You're more likely to become pregnant in any particular month if you have intercourse during your most fertile period - the days just before and after you ovulate. Ovulation usually occurs about 14 days before the start of your next menstrual period. If you would like to figure out when you're likely to ovulate next, you'll need to know the approximate length of your menstrual cycle. Your menstrual cycle begins with the first day your period starts (called "Day 1"). It ends the day before your next period starts. The average length of a menstrual cycle is 28 days. However, it is not at all unusual for women to have longer or shorter cycles. If you're not yet pregnant and you want to determine the length of your cycle, use a calendar to track your menstrual cycle. If you know the length of your menstrual cycle, then you're ready to use our convenient Fertility Calculator to find out your next expected ovulation date and most fertile period. Just fill out the form below, and click on "Calculate."

My menstrual cycle length:

The first day of my last menstrual period was: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 2003 2004 2005 2006 2007 2008 2009

Source:

http://www.nichd.nih.gov/