In Australia, the combined first trimester screening is used to estimate the risk of the fetus having Down syndrome (trisomy 21) as well as other chromosomal abnormalities like trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome). This combined screening involves both a blood test and an ultrasound:
- Blood Test: Measures levels of two pregnancy-specific substances in the mother’s blood:
- Free beta human chorionic gonadotropin (free β-hCG)
- Pregnancy-associated plasma protein-A (PAPP-A)
- Ultrasound: Called the “nuchal translucency” (NT) ultrasound, this test measures the fluid-filled space at the back of the fetus’s neck. An increased amount of fluid here can be associated with chromosomal abnormalities.
The combined first trimester screening is typically done between 11 and 13 weeks of pregnancy. The optimal time is usually around 12 weeks.
It’s essential to understand that this screening provides a risk estimate; it doesn’t definitively diagnose or rule out chromosomal abnormalities. If the combined first trimester screening indicates a higher risk, further diagnostic tests like NIPT, chorionic villus sampling (CVS) or amniocentesis might be recommended.
Women should discuss with their healthcare providers about the available screenings, their benefits, risks, and limitations, and make an informed decision based on their personal circumstances and preferences.