Your Chances of Getting Pregnant on Each Day of Your Cycle

8 min read

Your chances of getting pregnant on each day of your cycle

You’re trying for a baby and you want to know which days are actually worth it. Or you had sex on day 10, or day 20, and you’re wondering what the odds really were. Most answers online give you one number, or a calendar with six green days and no explanation.

Here’s the short version. Pregnancy can only start from sex in about six days of each cycle: the five days before ovulation and the day of ovulation itself. Within those days the chance rises as ovulation gets closer, from about 10% five days before to about 30% in the last two days before and on the day itself. Outside them, the chance is close to zero. The hard part is that ovulation doesn’t land on the same cycle day for everyone, or even every month, so “day 14” is only an example.

Chance of pregnancy by day relative to ovulation

The best-known numbers come from a study by Allen Wilcox and colleagues, published in the New England Journal of Medicine in 1995. They followed 221 healthy women in North Carolina who were trying to get pregnant. Each woman collected a urine sample every morning, so the researchers could pinpoint the day of ovulation from hormone levels, and recorded the days she had sex. Across 625 cycles, every pregnancy came from sex in the six days ending on ovulation day.

Day you have sex Chance a pregnancy starts
6 or more days before ovulation No pregnancies seen
5 days before ovulation 10%
4 days before ovulation 16%
3 days before ovulation 14%
2 days before ovulation 27%
1 day before ovulation 31%
Ovulation day 33%
Day after ovulation No pregnancies seen

The numbers are for sex on that one day in a cycle where pregnancy was possible at all, from the study’s statistical model (Table 1 in the paper). A few things to keep in mind when reading them:

  • They count every pregnancy the urine tests picked up, including very early ones. About a third of those ended before birth, which is common in early pregnancy.
  • The study couldn’t rule out a chance of up to 12% on the seventh day before ovulation or the day after it. It just didn’t see any pregnancies on those days.
  • The peak is spread across the last three days, not one magic day. Days 2 and 1 before ovulation are as good as ovulation day itself.

That pattern makes sense once you know that sperm can survive for up to about five days, while the egg lasts a much shorter time. Sex before ovulation means sperm are already waiting when the egg is released.

Chance of pregnancy by cycle day

The table above needs one thing you don’t usually know in advance: the day you ovulate. Counting by cycle day, from the first day of your period, is what most people actually want, so the same research team went back to the data.

In a 2000 study in the BMJ, Wilcox and colleagues worked out how likely women were to be in their fertile window on each day of the cycle. Ovulation in their data happened as early as day 8 and as late as day 60.

Cycle day Chance you’re in your fertile window
Day 2 Under 1%
Day 4 About 2%
Day 7 About 17%
Days 6 to 21 At least 10% on every one of these days
Days 12 and 13 About 54%, the peak
Fifth week of the cycle 4 to 6% of women who hadn’t had their period yet

Two things stand out. First, even at the peak, only about half of women were in their fertile window. Second, the textbook advice that the fertile days fall between day 10 and day 17 only fully fitted about 30% of women. Most reached their fertile window earlier, and some much later. Even women who called their cycles regular had a 1 to 6% chance of being fertile on the day their next period was due.

A year later, the same group published a paper estimating the chance of pregnancy from one act of sex on each cycle day. Taken across the whole cycle, a single random act had about a 3.1% chance of leading to pregnancy, and because ovulation is sometimes late, the risk never fully dropped to zero even into the sixth week.

How to find your own high-chance days

Here’s the logic most ovulation calendars and pregnancy calculators run on. If you want a calculator, this is it, worked by hand.

  1. Take the length of your usual cycle, counted from one day one to the next.
  2. Your next period is expected that many days after your last one started.
  3. Ovulation is estimated at about two weeks before that next period. The NHS says it’s usually about 14 days before the period starts.
  4. The six days ending on that estimate are your likely fertile window, and the last three are your highest-chance days.
Usual cycle length Estimated ovulation Likely fertile days Highest-chance days
26 days Around day 12 Days 7 to 12 Days 10 to 12
28 days Around day 14 Days 9 to 14 Days 12 to 14
30 days Around day 16 Days 11 to 16 Days 14 to 16
32 days Around day 18 Days 13 to 18 Days 16 to 18
35 days Around day 21 Days 16 to 21 Days 19 to 21

This is an estimate, not a measurement. The BMJ data shows why: real ovulation moves around, so it’s worth treating the days either side as possible too. If you’d rather not count, Lua’s ovulation calculator does the calendar part from your last period and cycle length, and what an ovulation calendar actually shows explains what that kind of estimate can and can’t tell you.

What the cycle-day numbers mean if you’re trying

You don’t need to hit one perfect day. In the 1995 study’s model, sex every other day did almost as well as daily sex. It put the chance of a pregnancy starting in one cycle at about 37% with daily sex, 33% with sex every other day and 15% with sex about once a week. The NHS gives similar advice: if you’re 39 or under and have sex without contraception every 2 to 3 days, you’re likely to get pregnant within a year.

Those per-cycle odds add up over months. Our chances of getting pregnant calculator shows how that works over time, using population averages.

Your body can also narrow the window down in the cycle you’re in. Stretchy, clear cervical mucus tends to show up in the days before ovulation, and a positive ovulation test picks up the hormone surge that comes just before it. A basal body temperature rise comes after ovulation, so it confirms it rather than predicting it, which is still useful for learning your pattern over a few cycles. If you think you ovulated and you’re counting down, how long after ovulation does your period come covers the second half of the cycle.

Where Lua fits

Lua: Period & Birth Control starts with the calendar estimate. It places ovulation 15 days before your next expected period and shows a fertile window from 5 days before that day to 1 day after. In trying-to-conceive mode, which is part of Lua+, you can log temperature, cervical mucus and ovulation tests, and Lua shows an ovulation confidence from 0 to 100%. When at least two of your signals agree closely enough for Lua to rate ovulation as likely or high, it moves that cycle’s estimated ovulation day and fertile window to the date your signals point to. It’s still an estimate, but it’s built from your own body that month. More on that on the trying to conceive page.

Sources

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