How the chance of getting pregnant is estimated
You can only get pregnant from sex in a short part of each cycle: the six days ending on the day you ovulate. The calculator estimates your ovulation day from your last period and cycle length, then shows which part of the cycle the date you had sex falls in.
| When you had sex | Chance of pregnancy |
|---|---|
| More than 5 days before ovulation | Unlikely, but possible if you ovulated early |
| 3 to 5 days before ovulation | Possible (about 10% five days before) |
| 2 days before ovulation to ovulation day | Highest (up to about 33%) |
| After ovulation | Unlikely, but possible if you ovulated late |
The estimate assumes your ovulation happens 14 days before your next period. Real cycles vary, so treat the result as a guide to how likely pregnancy is, never as proof that it can't happen.
What research says about the best days to get pregnant
In a study of 221 women trying to conceive, pregnancy only followed sex during the six days ending on the estimated day of ovulation. The chance from sex on a single day ranged from about 10% five days before ovulation to about 33% on ovulation day (Wilcox et al., 1995).
A later re-analysis of that study and an older one found the same six-day window, with the highest chance on the day before ovulation and a chance close to zero after ovulation (Dunson et al., 1999). Because sperm can survive for several days, sex before ovulation counts as much as sex on the day itself.
If you don't want to be pregnant
Emergency contraception can prevent pregnancy after unprotected sex. The NHS says the levonorgestrel pill must be taken within 3 days (72 hours), and the ulipristal acetate pill or a copper IUD within 5 days (120 hours). The IUD is the most effective option. You can get emergency contraception from pharmacies, GP surgeries and sexual health clinics (NHS).
A calendar estimate is not contraception. Ovulation can come earlier or later than predicted, so a result of “unlikely” still means pregnancy is possible.
If you're trying to conceive
You don't need to pinpoint ovulation. The NHS says more than 8 in 10 couples where the woman is under 40 will conceive within a year with regular unprotected sex, every 2 or 3 days (NHS). To plan ahead, the ovulation calculator shows your fertile window for the next cycle.
When to take a pregnancy test
Most home pregnancy tests work from the first day of a missed period. If you don't know when your period is due, the NHS suggests testing at least 21 days after the unprotected sex (NHS). The pregnancy test calculator shows the earliest day for each kind of test.
Frequently asked questions
Sources and medical references
The timing figures come from two peer-reviewed studies; the testing and emergency contraception advice is the NHS's.
- NIH (PubMed)Timing of sexual intercourse in relation to ovulation: effects on the probability of conception (Wilcox et al., NEJM 1995)Read the source
- NIH (PubMed)Day-specific probabilities of clinical pregnancy based on two studies with imperfect measures of ovulation (Dunson et al., Human Reproduction 1999)Read the source
- NHSEmergency contraceptionRead the source
- NHSDoing a pregnancy testRead the source
- NHSInfertilityRead the source