During the first 14 days of a woman's fertility cycle, follicles grow, one is selected, and surrounding cells produce estrogen.

“So, a group of eggs or follicles are starting to grow. One gets selected. We don't really understand. We don't understand why. It's random. Wow. Um, and that follicle that's egg is in a follicle. Follicles is kind of um fluid fil like structure that surrounds each egg. So that continues to grow and then eventually uh what's happening hormonally? So those cells surrounding the eggs are producing estrogen. That estrogen is preparing acting on the uterine lining to build up the lining to potentially support a pregnancy.”