A 28-year-old primigravida presents at 12 weeks gestation for first-trimester ultrasound. The sonographer identifies a twin pregnancy with a single placenta, two fetuses, and the structure marked **A** — a single shared amniotic cavity with no dividing membrane visible between the twins. Which of the following best describes the timing of embryonic splitting that resulted in this pregnancy configuration?
A. Splitting occurred between days 8 and 13 post-fertilization, after chorion and amnion formation but before primitive streak formation
B. Splitting occurred at day 6 post-fertilization, resulting in monochorionic diamniotic twins
C. Splitting occurred after day 13 post-fertilization, resulting in conjoined twins
D. Splitting occurred before day 3 post-fertilization, resulting in dichorionic diamniotic twins
Explanation
Why option B is right
Monochorionic monoamniotic (MCMA) twins arise when embryonic splitting occurs between days 8 and 13 post-fertilization, after both the chorion and amnion have already formed but before primitive streak formation. This timing is the defining embryological feature of MCMA twinning. The absence of a dividing membrane (structure A) — the hallmark finding on ultrasound — directly reflects this late splitting within an already-formed amniotic cavity. ACOG Practice Bulletin 169 and SMFM Twin Pregnancy guidelines emphasize that early first-trimester ultrasound (11–14 weeks) is essential to identify chorionicity and amnionicity, as these determine all subsequent risk stratification and management.
Why each distractor is wrong
Option A (splitting before day 3): Splitting before day 3 post-fertilization results in dichorionic diamniotic twins, which have two separate placentas, two chorions, and two amniotic cavities. This does not match the single placenta and single amniotic cavity seen in this case.
Option C (splitting after day 13): Splitting after day 13 post-fertilization results in conjoined twins, not monoamniotic twins. The primitive streak has already formed, preventing complete separation of the embryos.
Option D (splitting at day 6): Splitting at day 6 post-fertilization results in monochorionic diamniotic twins, which have a single placenta but two separate amniotic cavities divided by a thin membrane. This does not match the absence of a dividing membrane seen in structure A.
High-YieldNEET PG
MCMA twins (1% of monozygotic twins, ~1:10,000 pregnancies) result from embryonic splitting between days 8–13; the absence of an intertwin dividing membrane on ultrasound is pathognomonic and mandates intensive surveillance and elective cesarean delivery at 32–34 weeks due to near-universal cord entanglement and 10–25% perinatal mortality.
ACOG Practice Bulletin 169; SMFM Twin Pregnancy Guidelines
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