Question

Implantation occurs on which menstrual cycle day?

• A. 5-7 days
• B. 20-22 days
• C. 14-18 days
• D. 26-28 days

Hint:

Ovulation is day 14; add 6-7 days of pre-implantation development.

Answer 1

The Correct Option is B

Step 1: Separate the two timelines that students often confuse: time measured from fertilization versus time measured from the first day of the period. The options here are menstrual cycle days, so we count from the start of menstruation.

Step 2: Establish ovulation. For the textbook 28-day cycle, the luteal phase is fixed at 14 days, so ovulation falls on cycle day 14. The egg is fertilised in the ampulla shortly after, within a day.

Step 3: Add the pre-implantation development. The conceptus travels and develops for about a week, and the blastocyst attaches to the endometrium roughly 6 to 7 days after fertilisation. Day 14 plus about 7 days equals about day 21.

Step 4: That places implantation in the 20 to 22 day window of the cycle. The 5-7 day choice is a trap because it is the interval after fertilisation, not the cycle day.

\[\boxed{\text{20-22 days}}\]