WBR0333

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Reproductive
Prompt	[[Prompt::A 28 year old pregnant woman presents to her gynecologist's office for routine pregnancy visit. In the office, the gynecologist performs a pregnancy ultrasound. He informs the patient that she has twins. The physician then explains to the patient that most probably the cleavage of the fertilized egg occurred between day 4 and day 8 post-fertilization. Which of the following findings on ultrasound is most likely present in this patient?]]
Answer A	AnswerA::Dichorionic diamniotic twin pregnancy
Answer A Explanation	AnswerAExp::Dichorionic diamniotic twin pregnancy occurs when cleavage takes place between 0-4 days post-fertilization.
Answer B	AnswerB::Monochorionic diamniotic twin pregnancy
Answer B Explanation	AnswerBExp::Monochorionic diamniotic twin pregnancy occurs when cleavage takes place between 4-8 days post-fertilization.
Answer C	AnswerC::Dichorionic monoamniotic twin pregnancy
Answer C Explanation	AnswerCExp::Dichorionic monoamniotic twin pregnancy is not possible.
Answer D	AnswerD::Monochorionic monoamniotic twin pregnancy
Answer D Explanation	AnswerDExp::Monochorionic monoamniotic twin pregnancy occurs when cleavage takes place between 8-12 days post-fertilization
Answer E	AnswerE::Monochorionic monoamniotic conjoined twin pregnancy
Answer E Explanation	[[AnswerEExp::Monochorionic monoamniotic conjoined twin pregnancy occurs when cleavage takes place > 13 days post-fertilization. If no cleavage occurs whatsoever, no twinning will take place.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::While dizygotic twins always form from 2 separately fertilized eggs, monozygotic twins form from 1 fertilized egg that later splits into 2 zygotes very early during pregnancy. The time post-fertilization by which the fertilized egg cleaves is very 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. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::monochorionic, WBRKeyword::dichorionic, WBRKeyword::monoamniotic, WBRKeyword::diamniotic, WBRKeyword::twin, WBRKeyword::pregnancy, WBRKeyword::conjoined, WBRKeyword::twins, WBRKeyword::twin, WBRKeyword::fertilization, WBRKeyword::cleave, WBRKeyword::cleavage, WBRKeyword::fertilized, WBRKeyword::egg, WBRKeyword::4, WBRKeyword::8, WBRKeyword::12, WBRKeyword::13, WBRKeyword::days, WBRKeyword::post-fertilization, WBRKeyword::post, WBRKeyword::zygote, WBRKeyword::twinning, WBRKeyword::monozygote, WBRKeyword::monozygotic
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::