Ralph Waldo Emerson (1803-1882), perhaps best known for his essay Self-Reliance (1841), was one of America's most influential thinkers and writers. After graduating from Harvard Divinity School, he followed nine generations of his family into the ministry but practiced for only a few years.
The first menstrual period is called  menarche .. The menstrual cycle is about four weeks long, starting on the first day of bleeding and ending when the next period begins. However, it can vary greatly when a girl first starts her period. It may skip months or come several times per month in the beginning.
Add up the number of days, divide by 4 months, and you get an average of 30 days. With that number, you can predict the first day of your next period when adding 30 days to the last. So, if my last period started on February 2, I add 30 days and come up with March 4 for the first day of my next period.