Screening for open neural tube defects (ONTDs) by MSAFP began in the 1970s and screening for fetal chromosomal anomalies began with amniocentesis in the mid-1960s. Maternal age, ≥ 35 years at the expected date of delivery, was used as the lower threshold of who should be offered prenatal diagnostic testing for chromosomal anomalies. This age was chosen as it was the point at which the risk of a pregnancy loss was less than the chance of identifying a pregnancy with a significant chromosomal abnormality.
Maternal serum screening enabled women of all ages to pursue screening for chromosomal abnormalities. It also provided women who were concerned about the chance of miscarriage associated with a diagnostic test the opportunity to obtain information without the added risk. Screening provides women with an individual risk by adjusting their age related risk (in the case of chromosomal abnormalities) or population risk (open neural tube defects) with the biochemical results specific to their pregnancy.
Prenatal screening programs are applicable to the population as a whole. Diagnostic testing programs are targeted to specific high risk populations. Screening cut-offs are one way to define who is considered at high risk and who should be offered diagnostic testing.
Prenatal screening has made many advances in Canada. The current maternal age at the expected date of delivery for women wishing to access prenatal diagnostic testing has increased to ≥ 40 years in many centres across Canada. This change is reflective of the better detection rates and lower false positive rates with the current screening options.