Prenatal screening relates primarily to the care of pregnant women, but not exclusively during the first trimester.

The key challenge is early detection of chromosomal abnormalities, and particularly trisomy 21 (three copies of chromosome 21 instead of two), but also of the two other frequently-occurring trisomies, involving chromosomes 13 and 18. Chromosome 21 is one of the smallest on the genome and is gene poor. An extra copy is therefore better tolerated than a copy of one of the larger chromosomes. These three trisomies alone (13, 18 and 21) are responsible for the majority of chromosomal disorders during pregnancy.

As a rule, combined first-trimester screening (“first-trimester test”) is offered to all pregnant women in order to identify the presence of any chromosomal anomalies. It consists of determining the presence of two substances in the blood (biomarkers that correlate with the chromosomal abnormalities in question), then an ultrasound examination that measures nuchal translucency (the distance between two folds in the back of the baby's neck, which also correlates with chromosomal aberrations); the mother's age, which is a risk factor, is also taken into account. These three components are used to calculate the risk of the baby being affected by one of the three trisomies.

Depending on this risk, other tests may be offered:

non-invasive prenatal screening (NIPT), which examines the baby's DNA by testing the mother’s blood.
an invasive prenatal diagnostic test (IPNDT, using chorionic villus sampling or amniotic fluid cells).

It is important to specify that this first-trimester test may conceal non-chromosomal anomalies, e.g. heart malformations; it is therefore neither specific, nor exhaustive and always requires monitoring and verification by other methods for results that are not considered to involve low or very low risk.

Genesupport - Prenatal screenings