Taken from American Pregnancy Association:
Tracking Ovulation:
A woman's monthly cycle is measured from the first day of her menstrual period until the first day of her next period. On average, a woman's cycle normally is between 28-32 days, but some women may have much shorter cycles or much longer ones. Ovulation can be calculated by starting with the day the last menstrual period (LMP) starts or by calculating 12-16 days from the next expected period. Most women ovulate anywhere between Day 11 - Day 21 of their cycle, counting from the first day of the LMP. This is what many refer to as the \"fertile time\" of a woman's cycle, because sexual intercourse during this time increases the chance of pregnancy. Ovulation can occur at various times during a cycle, and may occur on a different day each month.
The Ovulation Cycle Divided into Two Parts:
The first part of the ovulation cycle is called the follicular phase. This phase starts the first day of the last menstrual period (LMP) and continues until ovulation. This first half of the cycle can differ greatly for each woman lasting anywhere from 7 days until 40 days. The second half of the cycle is called the luteal phase and is from the day of ovulation until the next period begins. The luteal phase has a more precise timeline and usually is only 12-16 days from the day of ovulation. This ultimately means that the day of ovulation will determine how long your cycle is. This also means that outside factors like stress, illness, and disruption of normal routine can throw off your ovulation which then results in changing the time your period will come. So the old thought that stress can affect your period is only partly true. Stress can affect your ovulation which ultimately determines when your period will come, but stress around the time of an expected period will not make it late—it was already determined when it would come 12-16 days earlier!
