View source for WBR0333

{{WBRQuestion
|QuestionAuthor={{Rim}}
|ExamType=USMLE Step 1
|MainCategory=Physiology
|SubCategory=Reproductive
|MainCategory=Physiology
|SubCategory=Reproductive
|MainCategory=Physiology
|SubCategory=Reproductive
|MainCategory=Physiology
|MainCategory=Physiology
|SubCategory=Reproductive
|MainCategory=Physiology
|SubCategory=Reproductive
|MainCategory=Physiology
|SubCategory=Reproductive
|MainCategory=Physiology
|SubCategory=Reproductive
|MainCategory=Physiology
|MainCategory=Physiology
|SubCategory=Reproductive
|Prompt=A 28 year old pregnant woman 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 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?
|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. 
<br>0-4 days: Dichorionic diamniotic twin pregnancy
<br>4-8 days: Monochorionic diamniotic twin pregnancy
<br>8-12 days: Monochorionic monoamniotic twin pregnancy
<br> > 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.
|AnswerA=Dichorionic diamniotic twin pregnancy
|AnswerAExp=Dichorionic diamniotic twin pregnancy occurs when cleavage takes place between 0-4 days post-fertilization.
|AnswerB=Monochorionic diamniotic twin pregnancy
|AnswerBExp=Monochorionic diamniotic twin pregnancy occurs when cleavage takes place between 4-8 days post-fertilization.
|AnswerC=Dichorionic monoamniotic twin pregnancy
|AnswerCExp=Dichorionic monoamniotic twin pregnancy is not possible.
|AnswerD=Monochorionic monoamniotic twin pregnancy
|AnswerDExp=Monochorionic monoamniotic twin pregnancy occurs when cleavage takes place between 8-12 days post-fertilization
|AnswerE=Monochorionic monoamniotic conjoined twin pregnancy
|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.
|RightAnswer=B
|WBRKeyword=monochorionic, dichorionic, monoamniotic, diamniotic, twin, pregnancy, conjoined, twins, twin, fertilization, cleave, cleavage, fertilized, egg, 4, 8, 12, 13, days, post-fertilization, post, zygote, twinning, monozygote, monozygotic
|Approved=No
}}