Fortunately, learning about the menstrual cycle is a lot easier than studying the Krebs cycle, and considerably more interesting than the carbon cycle. Here’s how it works: In a typical 28 day cycle, Day 1 represents the first day that menstrual bleeding begins. Between Day 7 and Day 11, the lining of the uterus thickens and becomes awash in hormones like estrogen. On Day 14, the egg is released, signaling the start of the second half of the cycle, also known as ovulation. If the egg comes into contact with a single male sperm, then the process that leads to pregnancy begins. If not, the egg disintegrates and the lining of the uterus sloughs off to become the next menstrual flow. A woman is most likely to become pregnant around two weeks before the expected date of her next period. Since sperm can last up to five days inside a woman’s body and an unfertilized egg can last only 24 hours, there is about a six day window when a woman is at her most fertile.