Conception Calculator
Work backwards to the estimated conception date from a due date, a last period or a birth date, with the fertile window that produced it and an honest error range.
Conception Calculator: with the default inputs, estimated conception date is December 18, 2025.
The date matching the method above.
First day of one period to the first day of the next. Longer cycles push ovulation — and conception — later.
The day the egg was most likely fertilised — ovulation day.
- Fertile window that produced it
- Dec 13, 2025 to Dec 18, 2025The five days before ovulation plus ovulation day (Wilcox et al., 1995).
- Plausible conception range
- Dec 13, 2025 to Dec 20, 2025Roughly ±5 days, from the width of the fertile window and normal variation in ovulation.
- First day of the last period
- December 4, 2025
- Estimated due date
- September 10, 2026
- Likely implantation window
- Dec 24, 2025 to Dec 30, 2025Six to twelve days after fertilisation (Wilcox et al., 1999).
Assumptions
- Ovulation is assumed to happen 14 days before the next period, so cycle length minus 14 days after the last one. The luteal phase is fairly constant within a person; the follicular phase is what varies.
- Conception is dated to ovulation day; fertilisation actually occurs within about 24 hours of it.
- Due dates use Naegele's rule — last period plus 280 days — adjusted for cycle length.
- The fertile window is the six days ending on ovulation day (Wilcox et al., 1995).
- This is a calendar estimate, not a diagnosis or a measurement. For anything consequential, a first-trimester dating scan is the standard.
| Event | Date | Basis |
|---|---|---|
| Fertile window opens | Dec 13, 2025 | Five days before ovulation — sperm survive that long in the reproductive tract |
| Ovulation / conception | Dec 18, 2025 | Cycle length minus a 14-day luteal phase (28 − 14 days after the last period) |
| Implantation | Dec 24, 2025–Dec 30, 2025 | 6–12 days after fertilisation |
| First missed period | Jan 1, 2026 | One full cycle after the last period began |
| Earliest reliable home test | Jan 1, 2026 | Day the period is due; earlier tests miss many pregnancies |
| Estimated due date | Sep 10, 2026 | Naegele's rule — last period plus 280 days |
How this is worked out
The formula
Conception (ovulation) = first day of last period + (cycle length − 14 days) Conception = estimated due date − 266 days Due date (Naegele's rule) = first day of last period + 280 days + (cycle length − 28) Fertile window = ovulation − 5 days … ovulation
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Work backwards from
- Choose one of 3 options.An estimated due date · The first day of the last menstrual period · The baby's actual birth date
- Date
- The date matching the method above.defaults to today
- Average cycle length
- First day of one period to the first day of the next. Longer cycles push ovulation — and conception — later.from 20 to 45 · whole numbers only · defaults to 28
What you get back
- Estimated conception datemain answer
- The day the egg was most likely fertilised — ovulation day.
- Fertile window that produced it
- The five days before ovulation plus ovulation day (Wilcox et al., 1995).
- Plausible conception range
- Roughly ±5 days, from the width of the fertile window and normal variation in ovulation.
- First day of the last period
- Estimated due date
- Likely implantation window
- Six to twelve days after fertilisation (Wilcox et al., 1999).
What this assumes
- Ovulation is assumed to happen 14 days before the next period, so cycle length minus 14 days after the last one. The luteal phase is fairly constant within a person; the follicular phase is what varies.
- Conception is dated to ovulation day; fertilisation actually occurs within about 24 hours of it.
- Due dates use Naegele's rule — last period plus 280 days — adjusted for cycle length.
- The fertile window is the six days ending on ovulation day (Wilcox et al., 1995).
- This is a calendar estimate, not a diagnosis or a measurement. For anything consequential, a first-trimester dating scan is the standard.
About this calculator
Pregnancy is dated from the first day of your last period, not from sex — a convention that predates anyone being able to see an ovary, and one that puts you "two weeks pregnant" before conception happened. This calculator undoes that convention. Give it a due date, a last period, or a birth date, and it works back to the day the egg was most likely fertilised, plus the window of days that could have produced it.
How to use it
Pick what you actually know. A due date from a first-trimester scan is the most reliable starting point; a last menstrual period is next; a birth date is the loosest, because babies arrive across a five-week spread. Set your usual cycle length — a 32-day cycle ovulates about four days later than a 28-day one, and everything downstream shifts with it.
How to read the results
- Estimated conception date is ovulation day. The egg lives about 12–24 hours after release, so fertilisation happens on that day or the one after.
- The fertile window is the five days before ovulation plus ovulation day. That six-day span comes from Wilcox, Weinberg and Baird's 1995 study of 625 cycles: no pregnancies were observed from intercourse more than five days before ovulation, because sperm do not survive longer than that. Conception odds peak in the two days before ovulation.
- The plausible range widens the answer to about a week, which is honest. Ovulation day shifts between cycles even in regular ones — only about a third of people ovulate on the day a calendar predicts.
- Implantation happens 6–12 days after fertilisation. A pregnancy test cannot turn positive before it, which is why testing early so often gives a false negative.
Why the uncertainty is real, not a disclaimer
Two separate things vary. Ovulation day varies between cycles by several days even in women with clockwork periods. And gestation length itself varies: in a study of 125 pregnancies with an ovulation date confirmed by hormone measurement, the time from conception to birth spanned 37 days once preterm births were excluded. So working backwards from a birth date puts the true conception date somewhere in a fortnight-wide band, not on a single day.
That is why obstetric practice does not date pregnancies from a calculator. A crown-rump length measured before 14 weeks dates a pregnancy to within about five to seven days, and a scan date overrides a period date when they disagree by more than that.
If the date matters
For a paternity question, a workplace or immigration timeline, or any decision with consequences, this tool is background reading, not evidence. Ask for the dating scan report — it will state a gestational age and the date it was assigned, which is the number a clinician will stand behind.
Frequently asked questions
▸How do I calculate my conception date from my due date?
Subtract 266 days — 38 weeks — from the due date. Due dates are set 280 days after the last period, and conception is dated two weeks after that period begins.
▸Can a conception date be exact?
Only with IVF, or with ovulation confirmed by hormone testing. Otherwise the honest answer is a window of about six days, because sperm survive up to five days and ovulation day moves between cycles.
▸Can I work out conception from my baby's birth date?
Approximately. Subtract 266 days for a mid-point, but expect several weeks of uncertainty — gestation length from a confirmed ovulation varied by 37 days in Jukic's 2013 study of healthy pregnancies.
▸Does a longer cycle change the conception date?
Yes. Ovulation happens roughly 14 days before your next period, not 14 days after the last one. A 35-day cycle ovulates around day 21, so conception and the due date both shift a week later.
▸Which is more accurate, my period date or an ultrasound?
An ultrasound before 14 weeks, easily. It dates a pregnancy to within five to seven days and is what clinicians use when the two disagree.
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