WBR0513

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian and Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Reproductive
Prompt	[[Prompt::A couple has been trying to conceive a child for 5 years with no success. They decide to explore in-vitro fertilization (IVF) and ask their primary care physician for advice. The physician describes the process explaining that after fertilization the embyro is directly introduced into the uterus. Assuming fertilization occurs at day 0, at which day post-fertilization does an embryo undergoes implantation in the uterus?]]
Answer A	AnswerA::Day 2
Answer A Explanation	AnswerAExp::On day 2, the zygote is still within the fallopian tubes and has not descended to the level of the endometrium.
Answer B	AnswerB::Day 4
Answer B Explanation	AnswerBExp::On day 4, the morula is still within the fallopian tubes and has not descended to the level of the endometrium.
Answer C	AnswerC::Day 6
Answer C Explanation	AnswerCExp::On day 6, the blastocyst is in the endometrium and implantation starts.
Answer D	AnswerD::Day 10
Answer D Explanation	AnswerDExp::On day 10, implantation has occured and hCG is secreted by the syncytiotrophoblast.
Answer E	AnswerE::Day 12
Answer E Explanation	AnswerEExp::On day 12, implantation has occured and hCG is secreted by the syncytiotrophoblast.
Right Answer	RightAnswer::C
Explanation	[[Explanation::During a normal pregnancy, fertilization is considered to occur at day 0. The fertilization process typically occurs in the ampullary-ischmic junction of the fallopian tubes. After fertilization, the oocyte proceeds with meiosis II, which was previously arrested following ovulation. The fertilized egg continues to descend along the fallopian tube as mitotic division of the embryo proceeds and gives rise to the morula on day 3 and the blastocyst on day 5. On day 6, the blastocyst reaches the endometrium, and implantation starts. Soon after implantation takes place, hCG is secreted and pregnancy then becomes detectable by serum and urinary concentrations of hCG within 10-14 days post-fertilization. Educational Objective: Blastocyst implantation occurs on day 6 post-fertilization. References: Cunningham F, Leveno K, Bloom S et al. Williams Obstetrics 24/E. Chapter 5: Implantation and Placental Development. McGraw-Hill Professional; 2014. First Aid 2014 page 552]]
Approved	Approved::Yes
Keyword	WBRKeyword::Implantation, WBRKeyword::Blastocyst, WBRKeyword::Morula, WBRKeyword::hCG, WBRKeyword::Zygote, WBRKeyword::Fertilization, WBRKeyword::Ampulla, WBRKeyword::Oocyte, WBRKeyword::Sperm, WBRKeyword::IVF
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::