This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
Radiation exposure during which period of Pregnancy is highly lethal 
Select and explain. 
A. During fetal cardiac development, B. Before implantation, C. During closure of proximal neural tube, D. None of the above
B: Before implantation
Answer: B: Before implantation 
Explanation: Lethality risk is highest during the pre-implantation period - up to 10 days post conception. This is likely due to blastomere destruction caused by chromosomal damage.