pregnancies showing low concentrations of MSAFP (<0. 75 multiples of the median) and hCG (<0.5 multiples of the mean) were considered positive. If gestational age was confirmed, genetic counselling and invasive prenatal diagnosis were offered.

Between January 1995 and December 1996, 19,491 women were screened, of whom 252 were found to be positive (1.25%). 145 invasive procedures were carried out, and 3 cases were detected. The odds of a fetus being affected after a positive screen result were 1 in 36.

Measurement of low concentrations of MSAFP and hCG in pregnant women less than 37 years of age is an effective screening test for Edward's syndrome, but with regard to the natural history of this genetic condition, it can result in overuse of invasive tests which in turn can harm chromosomally normal pregnancies. This stresses the need to evaluate these pregnancies further with detailed ultrasonographic assessment and selective fetal karyotyping only.