Which Genetic Tests Are Performed at Which Weeks of Pregnancy?

Prenatal genetic assessment is not a single test. Carrier screening, chromosome screening, ultrasound and, when indicated, diagnostic testing answer different questions. The appropriate timeline is therefore personalised according to gestational age, family history, previous pregnancies and ultrasound findings.

Before pregnancy or early in pregnancy

Carrier screening for inherited conditions such as SMA, cystic fibrosis and haemoglobin disorders can be performed before or during pregnancy. Preconception testing allows more time to consider partner testing and reproductive options. If a familial genetic variant is known, targeted testing for that variant may be more appropriate than a general panel.

From 10 weeks: NIPT

NIPT uses placental cell-free DNA in maternal blood to estimate the chance of selected chromosome conditions, primarily trisomy 21, 18 and 13. It is available from 10 weeks. NIPT is screening: a high-risk result requires diagnostic evaluation, and a low-risk result does not exclude all genetic conditions.

10–13 weeks: first-trimester combined screening

Maternal serum markers, nuchal translucency ultrasound and clinical information are combined. The ultrasound also provides information about gestational dating, viability, multiple pregnancy and certain early structural findings.

10–13 weeks: chorionic villus sampling

CVS is a diagnostic procedure that obtains cells from the placenta. It may be considered after a high-risk screen, for a known familial variant or with specific ultrasound findings. The chromosome or gene analysis is selected according to the clinical question.

15–22 weeks: second-trimester screening

The quadruple serum screen can generally be performed between 15 and 22 weeks to estimate the chance of certain chromosome conditions and neural tube defects. Local practice and the clinical situation influence the exact approach.

15–20 weeks: amniocentesis

Amniocentesis is usually performed between 15 and 20 weeks using a sample of amniotic fluid. Karyotyping, chromosomal microarray or a specific gene test may be selected according to the indication. The small procedure-related risk of pregnancy loss should be discussed with a specialist.

18–22 weeks: detailed fetal anatomy ultrasound

The fetal brain, spine, heart, face, abdomen and limbs are assessed. This examination is not a genetic blood test. NIPT and serum screening do not replace the anatomy ultrasound, and the ultrasound does not replace genetic screening or diagnosis.

How is the appropriate test selected?

Understand the distinction between screening and diagnostic testing.
After counselling, select the most appropriate screening pathway rather than several independent screens for the same purpose.
Consider targeted testing when there is a known familial condition or a relevant ultrasound finding.
Do not treat a high-risk screening result as a definitive diagnosis.