Reverse conception date calculator
Runs the due-date arithmetic backwards. Enter a due date, or a date of birth with the gestational age at birth, and it returns the window in which conception happened, plus the implied last period, implantation window and missed period. It gives a window rather than a day, because a day would be a guess.
Reverse conception date calculator
No sign-up · PrivateThe due date on your notes, app or scan report. A date set by an early scan gives a much better answer than one set from a remembered period.
The day the baby was actually born.
Optional, but it changes the answer completely. It is on the discharge summary or red book, written like “39+4”. Leave blank if you do not have it.
The number after the plus sign in “39+4”. Ignored if the weeks box above is empty.
Conception most likely happened between
Due date you entered: 1 July 2026
Most likely single day (ovulation): 8 October 2025
Widest plausible window, allowing 7-day sperm survival: 1 to 8 October 2025
How wide the likely window is: 6 days
Pregnancy day 0 — last period, or its equivalent: 24 September 2025
The period that did not arrive: around 22 October 2025
Implantation, typically 6 to 12 days after ovulation: 14 to 20 October 2025
Nothing you type leaves your device. The whole calculation runs in your browser.
How this is calculated
FormulaHow the conception window is calculated
This runs the due-date arithmetic backwards. It returns a window, never a single day, because a single day is a claim the arithmetic cannot support.
From a due date
estimated ovulation = due date − 266 days
day 0 (last period, or its equivalent) = due date − 280 days
conception window = ovulation − 5 days … ovulation
266 is 280 − 14: Naegele's 40 weeks, minus the fourteen days between the first day of a textbook 28-day period and ovulation. The six-day window is the fertile window established by Wilcox, Dunson and Baird in the BMJ in 2000 — “the five days before ovulation and the day of ovulation itself”. Intercourse outside those six days does not produce a pregnancy, so those six dates are the ones that can have caused this one.
A wider, more cautious bound uses the NHS figure for sperm survival instead: ovulation − 7 … ovulation, because “sperm can survive in the fallopian tubes for up to 7 days after sex”.
From a date of birth, with the gestational age at birth
day 0 = date of birth − gestational age at birth
estimated ovulation = day 0 + 14 days
This is the narrow version of the answer, and it is the one to use if you have the figure. The gestational age at birth is on the discharge summary or in the red book, written like 39+4, and it almost always came from a dating scan. A baby born on 3 March at 39+4 — 277 days — gives day 0 on 30 May of the previous year and ovulation on 13 June.
From a date of birth alone
estimated ovulation = date of birth − 268 days
plausible range = date of birth − 284 days … date of birth − 247 days
Without a gestational age, the honest answer is the observed distribution. Jukic and colleagues (Human Reproduction, 2013) measured ovulation directly with daily hormone samples and found a median of 268 days from ovulation to birth. In the same data, after excluding preterm births and pregnancies with complications, gestation still varied by 37 days — from 247 to 284 days after ovulation. That 37-day spread is the answer, and the median is only where the middle of it sits. The same paper notes that only about 4% of women deliver at 280 days and only about 70% within 10 days of the estimated date.
Everything else on the page
- Implantation:
ovulation + 6 … ovulation + 12 days. - The period that did not arrive:
ovulation + 14 days. The NHS puts ovulation “usually about 14 days before the start of your next period”, and elsewhere gives the fuller range of “around 10 to 16 days before the next period”. - Due date implied:
ovulation + 266 days.
Why it stays a window
Three separate uncertainties stack up, and none of them can be removed by arithmetic. First, conception follows intercourse anywhere within a six-day window, so the day of sex and the day of fertilisation are routinely several days apart. Second, ovulation itself moves: the NHS gives 10 to 16 days before the next period in most women, and Wilcox found that more than 70% of women are in their fertile window before day 10 or after day 17 of the cycle. Third, a due date set from a first-trimester scan is itself accurate to about ±5 to 7 days, and one remembered from a last period is looser still.
All arithmetic runs on whole days in UTC inside your browser. Nothing is transmitted or stored.
Running the calendar backwards
Most pregnancy calculators go forwards: give them a date and they give you a due date. This one goes the other way. You give it the due date, or the birthday, and it works out when the pregnancy is likely to have started.
People arrive at this question for all sorts of reasons. Curiosity, mostly — there is something quietly satisfying about knowing roughly which week of your life a person began in. Sometimes it is practical: reconciling a date with a holiday, a house move, an illness, a course of medication, a night out you would rather forget. Sometimes it is a paternity question. The arithmetic is the same in every case, and so is the honest answer: it is a window, and the window is days wide.
The six days that matter
In 2000 Wilcox, Dunson and Baird published a study in the BMJ that has shaped how the fertile window is understood ever since. Working from 221 women whose ovulation was pinned down by daily urinary hormone measurements across 696 cycles, they established that conception can only follow intercourse in a six-day window: the five days before ovulation, plus the day of ovulation itself.
That is the whole reason a conception date cannot be a single date. Sperm can wait. The NHS puts the outer limit at seven days — “sperm can survive in the fallopian tubes for up to 7 days after sex” — while an egg survives only about a day after release. So the day the pregnancy began and the night that caused it can be the best part of a week apart, and there is no way to tell from a calendar which of those days it was.
Working back from a due date
Subtract 266 days from the due date and you have the estimated day of ovulation. Subtract 280 and you have day 0 of the pregnancy — the first day of the last period, or the date standing in for it. The six-day conception window is the six days ending on the ovulation day.
How good that answer is depends entirely on how the due date was set. A date from a first-trimester dating scan is accurate to about five to seven days, so read the window as roughly a week wide before the fertile window is even added. A date carried over from a remembered last period inherits every uncertainty in that memory and in the cycle it assumed.
Working back from a birthday
This is where the answer either gets much better or much worse, depending on one number.
With the gestational age at birth — the 39+4 written on the discharge summary or in the red book — the calculation is nearly exact. Subtract that many days from the birthday to get day 0, add fourteen days for ovulation, and you have a six-day window. It is worth digging the paperwork out.
Without it, you are working from a distribution rather than a date. Jukic and colleagues, whose 2013 study in Human Reproduction measured ovulation directly rather than inferring it, found a median of 268 days from ovulation to birth. But even after excluding preterm births and pregnancies with complications, gestation varied across 37 days — from 247 to 284 days. So a birthday alone gives you a five-week window, and any calculator that offers you a single day from a birthday is not telling you the truth.
Prematurity skews it in a predictable direction. If the baby arrived early, conception happened later than a birthday-only estimate suggests, by roughly the number of days the birth was early. A baby born at 30 weeks was conceived about ten weeks later than the arithmetic would guess.
If this is a paternity question
It is worth saying plainly, because it is why a good many people are reading this page. A conception calculator cannot settle paternity. The window is six days wide at best and five weeks wide at worst, dates overlap, and no amount of arithmetic separates two possibilities that both fall inside the same window.
What can answer it is a DNA test. Non-invasive prenatal paternity testing is available during pregnancy from a maternal blood sample, and a straightforward postnatal test is available after birth. If the question matters, that is the route to an answer, and a calculator is at best a way of narrowing down who to ask.
The two-week head start
One number on the results here surprises people every time: day 0 of the pregnancy is a fortnight before conception. That is not an error. Gestational age is counted from the first day of the last menstrual period, so on the day you conceived you were already, by convention, two weeks pregnant. It is the same offset that makes a six-week scan show a four-week-old embryo.
What this cannot do
This is an estimate built on averages, and averages are the one thing no individual pregnancy is obliged to follow. It cannot know whether you ovulated late that month, whether your cycle was disrupted, or whether the due date you were given was ever revised. It cannot account for twins, who are rarely carried to 40 weeks. And it is not a medical record — your maternity notes carry the dates your team actually worked from.
If you need a date for a medical reason, ask for it. Your notes, or your GP record, will have the dating-scan measurement and the gestational age it produced, and those are better than anything a calculator can reconstruct from the far end.
Sources
- The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study — BMJ (Wilcox AJ, Dunson D, Baird DD, 2000;321(7271):1259-1262), accessed
- Periods and fertility in the menstrual cycle — NHS, accessed
- Trying to get pregnant — NHS, accessed
- Length of human pregnancy and contributors to its natural variation — Human Reproduction (Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ, 2013;28(10):2848-2855), accessed
- Committee Opinion No. 700: Methods for Estimating the Due Date — American College of Obstetricians and Gynecologists (with AIUM and SMFM), accessed
- Due date calculator — NHS, accessed
- Ultrasound scans in pregnancy — NHS, accessed