Expert: John Locke - 11/10/2009

Question
my period started today so which day will i ovulate?

Answer
Thanks for using AllExperts. The menstrual cycle lasts around 28 days in most cases, with a variation of about one week. Within that time period, there are a number of distinct time points at which things occur. The time course that I'll describe assumes a time course of 28 days; I will mention the effect of longer or shorter cycles at the end.

The process begins with menstruation, which lasts up to about day 5. After menstruation is complete, the lining of the uterus begins to regrow in anticipation of potentially receiving a fertilized ovum. Ovulation occurs at day 14. The uterine lining continues to grow until day 26. If the ovum is not fertilized at that point, the uterine lining sloughs off and the cycle begins again.

The menstrual cycle is divided into two parts: follicular phase (before ovulation) and luteal phase (after ovulation). The luteal phase is always 14 days in length due to the limited lifetime of the ovum after it has been ovulated. The follicular phase can vary in length, and variations in the length of the follicular phase affect the overall length of the menstrual cycle. The most fertile period, in any case, occurs from 5 days before to 2 days after ovulation. Assuming a 28 day cycle, this would correspond to days 9 to 16.

As I mentioned at the start, the menstrual cycle is often assumed to be 28 days, but it can vary from 21 to 35 days, with all the variation occurring in the follicular phase. This complicates the issue of knowing when ovulation occurs, but there is a solution. If you know the length of time between menstruations, you will be able to determine the length of the cycle. Since ovulation always occurs 14 days before menstruation, count forward from the onset of your menstrual period until you reach a point 14 days from when your next period would begin; that would be the most likely date of ovulation.