What is the fertile window?
The fertile window is the six days in each cycle when sex can lead to pregnancy: the five days before ovulation plus ovulation day itself. It exists because sperm can survive up to five days, while an egg lives about a day. The chance of conception is highest in the two or three days up to and including ovulation.
Why the fertile window is six days
In a classic study of 221 women trying to conceive, who measured ovulation with daily urine hormones, every pregnancy came from sex during a six-day span ending on ovulation day. The chance was about 10% for sex five days before ovulation, rising to about 33% on ovulation day itself. No pregnancies came from sex the day after ovulation.
| Day relative to ovulation | Chance of conception (that study) |
|---|---|
| 5 days before | About 10% |
| 2 to 1 days before | Among the highest |
| Ovulation day | About 33% |
| The day after | Close to zero |
How to estimate your fertile window
- Find your likely ovulation day: about 14 days before your next period.
- Count back five days. Those six days are your estimated window.
- Watch for egg-white discharge: clear, slippery and stretchy. It shows up in the most fertile days.
- Use LH tests: a positive means ovulation is likely within a day or so.
- Use temperature to confirm afterwards: a small rise means ovulation has passed.
Our fertile window calculator does step 1 and 2 for you.
Why it moves from cycle to cycle
Calendar estimates assume ovulation happens on the same day each time. It doesn't. In the same research group, only about 30% of women had their whole fertile window inside days 10 to 17, and on every day from day 6 to day 21 at least 10% of women were in their window. Stress, illness, travel and conditions like PCOS make it even less predictable.
Because the window moves, a calendar can't tell you which days are safe. If you don't want to get pregnant, use contraception. Lulu shows fertile window estimates to help you understand your cycle, never as birth control. See our safe days explainer.
How the fertile window feels
Estrogen peaks during the fertile window. Many people notice more energy, a better mood, more confidence and a higher sex drive. It's the most outgoing stretch of the cycle for a lot of people, and the one right before the luteal phase, when PMS can start.
When to talk to a doctor
If you've been trying to conceive for 12 months (or 6 months if you're 35 or older), or if your cycles are very irregular or longer than 35 days, talk to a doctor. Irregular ovulation is common and often treatable.
Frequently asked questions
How many days are you fertile each month?
About six days: the five days before ovulation and ovulation day. See how many days are you fertile.
Can you get pregnant outside the fertile window?
Only if your window was somewhere other than you thought. Because ovulation moves, the real window can be earlier or later than a calendar says.
What are the most fertile days?
The two days before ovulation and ovulation day itself have the highest chances.
Do period apps know my fertile window?
They estimate it from past cycles and any signs you log. They can't see ovulation directly, so treat it as an estimate. See how accurate are ovulation calculators.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. NEJM (1995)
- Wilcox AJ, Dunson D, Baird DD. The timing of the fertile window in the menstrual cycle. BMJ (2000)
- NHS: Fertility in the menstrual cycle
- ACOG: Fertility Awareness-Based Methods of Family Planning
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
Fertile window calculator: when am I most fertile?Free fertile window calculator: see the days each cycle when pregnancy is most likely, for your next 4 cycles. Private, estimates only, not contraception.
When do you ovulate?When do you ovulate? Usually 12 to 14 days before your next period, not always on day 14. How to estimate it and the body signs that confirm it.
What are the signs of ovulation?Signs of ovulation: egg-white discharge, a positive LH test, a temperature rise, one-sided twinges and higher sex drive. What each means and how to read them.Can you get pregnant on your period?Can you get pregnant on your period? It's possible but less likely. How sperm survival and early ovulation make it happen, with the real numbers.
Ovulation phase: signs, timing and feelingsThe ovulation phase explained: when you ovulate, how long it lasts, the signs and symptoms to look for, and why mood, energy and libido often peak.