WBR0333

Author	[[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Alison Leibowitz)]]
Exam Type	USMLE Step 1
Main Category	Physiology
Sub Category	Reproductive
Prompt	A pregnant 28-year-old female presents to her gynecologist's office for a routine pregnancy visit. In the office, the gynecologist performs a pregnancy ultrasound. He informs the patient that she has twins. Then he explains to the patient that the cleavage of the fertilized egg likely occurred between day 4 and day 8 post-fertilization. Which of the following ultrasound findings is most likely present in this patient?
Answer A	Dichorionic diamniotic twin pregnancy
Answer A Explanation	Dichorionic diamniotic twin pregnancy occurs when cleavage takes place between 0-4 days post-fertilization.
Answer B	Monochorionic diamniotic twin pregnancy
Answer B Explanation	Monochorionic diamniotic twin pregnancy occurs when cleavage takes place between 4-8 days post-fertilization.
Answer C	Dichorionic monoamniotic twin pregnancy
Answer C Explanation	Dichorionic monoamniotic twin pregnancy is not possible.
Answer D	Monochorionic monoamniotic twin pregnancy
Answer D Explanation	Monochorionic monoamniotic twin pregnancy occurs when cleavage takes place between 8-12 days post-fertilization
Answer E	Monochorionic monoamniotic conjoined twin pregnancy
Answer E Explanation	Monochorionic monoamniotic conjoined twin pregnancy occurs when cleavage takes place > 13 days post-fertilization.
Right Answer	B
Explanation	While dizygotic twins always form from 2 separately fertilized eggs, monozygotic twins form from 1 fertilized egg that later splits into 2 zygotes during early pregnancy. The time post-fertilization by which the fertilized egg cleaves is important for determining the number of chorions and amnions. 0-4 days: Dichorionic diamniotic twin pregnancy 4-8 days: Monochorionic diamniotic twin pregnancy 8-12 days: Monochorionic monoamniotic twin pregnancy > 13 days: Monochorionic monoamniotic conjoined twin pregnancy Educational Objective: Monochorionic diamniotic twin pregnancy occurs when the fertilized egg cleaves at 4-8 days post-fertilization. References: First Aid 2014 page 555
Approved	Yes
Keyword	monochorionic, dichorionic, monoamniotic, diamniotic, pregnancy, conjoined twins, fertilization, cleavage, fertilized, post-fertilization, post, zygote, monozygote, monozygotic
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