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Question 7

A 32-year old nulligravid with unexplained infertility undergoes IVF with a single blast transfer. Her HCG levels rise appropriately. Seven-week transvaginal ultrasound reveals 1 chorion with 1 amnion, 1 yolk sac and 2 separate fetal poles, both with appropriate fetal heart motion. The most likely diagnosis is:

Answer Choices:

Dichorionic/diamniotic twins

Monochorionic/diamniotic twins

Monochorionic/monoamniotic twins

Conjoined twins

Correct Answer:

Monochorionic/monoamniotic twins


  • Monozygotic twinning after single embryo transfer is rare but can occur

  • Monoamniotic twins occur when an embryo splits about 9-13 days after fertilization

  • A single chorion and single amnion are diagnostic of mono/mono twins

  • The number of yolk sacs often corresponds to the number of amnions, but not always

  • Monochorionic/diamniotic twins will have to amnions and a “T-sign”