Can you get pregnant on your period? Yes, but it is unlikely. Ovulation doesn't usually happen during a period. The catch is that sperm can wait for it, for up to about 5 days. The NHS says: "It's possible (although not very likely) to get pregnant soon after your period finishes if you ovulate early or have a short menstrual cycle."
So if your cycle is short, your period is long, or you ovulate early, sex late in your period can land inside your fertile days. A period is not contraception.
What did I get wrong about my own cycle?
I didn't know my own ovulation window. I knew ovulation happened somewhere in the middle of a cycle, and that was about it. When I started trying, I didn't realise the tiny ovulation window and wasn't aware exactly where that fell.
I'd assumed the fertile window was basically one day, ovulation day itself. I'd been treating my cycle like one number on a calendar. My signs weren't identical every single month. A stressful few weeks, a period of training harder than usual, even travel, and the pattern I'd normally recognise showed up later, fainter, or not quite as clearly some cycles.
I wasn't aware of the actual window at all, or where it fell.
What are the chances of getting pregnant on your period?
The chance is low on the first days of a cycle and rises as the days go on. The best day-by-day figures we could find come from a 2015 re-analysis of two North Carolina studies, which modelled the chance of clinical pregnancy from one act of unprotected sex.
| Cycle day | Chance from one act of sex |
|---|---|
| Day 1 | 0.1% |
| Day 2 | 0.1% |
| Day 3 | 0.2% |
| Day 4 | 0.5% |
| Day 5 | 0.9% |
| Day 6 | 1.1% |
| Day 7 | 2.1% |
| Day 13 (the peak) | 9.7% |
Source: Li, Wilcox and Dunson, Contraception, 2015, Table 3, all women. Modelled estimates from US women with no known fertility problems.
You may see 33% quoted for ovulation day, including in our guide to the best time to get pregnant. That figure counts from a measured ovulation. These figures count by cycle day across all women, whenever they ovulated, so the peak is lower.
That's about 1 in 1,000 on day 1 and 1 in 50 on day 7. These are averages across all cycle lengths, not your personal odds. They count cycle days, not bleeding days, so day 7 only applies to your period if you're still bleeding then.
What 5 things decide whether you can get pregnant on your period?
1. How long is your cycle?
The shorter your cycle, the closer ovulation sits to your period. The NHS says regular cycles from 21 to 35 days are normal, and ovulation is usually about 14 days before your next period, though this depends on your cycle length. On a 21-day cycle, that sum gives day 7.
2. How long does your period last?
A longer bleed closes the gap from the other side. Periods last 2 to 7 days, usually about 5. On a 24-day cycle, a 5-day period only just touches the fertile days, while a 7-day period overlaps them on days 5, 6 and 7.
3. How long can sperm survive?
Sperm can survive for up to about 5 days in the body, though the chance of pregnancy is highest close to ovulation. In a 1995 study of 221 women trying to conceive, conception only followed sex in the 6 days ending on ovulation, so up to 5 days before it, and only 6% of pregnancies could be firmly tied to sperm 3 or more days old.
4. When do you really ovulate?
Ovulation moves around, even in regular cycles. In a study of 213 women, an estimated 2% were in their fertile window by day 4 and 17% by day 7, and only about 30% had the whole window between days 10 and 17. The authors put it plainly: "Women should be advised that the timing of their fertile window can be highly unpredictable, even if their cycles are usually regular."
More on the real signs of ovulation.
5. Is the bleeding really a period?
Not every bleed is a period. The NHS lists implantation bleeding as a possible early sign of pregnancy: a bleed similar to a very light period, around the time your period is due. One small study questioned the cause, but 14 of its 151 women with clinical pregnancies still bled in the first 8 weeks. More on implantation bleeding and ovulation bleeding. If a bleed is unusually light or you have any doubt, take a pregnancy test. The NHS says tests are most reliable from the first day of a missed period, or at least 21 days after unprotected sex if you don't know when your period is due.
How does cycle length change your chances of getting pregnant on your period?
Short cycles are where a period and fertile days can overlap. On a 28-day or 35-day cycle, these sums show little or no overlap.
| Cycle length | Likely ovulation | Fertile days | Overlap with a 5-day period | Overlap with a 7-day period |
|---|---|---|---|---|
| 21 days | Day 7 | Days 2 to 7 | Days 2 to 5 | Days 2 to 7 |
| 24 days | Day 10 | Days 5 to 10 | Day 5 | Days 5 to 7 |
| 28 days | Day 14 | Days 9 to 14 | None | None |
| 35 days | Day 21 | Days 16 to 21 | None | None |
Worked out by us from NHS figures (ovulation about 14 days before the next period, periods of 5 and 7 days) and the 6-day window in Wilcox et al, NEJM, 1995. This is arithmetic, not measured data.
Treat this as a guide. In an app study of 612,613 ovulatory cycles, funded by the app's maker, cycles of 21 to 24 days had an estimated ovulation day of about 12.4 on average, so the 14-day sum probably overstates overlap in short cycles.
Dr Natalie Crawford, a US reproductive endocrinologist, works through this same cycle-length sum from 3:40 in this video, and says that with longer bleeding and shorter cycles you "very well could ovulate while you're still bleeding".
Myth or fact: what do people get wrong?
Most of what gets repeated is wrong or overstated. Tap each card.
“You can't get pregnant on your period.” Myth or fact? Tap to see.
False. The chance is low but not zero: about 0.1% on cycle day 1, rising to 2.1% on day 7 for one act of sex, in modelled figures.
“Your period is a safe time to skip contraception.” Myth or fact? Tap to see.
False. We couldn't find any published figure that supports period-only sex as a method. Even fertility awareness is only 76% effective if the instructions aren't followed exactly, and it doesn't protect against STIs.
“Period blood kills sperm.” Myth or fact? Tap to see.
Not proven, so treat it as false. Our PubMed searches found no human study showing this. Sperm can survive for up to about 5 days in the body, and motile sperm have been recovered from cervical mucus after 5 days.
“If you're bleeding, you can't be pregnant.” Myth or fact? Tap to see.
False. 9% of women with clinical pregnancies in one study recorded bleeding in the first 8 weeks, and the NHS describes a very light bleed around the time a period is due.
Dr Kate Shaw, an ob/gyn at Stanford, covers the first myth from 3:00 in this 2022 Stanford Center for Health Education video, saying it is still possible, "especially true for those with shorter menstrual cycles or who have more bleeding days".
What does this look like in real life?
These four situations show the sums on a calendar. They are arithmetic, not a personal risk figure.
Copper is bleeding, green is the usual fertile days worked out from the sums above, a split square is both, and a dark outline marks the day of sex.
21-day cycle, 7-day period, sex on day 6
With ovulation 14 days before the next period, 21 minus 14 gives day 7, so day 6 is one day before and inside the fertile window. At the app study's average for 21 to 24 day cycles (about day 12.4), day 6 is about 6 days before ovulation, just outside the window, which shows why you can't rely on the sums alone.
What it means: a real possibility. Use contraception if you don't want a pregnancy, and speak to a pharmacist or clinic about emergency contraception within 3 to 5 days.
28-day cycle, 5-day period, sex on day 3
28 minus 14 gives day 14, which is 11 days away. If you ovulate 16 days before your next period, the top of the NHS range, 28 minus 16 gives day 12, 9 days away. Both are beyond the 5 days sperm can survive.
What it means: this is the low-risk end, but not zero. The modelled figure for day 3 in women with regular cycles is 0.3% (0.2% across all women, as in the table above), because real cycles vary.
28-day cycle, 5-day period, light bleeding on day 13
That isn't when a period is due. Day 13 (copper outline) falls in the usual fertile days, so assuming your period has come early could be a mistake. In one study of women with regular cycles, followed for 2 cycles, 4.8% had midcycle bleeding.
What it means: treat it as possibly fertile, and see a GP about bleeding between periods.
Trying to conceive: 24-day cycle, 7-day period
Fertile days run from about day 5 to day 10, so days 5 to 7 overlap with bleeding. Waiting until the bleeding stops could miss part of the window.
What it means: don't time sex around the end of your period. The NHS says sex every 2 to 3 days makes pregnancy within a year likely if you're 39 or under.
What if you're trying to get pregnant, or trying not to?
Tommy's says, when asked if you can get pregnant at any time in your cycle, "Yes, but you're least likely to get pregnant just before or just after your period."
If you're trying to conceive: the NHS says that if you're 39 or under and have sex without contraception every 2 to 3 days, it's likely you'll get pregnant within a year. See the best time to get pregnant for more on timing.
If you're not: use contraception throughout your cycle, including your period. The emergency pill is free from most sexual health clinics, GP surgeries and pharmacies, and an IUD from most sexual health clinics and GP surgeries. You need to take levonorgestrel within 3 days or ulipristal acetate within 5 days, and an IUD can be fitted within 5 days. The sooner the better.
When should you see a GP?
See a GP if your periods are irregular or last longer than 7 days, or you bleed between periods. Also go if you've missed 3 periods in a row, bleed after sex, or have been trying for 1 year or more through sex (sooner if you're 36 or over, or already know you may have fertility problems).
If you're pregnant and bleeding, the NHS says:
- Get urgent help for light bleeding or spotting: call your maternity unit, an early pregnancy unit if you are under 20 weeks, or NHS 111.
- Call 999 if you are bleeding and have severe tummy pain, shoulder pain, feel sick, faint or dizzy, or the bleeding is heavy (soaking a pad soon after putting it on).
Frequently asked questions
What are the chances of getting pregnant on your period?
For one act of unprotected sex, modelled figures give 0.1% on cycle day 1, rising to 2.1% on day 7, against a peak of 9.7% on day 13. They are averages across all cycle lengths, and they count cycle days, not bleeding days, so your own chance may be higher or lower.
Does period mean 100% not pregnant?
No. The NHS lists a bleed similar to a very light period, around the time your period is due, as a possible early sign of pregnancy. In one study, 9% of women with clinical pregnancies recorded bleeding in the first 8 weeks. That bleeding was generally light and unlikely to be mistaken for a period. If a bleed is unusually light or you have any doubt, take a pregnancy test. The NHS says tests are most reliable from the first day of a missed period, or at least 21 days after unprotected sex if you don't know when your period is due.
Can sperm survive in period blood?
There is no good direct data: our PubMed searches found no study measuring human sperm survival in menstrual blood. Sperm can survive for up to about 5 days in the body, and motile sperm have been recovered from cervical mucus 112 and 120 hours after insemination. So don't assume blood kills or washes out sperm.
When is your lowest risk of getting pregnant?
Tommy's says you are least likely to get pregnant just before or just after your period, though on a short cycle the days just after a period can be fertile. The lowest modelled figures are 0.1% on cycle days 1 and 2. Lowest doesn't mean zero, and no day is guaranteed safe, because ovulation timing can move from cycle to cycle.
Keep Reading: Helpful TTC Guides
More on timing:
- How long do sperm stay in the body?
- Best time to get pregnant
- Ovulation symptoms: the real signs I missed
