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
In transvaginal ultrasound, earliest detection of gestation sac is by : 
Select and explain. 
A. 21 days after ovulation, B. 21 days after implantation, C. 28 days post ovulation, D. 14 days after ovulation
D: 14 days after ovulation
Answer: D: 14 days after ovulation 
Explanation: Ans. is d i.e. 14 days after ovulation The first definitive sonographic finding to suggest early pregnancy is visualization of the gestational sac. Using transvaginal transducers with frequency of 5 MHz, the size threshold for sac detection is 2 - 3 mm, corresponding to 4 weeks + 1 day gestational age to 4 weeks + 3 days gestational age." To understand how many days after ovulation, you should first understand that gestational age is calculated from 1st day of last menstrual period. As is evident from above diagram gestational sac is visualised approximately 15-17 days after ovulation (or within 1-3 days of missed period) but since it is not given in options so the nearest possible answer is 14 days after ovulation.