Fertile window for conceiving a pregnancy

Your "fertile window" is your day of ovulation and the 5 days before it. Because a human egg typically lives for only 12 to 24 hours after ovulation, you are unlikely to become pregnant by having sexual intercourse the day after you ovulate.1

Traditional medical guidelines for calculating ovulation are based on the notion that ovulation is 14 days before the next menstrual period starts, or day 15 of a 28-day cycle. However, recent research suggests that only about 10% of women ovulate on this day. The date of ovulation varies widely from woman to woman, ranging in this study from 7 to 19 days before the beginning of the menstrual period.2 Women with irregular cycles have an even greater range of possible ovulation days.

You can most accurately predict your 6-day fertile window by monitoring your cervical mucus, your basal body temperature, and your luteinizing hormone (LH) changes.1

  • During the 5 to 6 days before and the day of ovulation, the cervix produces a type of mucus that is stretchy, slippery, thin, and clear. This is the type of fertile mucus that helps sperm easily travel into your reproductive tract. You are likely to notice this type of mucus when using toilet paper after urinating.1 If not, place your fingertip just inside your vagina, then check the consistency and appearance of the mucus.
  • About 24 hours before you ovulate, your luteinizing hormone (LH) level rises. You can confirm that ovulation is approaching by testing your urine for high LH with a home ovulation predictor kit. Note, however, that by the time your LH rises, most of your 6-day fertile window has passed. (Also note that an LH surge can happen as soon as 16 hours or as long as 48 hours before ovulation.)1 It's best to have sex as soon as possible after learning that your LH level is up.
  • Just before an egg is released (ovulation), your basal body temperature (BBT) decreases slightly; it then increases sharply 24 hours after ovulation. After 2 to 3 months of carefully measuring and recording your BBT every morning before getting out of bed, you may be able to identify a pattern that helps estimate when you are ovulating.

If you have menstrual cycles that are unusually long (more than 42 days) or short (less than 21 days), monitoring your cervical mucus is your best bet for identifying your fertile window; an ovarian monitor LH test won't work well for you.

When you have identified your 6-day fertile window, abstain from sex for 5 days before it starts. (Not ejaculating for a few days helps build up sperm count.) Then have sexual intercourse each day of your fertile window, including ovulation day. If your partner has a low sperm count, have sex every other day, since frequent ejaculation does temporarily lower sperm count.


  1. Stanford JB, et al. (2002). Timing intercourse to achieve pregnancy: Current evidence. Obstetrics and Gynecology, 100(6): 1333–1341.
  2. Wilcox AJ, et al. (2000). The timing of the "fertile window" in the menstrual cycle: Day-specific estimates from a prospective study. BMJ, 321(7271): 1259–1262.

Last Updated: March 21, 2008

Author: Bets Davis, MFA & Sandy Jocoy, RN

Medical Review: Sarah Marshall, MD - Family Medicine & Kirtly Jones, MD - Obstetrics and Gynecology

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