ALPPI FIT LAB
Conception Date Calculator
Wondering when your little one got started? This conception date calculator estimates when you conceived from your due date, the first day of your last period, or an ultrasound. It also shows the likely conception window and your full pregnancy timeline.
- 3 ways to calculate
- Conception window
- Pregnancy timeline
When Did I Conceive?
Calculate from your due date, last period or an ultrasound
Enter your date above, then tap Find my conception date.
Key takeaways
- Conception usually happens about 266 days (38 weeks) before your due date, or about 2 weeks after the first day of your last period.
- The calculator gives a likely window rather than one exact day, because ovulation varies and sperm can live up to 5 days.
- A first-trimester ultrasound is the most accurate way to date a pregnancy.
- A conception date alone can't confirm paternity. Only a DNA test can.
How to use the conception date calculator
- Pick what you know. Choose your due date, the first day of your last menstrual period (LMP), or the date and result of an ultrasound.
- Enter your dates. If you use your last period, add your average cycle length so we can adjust for earlier or later ovulation.
- See your results. You'll get your estimated conception date, the likely window when you conceived, the days intercourse most likely led to pregnancy, and your full pregnancy timeline.
Which method is most accurate? If you had a first-trimester ultrasound, use it. ACOG considers an early ultrasound the most accurate way to date a pregnancy, and your provider may have changed your due date based on it.
How conception dates are calculated
Pregnancy is counted from the first day of your last period, not from conception. For a typical 28-day cycle, ovulation and conception happen about two weeks after that date, or about 38 weeks (266 days) before your due date.
Last period + (cycle length − 14) days = estimated conception date
Ultrasound date − gestational age at scan + 14 days = estimated conception date
For a longer or shorter cycle, we shift the date by the difference from 28 days. A 32-day cycle usually means ovulating about 4 days later.
Because an egg survives only 12 to 24 hours but sperm can live up to five days, the sex that led to pregnancy may have happened several days before conception itself.
Conception
The moment sperm fertilizes the egg, usually within a day of ovulation, in the fallopian tube.
Implantation
The embryo attaches to the uterine lining about 6 to 12 days after conception. Some people notice light spotting.
Positive test
hCG rises after implantation. Most home tests turn positive around the time of a missed period.
Conception date example
Say your due date is July 11, 2027. Counting back 266 days gives an estimated conception date of October 18, 2026.
Conception most likely happened between October 16 and 20, and the sex that led to pregnancy most likely happened between October 13 and 20.
| Due date | Estimated conception | Likely conception window | Likely intercourse dates |
|---|---|---|---|
| Jan 15, 2027 | Apr 24, 2026 | Apr 22–26 | Apr 19–26 |
| Apr 1, 2027 | Jul 9, 2026 | Jul 7–11 | Jul 4–11 |
| Jul 11, 2027 | Oct 18, 2026 | Oct 16–20 | Oct 13–20 |
| Oct 1, 2027 | Jan 8, 2027 | Jan 6–10 | Jan 3–10 |
How accurate is a conception date?
A conception calculator gives a best estimate, not an exact date. Several things affect accuracy:
- Ovulation varies. Even people with regular cycles can ovulate a few days early or late in any month.
- Sperm lifespan. Sperm can survive up to five days, so sex on Monday can lead to conception on Thursday.
- Due dates are estimates. Only about 4% of babies arrive on their due date. A due date set by early ultrasound is the most reliable starting point.
- Irregular cycles. If your cycles vary a lot, the last-period method is less reliable. Use your ultrasound or due date instead.
Trying to work out paternity? A conception calculator can't reliably tell apart intercourse dates that are only a few days apart. Only a DNA test can confirm paternity.
Conception dates after IVF or IUI
With assisted conception, your dates are known much more precisely. For IUI, conception is close to the insemination date. For IVF, fertilization happens on egg retrieval day.
Use our IVF Due Date Calculator, which accounts for 3-day and 5-day embryo transfers. If you're expecting more than one baby, try the Twin Pregnancy Due Date Calculator.
Frequently asked questions
How do I calculate when I conceived?
Subtract 266 days (38 weeks) from your due date, or add about 14 days to the first day of your last period if you have a 28-day cycle. For longer or shorter cycles, shift the date by the difference from 28 days. The calculator above does this for you and shows the likely window too.
Can a conception calculator tell me the exact day I got pregnant?
No. It gives the most likely date and a window of a few days. Ovulation timing varies, and sperm can live up to five days, so the sex that led to pregnancy could have happened several days before conception.
Is conception 2 weeks after my last period?
For a typical 28-day cycle, yes: ovulation and conception usually happen around day 14. If your cycle is longer, you probably ovulated and conceived later. If it's shorter, earlier.
How many weeks pregnant am I at conception?
About 2 weeks. Doctors count pregnancy from the first day of your last period, so you're already considered "2 weeks pregnant" on the day you conceive. Your due date is 40 weeks from your last period, or 38 weeks after conception.
Is the conception date the same as the implantation date?
No. Conception (fertilization) happens first. The embryo then travels to the uterus and implants about 6 to 12 days later. Pregnancy hormones (hCG) start rising only after implantation.
Can an ultrasound tell when I conceived?
An early ultrasound, ideally before 14 weeks, measures your baby to estimate gestational age within about 5 to 7 days. From that, you can work out an estimated conception date. Later ultrasounds are less precise for dating.
Can I use this calculator to find out who the father is?
Not reliably. If the possible dates are more than a couple of weeks apart, a calculator may point to one window, but it can't rule out the other, because cycles and sperm survival vary. A prenatal or postnatal DNA test is the only way to confirm paternity.
How accurate is my due date?
Very few babies arrive exactly on their due date. Most are born within about two weeks of it. A due date confirmed by first-trimester ultrasound is the most accurate.
Sources
- ACOG Committee Opinion 700: Methods for Estimating the Due Date
- Wilcox AJ, et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995
- Wilcox AJ, et al. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med. 1999
- Office on Women's Health: Trying to conceive
This calculator provides estimates for general information only and is not medical advice. Your healthcare provider's dating, especially from an early ultrasound, should always take priority.
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