Chorionic Villus Sampling (CVS)

Definition

Timing

• Performed between 10 and 13 weeks of gestation (ideally not before 10 weeks - earlier sampling is associated with increased risk of limb-reduction defects/oromandibular-limb hypogenesis).
• Key exam point: CVS gives results weeks earlier than amniocentesis (done at 15-16 weeks), permitting earlier reassurance or earlier, safer termination if abnormal - The Developing Human: Clinically Oriented Embryology, p. 317.

Indications

• Advanced maternal age / increased risk of aneuploidy
• Previous child with chromosomal abnormality
• Parental balanced translocation/chromosomal rearrangement
• Known single-gene disorder in family (needs DNA-based diagnosis) - e.g., thalassemia, hemophilia, muscular dystrophy, inborn errors of metabolism
• Abnormal first-trimester screening (NT, biochemical markers)
• Rapid karyotype needed for early decision-making

Technique

• Ultrasound first confirms fetal viability, size, and placental location; bladder moderately filled to help straighten an anteverted uterus (overfilling is avoided as it lengthens the sampling path).
• Patient in lithotomy position; vulva, vagina, and cervix prepped with povidone-iodine.
• A malleable catheter (distal 3-5 cm curved) is passed transcervically under continuous ultrasound guidance to the distal edge of the placenta, parallel to the chorionic membranes.
• Stylet removed, a 20 mL syringe with nutrient medium attached, negative pressure applied while withdrawing the catheter to aspirate villi.
• Uterine contractions can obstruct catheter passage - the procedure is delayed until the contraction resolves (Fig. 30.11 in Creasy & Resnik).

• A needle is inserted through the maternal abdominal and uterine walls under continuous ultrasound guidance into the chorionic frondosum, kept parallel to the chorionic plate.
• Sample obtained by back-and-forth needle movement while maintaining negative pressure, needle tip kept in view throughout - Creasy & Resnik, p. 681; The Developing Human, p. 317.
• Preferred when the placenta is posterior/fundal or transcervical access is difficult (e.g., cervical stenosis, active infection, myomas).

Advantages over amniocentesis

• Earlier diagnosis (first trimester vs second trimester)
• Larger tissue yield allows faster cytogenetic/molecular turnaround
• Avoids amniotic cavity puncture (no risk of early oligohydramnios-related clubfoot, which is linked to fluid leakage after early amniocentesis)

Limitations / Pitfalls (favorite exam points)

• Confined placental mosaicism (CPM): mosaic result on CVS that does not reflect the true fetal karyotype - requires confirmatory amniocentesis in ~1-2% of cases.
• Maternal cell contamination: can give false results if maternal decidual tissue is inadvertently sampled instead of pure villi.
• Cannot detect neural tube defects (no AFP/acetylcholinesterase assay possible, unlike amniocentesis) - maternal serum AFP or later ultrasound is still needed.
• Not useful for assessing fetal lung maturity or bilirubin/Rh isoimmunization studies.

Complications

• Pregnancy loss: comparable to amniocentesis when performed by experienced operators. The Canadian Collaborative trial showed 7.6% loss (CVS) vs 7.0% (amniocentesis) after adjusting for background first-trimester loss (RR 1.10, 95% CI 0.92-1.30). A meta-analysis by Salomon et al. (cited in Creasy & Resnik) confirms similar procedure-related loss rates between CVS and amniocentesis once background loss is accounted for.
• Repeated catheter passes significantly raise miscarriage risk (10.8% loss with ≥3 passes vs 2.9% with a single pass) - Creasy & Resnik's Maternal-Fetal Medicine, p. 681.
• Limb reduction defects/oromandibular hypogenesis if performed before 10 weeks.
• Vaginal spotting (more common with transcervical route), chorioamnionitis (rare), Rh sensitization (Rh-negative women need anti-D prophylaxis).

Diagnostic yield

• Fetal loss rate overall ~0.5-1%, comparable to amniocentesis.
• Can diagnose chromosomal abnormalities, inborn errors of metabolism, and X-linked disorders (Langman's Medical Embryology, The Developing Human, p. 317-318).