Very often the first indication a woman has that she is pregnant is a missed menstrual period. In which week of embry- onic development will a woman experience her first missed menstrual period?

A. Given a regular 28-day menstrual cycle, a woman who starts menses on, say, February 1 will ovulate on February 14, and the secondary oocyte will be fertilized, if she becomes pregnant, within 24 hours. So, the zygote undergoes week 1 of development from February 15 to 21. Week 2 of development is from February 22 to 28. On the next day, March 1, the woman would enter her next menstrual cycle if she were not pregnant, but because she is pregnant, she does not menstruate. Therefore, this first missed menstrual period corresponds with the start of week 3 of embryonic development. The embryonic period (week 3–week 8) is a time of high susceptibility to teratogens.