Ovulation calculator for a 40-day cycle
Ovulation ~Wednesday, November 11 · fertile Fri, Nov 6 – Thu, Nov 12
Ovulation ~Monday, December 21 · fertile Wed, Dec 16 – Tue, Dec 22
Ovulation ~Saturday, January 30 · fertile Mon, Jan 25 – Sun, Jan 31
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With a 40-day cycle, you most likely ovulate around cycle day 26 (day 1 is the first day of your period). Your fertile window is roughly days 21 to 27, and your best chance of conceiving is from day 24 to day 26. Enter your last period above to turn that into dates.
When do you ovulate with a 40-day cycle?
Ovulation usually happens about 14 days before your next period, not 14 days after the last one. The part of the cycle after ovulation, the luteal phase, is fairly steady at 12 to 14 days for most people, while the part before it stretches or shrinks. So with a 40-day cycle you count back 14 days from day 41 (the first day of your next period) and land on day 26.
| Cycle day | What usually happens in a 40-day cycle |
|---|---|
| 1 to 5 | Your period (most people bleed 3 to 7 days) |
| 6 to 20 | Follicular phase: estrogen rises, energy and mood usually lift |
| 21 to 25 | Fertile window opens: sperm can survive up to 5 days |
| 26 | Estimated ovulation, the most fertile day along with the 2 days before |
| 27 to 40 | Luteal phase: progesterone rises; PMS often shows up in the last week |
Your fertile window with a 40-day cycle
An egg lives for about 12 to 24 hours after ovulation, but sperm can survive up to five days. That makes days 21 to 27 your fertile window, with the highest chances on days 24, 25 and 26. In the classic study of couples trying to conceive, sex on the two days before ovulation and on ovulation day itself carried the highest chance, about 27 to 33% per cycle.
With a 40-day cycle, start daily ovulation (LH) tests on cycle day 24, about 17 days before your next period is due, and keep going until you get a positive. Ovulation usually follows 24 to 36 hours later. See our ovulation test calculator.
Is a 40-day cycle normal?
A 40-day cycle is longer than the usual 21 to 35 days. It can be normal for you, especially in your teens or in perimenopause, but regularly longer cycles can be a sign of irregular ovulation (for example with PCOS or thyroid changes). It's worth tracking a few months and talking to a doctor.
Cycles also vary from month to month. If yours swing by several days, use the dates as a starting point and check signs of ovulation such as stretchy, clear discharge or a positive LH test. Our ovulation calculator for irregular periods works from your last few cycles instead of a single number.
40-day cycle compared with other cycle lengths
| Cycle length | Likely ovulation day | Fertile window |
|---|---|---|
| 36 days | day 22 | days 17 to 23 |
| 38 days | day 24 | days 19 to 25 |
| 40 days (yours) | day 26 | days 21 to 27 |
| 42 days | day 28 | days 23 to 29 |
| 44 days | day 30 | days 25 to 31 |
Calendar dates are estimates and are not a method of contraception. See a doctor if your cycles are often shorter than 21 or longer than 35 days, if periods stop for 3 months, or if you've been trying to conceive for 12 months (6 months if you're 35 or older).
Frequently asked questions
What day do I ovulate on a 40-day cycle?
Most likely around day 26, counting the first day of your period as day 1. It can move a day or two either way from cycle to cycle.
When is the best time to try to conceive with a 40-day cycle?
Have sex every 1 to 2 days from about day 21 to day 27, with the most fertile days being 24 to 26.
Can I get pregnant right after my period with a 40-day cycle?
It's less likely but possible: your fertile window usually opens around day 21, and sperm can survive up to five days, so an early ovulation or a long period could overlap.
When should I take a pregnancy test with a 40-day cycle?
From the day your period is due, day 41, for an accurate result. Use our pregnancy test calculator for the exact date.
Sources
- Wilcox AJ et al. Timing of sexual intercourse in relation to ovulation. NEJM (1995)
- Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine (2019)
- NHS: Periods and fertility in the menstrual cycle
- ACOG: Evaluating Infertility FAQ
This page is for information only and isn’t medical advice. If something worries you, please talk to a doctor.
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