How is first-trimester screening for trisomy 13, 18 and 21 performed?

The combined test at ~11-14 weeks uses nuchal translucency + maternal age + serum beta-hCG + PAPP-A.

Nuchal translucency is subcutaneous fluid behind the fetal neck, measured between 11 and 14 weeks. The combined test identifies about 90% of affected fetuses with a 5% screen-positive rate; not all fetuses with increased NT have a problem.

Clinical relevance. The combined test estimates aneuploidy risk so that diagnostic testing can be offered to high-risk pregnancies.

More Detail

First-trimester aneuploidy screening

The combined test screens for the common trisomies at the dating scan.

Components

• Nuchal translucency (NT) + maternal age + serum beta-hCG + PAPP-A (pregnancy-associated plasma protein A).

Nuchal translucency

• A collection of subcutaneous fluid behind the fetal neck, measured between 11 and 14 weeks.
• An increased NT raises the risk of trisomy and also of major structural defects (e.g. cardiac), but not all fetuses with increased NT have problems.

Performance

• The combined method detects about 90% of affected fetuses with a 5% screen-positive (false-positive) rate.
• A high-risk result leads to offering diagnostic testing (CVS or amniocentesis) or cell-free fetal DNA (NIPT).

Note

It is a screening test (risk estimate), not diagnostic - a screen-positive result requires a diagnostic test to confirm.