Antenatal screening for Down's syndrome

Many pregnant women wish to undergo antenatal testing for Down's syndrome—for reassurance that their unborn child does not have Down's syndrome, to allow the option of termination if it does, or to allow preparation for the birth of a baby with the condition. Unfortunately, invasive tests required to obtain tissue for fetal karyotyping (chorionic villus sampling, amniocentesis) cause loss of the pregnancy in about 1% of cases. The challenge of an antenatal screening programme is, therefore, to identify women in whom a risk of Down's syndrome is sufficiently high to justify such an invasive test and to minimise the risk of miscarrying a healthy baby.

Initially, invasive testing was offered only to women over 35 years, but this identified only one third of fetuses with Down's syndrome. Universal screening started with the observation that serum concentrations of α fetoprotein, used to screen for neural tube defects, tended to be lower when the fetus had Down's …