Prenatal diagnosis of chromosomal abnormalities--shifting paradigm.

It is very likely that in 10 years time, invasive prenatal diagnostic tests like amniocentesis and chorionic villus sampling (CVS) will join the club of forgotten obstetric procedures like vaginal breech delivery and rotational forceps delivery. In 1968, Henry Nadler1 and his team were the fi rst to report prenatal diagnosis of trisomy 21 from full karyotyping of cultured amniocytes obtained by amniocentesis. Over the next 40 years, both amniocentesis and CVS became commonplace diagnostic tools to obtain fetal cells for antenatal diagnosis of chromosomal abnormalities in high-risk pregnant mothers. The methods of screening and identifying the high-risk mother have changed over time. Initially, invasive testing methods like amniocentesis and CVS were offered to women of advanced maternal age (typically women above 35 years). It was soon realised that age was a poor screening method for fetal chromosomal abnormalities, especially Down syndrome, that most deliveries occurred in mothers aged less than 35, and while their individual risk is lower, this group makes a signifi cant contribution to missed diagnoses. Various screening tests were therefore introduced. The most sensitive and most recommended is the fi rst trimester combined screen (FTS), which includes ultrasound-measured fetal nuchal translucency and serum biochemistry at 11 to 14 weeks gestation. The FTS detects 85% to 90% of all fetuses with Down syndrome but has a signifi cant false positive rate (3% to 5%). These 3% to 5% of pregnant mothers require invasive diagnostic tests to confi rm the diagnosis, which will be truly positive in only 3% to 4% of these screenpositive women.