Invasive Prenatal Diagnostics

A screening test says "high risk" — now what? The next step is an invasive diagnostic procedure that gives a definitive answer, but it carries real risks the nurse must explain.

Invasive prenatal diagnostics — amniocentesis and chorionic villus sampling (CVS) — provide definitive genetic and chromosomal diagnoses, unlike screening tests that only estimate risk. CVS is performed at 10–13 weeks by aspirating placental chorionic villi; it gives early results but cannot detect neural tube defects. Amniocentesis is typically done at 15–20 weeks by inserting a needle through the abdominal wall into the amniotic sac under ultrasound guidance; it detects chromosomal abnormalities, neural tube defects (via alpha-fetoprotein in the fluid), and fetal lung maturity in late pregnancy (lecithin-to-sphingomyelin ratio ≥ 2:1 indicates maturity). Before either procedure, the nurse verifies informed consent, obtains a baseline fetal heart rate, confirms Rh status (Rh-negative mothers receive RhoGAM after the procedure to prevent isoimmunization), and ensures the client has a full bladder for CVS or an empty bladder for amniocentesis. Post-procedure monitoring includes assessing for vaginal bleeding, fluid leakage, cramping, fever, and decreased fetal movement. The client is instructed to rest and avoid strenuous activity for 24 hours. Both procedures carry a small risk of miscarriage — approximately 0.1–0.3% for amniocentesis and slightly higher for CVS.

Key Distinctions

Don't confuse CVS with amniocentesis timing: CVS is first trimester (10–13 weeks), amniocentesis is second trimester (15–20 weeks). Students mix up bladder prep — full bladder for CVS (tilts uterus for access), empty bladder for amniocentesis (prevents puncture). Screening tests estimate probability; invasive diagnostics give a definitive yes-or-no chromosomal diagnosis. A positive screening doesn't equal a diagnosis.

Clinical Pearl

Full for CVS, empty for amnio — match the bladder to the procedure. And always check Rh status: if negative, RhoGAM goes in before the client leaves.