Diagnostic Procedures in Multiples

Description, Technique, and Equipment

CVS and amniocentesis should be performed by a physician experienced with the procedures and ultrasound (US). Similar to singleton pregnancies, unsensitized Rh(D)-negative women should be given Rh ₀ (D) immune globulin after CVS and amniocentesis to prevent Rh sensitization.

When performing invasive prenatal karyotype analysis on monochorionic twin gestations, the question remains whether one or both fetuses should be sampled. Although monochorionic twins theoretically should have identical karyotypes, case reports exist showing monochorionic twins discordant for chromosomal abnormalities. Although this is a rare event, because of this reported risk of discordant karyotypes, we recommend sampling both sacs if the patient is undergoing invasive prenatal genetic testing.

Chorionic Villus Sampling in Multiple Gestations

Before a CVS procedure, a transvaginal US scan should be done to evaluate the number of embryos, chorionicity, fetal viability, and any early US evidence of fetal anomalies. CVS in multiple gestations is performed in the same way as for singletons (see Chapter 113 ). The procedure can be done using the transabdominal or the transcervical method, or both. If there are two separate placentas, both methods may be used to avoid twin-twin contamination. If the placentas are fused or the chorionicity is unclear, the transcervical catheter or the transabdominal spinal needle should be directed to the placental margin farthest from the fusion site or the area closest to the umbilical cord insertion site. This approach helps avoid sampling the same fetus twice. To eliminate contamination of the samples, the aspiration device should not pass through one placenta to reach the second placenta. Although deoxyribonucleic acid (DNA) polymorphism and cytogenetic results can assist in ensuring accuracy of the results, an amniocentesis may be needed later in the pregnancy if the CVS results are unclear.