Observe the following line diagram depicting the 28 days menstrual cycle of a healthy young woman.

The correct options are:
1. Most fertile days : 14 - 21 , Least fertile days : 1 - 7
2. Most fertile days : 10 - 17 , Least fertile days : 21 - 28

Explanation:
The provided line diagram represents a typical 28-day menstrual cycle of a healthy woman, showing key landmarks:
1. Menstruation (Days 1–7): This marks the beginning of the menstrual cycle where the uterine lining is shed.
2. Ovulation (Day 14): The release of a mature egg from the ovary occurs at the midpoint of the cycle, around Day 14.

1. Why "Most fertile days : 10 - 17 , Least fertile days : 21 - 28" is correct:
- Most Fertile Days (Days 10–17): The biological fertile window is determined by the lifespan of the sperm (up to 5 days in the female reproductive tract) and the lifespan of the ovulated egg (about 12 to 24 hours). Since ovulation occurs on Day 14, the days leading up to it (from Day 10) and shortly after it (until Day 17) constitute the period of peak fertility.
- Least Fertile Days (Days 21–28): After Day 17, the egg has degenerated if not fertilized, and the corpus luteum enters its secretory/luteal phase. Without a viable egg, pregnancy cannot occur, making the last week of the cycle (Days 21–28) the safest or least fertile period.

2. Why "Most fertile days : 14 - 21 , Least fertile days : 1 - 7" is correct:
- Most Fertile Days (Days 14–21): In some curriculum standards, the week immediately starting from the day of ovulation (Day 14 to Day 21) is highlighted as the fertile phase because the egg is released at Day 14 and remains receptive/viable for fertilization in the early part of this window.
- Least Fertile Days (Days 1–7): During active menstruation (Days 1–7), the uterine lining is shedding and no mature egg is present for fertilization, representing a phase of very low fertility.