How to use the pregnancy conception calculator
- Choose whether you know your last period or your due date.
- For a last period, enter its first day and your average cycle length. Leave the cycle at 28 days if you are unsure.
- For a due date, enter the date your doctor, midwife or dating scan gave you.
- Read the estimated conception date, the fertile window it fell in, and the timeline of early pregnancy dates.
How the conception date is estimated
Conception happens within a day of ovulation, so the calculator estimates the day you ovulated:
The 14 days come from the luteal phase, the time between ovulation and the next period, which is fairly steady at around two weeks. A 28-day cycle puts ovulation 14 days after the start of the last period. A 32-day cycle puts it 18 days after. From a due date, the count runs back 38 weeks, or 266 days, because a pregnancy lasts about 266 days from conception to the due date.
The fertile window is six days long because sperm can survive in the body for up to 5 days, while an egg lives for only about 12 to 24 hours after ovulation. Implantation timing comes from a well-known study of early pregnancy (Wilcox and colleagues, New England Journal of Medicine, 1999). It found implantation 6 to 12 days after ovulation, most often on day 8, 9 or 10.
Worked example
The calculator opens with a last period 42 days before today and a 28-day cycle. On October 29, 2026, the period started on September 17, 2026.
- Ovulation: September 17 + (28 − 14) = 14 days later, so October 1, 2026, the estimated conception date
- Fertile window: September 26 to October 1, with the most fertile days September 29 to October 1
- Implantation, if pregnant: October 7 to October 13
- Missed period and first reliable home test: October 15
- Due date: October 1 + 266 days = June 24, 2027
Gestational age vs. the baby's age
There are two ways to count a pregnancy, and they differ by two weeks. Doctors use gestational age, counted from the first day of the last period. The baby's actual age, sometimes called fetal age or conception age, is counted from conception and is about two weeks less. At your due date the gestational age is 40 weeks, but the baby is about 38 weeks old.
| Date in the example | Gestational age | Time since conception |
|---|---|---|
| Sep 17, 2026 (last period) | 0 weeks | − |
| Oct 1, 2026 (conception) | 2 weeks | 0 |
| Oct 15, 2026 (missed period) | 4 weeks | 2 weeks |
| Jun 24, 2027 (due date) | 40 weeks | 38 weeks |
How accurate is it?
The estimate assumes you ovulated 14 days before your next period was due, which is an average. Stress, illness, travel and irregular cycles can move ovulation by several days in either direction, so the real conception date may sit a few days either side of the one shown. A due date from a first-trimester ultrasound narrows it down, which is why entering that date usually gives a better answer than a last period. To look backward from a birth date and see the full range of uncertainty, use the conception calculator.
Frequently asked questions
Is the conception date the day we had sex?
Not necessarily. Sperm can live for up to 5 days, so sex several days before ovulation can lead to conception on ovulation day. The calculator shows the most likely day of fertilization and the whole fertile window around it.
When can I take a pregnancy test?
Most home tests are reliable from the first day of a missed period, about 14 days after conception. Testing earlier can give a false negative because the pregnancy hormone hCG only starts to rise after implantation. If a test is negative but your period still hasn't come, test again a few days later.
Why does my doctor say I'm further along than the conception date suggests?
Because pregnancy is dated from your last period, which is about two weeks before conception. A pregnancy conceived 4 weeks ago is described as 6 weeks along. Both describe the same pregnancy.
Can I use this to plan when to get pregnant?
It shows when your fertile days are likely to fall, which can help with timing. For planning ahead over several months, the ovulation calculator lists your next six fertile windows. If you have been trying for a year without success, or six months if you are 35 or older, talk to your doctor.
Related calculators
- Conception Calculator — Count back from a due date or birth date to the likely conception date, with an honest range of uncertainty.
- Due Date Calculator — Your estimated due date by Naegele's rule, adjusted for cycle length, or from a conception or IVF transfer date.
- Ovulation Calculator — Your estimated ovulation date and fertile window for the next 6 cycles, from your last period and cycle length.
- Pregnancy Calculator — How many weeks pregnant you are, your due date, trimester and key milestones, from your last period, conception, ultrasound or IVF.
Results are estimates for general information and are not medical advice. Talk to a doctor or other qualified professional about your health. See our disclaimer.