Conception Date Calculator

About the Conception Date Calculator

People want to know when a pregnancy began for all sorts of reasons: curiosity, a baby book, making sense of a scan, or matching the timing to a trip or a particular week. Working it out is straightforward arithmetic, but the answer is a range rather than a single day, and how wide that range is depends on where you start.

This conception date calculator works backwards from a due date, the first day of the last period, or a birth date. It gives the most likely date, the realistic range around it, and the window in which intercourse most likely led to conception — which is not the same thing, and is wider.

How to Use the Conception Date Calculator

Choose what you know.

Due date gives the best estimate, especially if it came from an early scan.

First day of last period works well with a known cycle length. Enter your usual cycle, measured from the first day of one period to the first day of the next.

Birth date is the least precise starting point, because babies are born across a five-week span. Add the weeks pregnant at birth if you have it — it is on most birth records and discharge notes — and the range narrows considerably.

How the Conception Date Is Worked Out

  From a due date:     conception ≈ due date − 266 days
  From a last period:  conception ≈ period + 14 days + (cycle length − 28)
  From a birth date:   conception ≈ birth − (weeks at birth − 2 weeks)

Why 266 and not 280? Pregnancy is dated from the first day of the last period, about two weeks before conception. The 280-day pregnancy therefore contains roughly 14 days before conception happened, leaving 266 from conception to the due date.

Step-by-Step Example

Due date 6 December 2026.

  6 December 2026 − 266 days  =  15 March 2026
  Likely range:                  12 to 18 March
  Intercourse window:            7 to 18 March

The intercourse window starts five days before the range because sperm can survive that long.

The same baby, from its birth date with no gestational age: born on 6 December, conception could have been anywhere from about 1 March (if born at 42 weeks) to 5 April (if born at 37 weeks) — five weeks rather than one.

Conception Is Not the Day of Sex

This is the point most often misunderstood.

Conception — fertilisation — can only happen within about a day of ovulation, because the egg survives only 12 to 24 hours.

Sperm, though, can survive in the reproductive tract for up to about five days. So intercourse up to five days before ovulation can lead to a pregnancy.

The result is that even a precise conception date points to a window of about six days in which the relevant intercourse happened. The calculator shows that window separately, so the two are not confused.

Why the Range Depends on the Starting Point

A scan-based due date is the most reliable starting point. A first-trimester scan measures the embryo to within about five to seven days, so the conception estimate is good to within about a week.

A period date assumes ovulation happened at the usual point in the cycle. Even in regular cycles it can shift by several days, so a period-based estimate inherits that uncertainty.

A birth date without the gestational age is the weakest, because a full-term birth can happen anywhere from 37 to 42 weeks. Knowing that a baby was born at, say, 39 weeks narrows a five-week range to about two.

What a Conception Date Cannot Do

It cannot establish paternity. The range is days wide, and intercourse can precede conception by up to five days, so two possible dates close together cannot be separated this way. Where paternity matters, a DNA test is the only reliable answer, and it can be done during pregnancy as well as after birth.

It also cannot confirm a pregnancy or its viability. A pregnancy test and, where needed, a scan are what do that.

Where Twins and IVF Fit In

Twins conceived naturally usually come from two eggs released in the same cycle, typically within a day of each other, so they share essentially the same conception window. Identical twins come from a single fertilised egg that splits, so they share one conception date by definition. Twins are, however, often born earlier than 40 weeks, which matters when working back from a birth date: the weeks at birth are especially important to enter.

IVF pregnancies are the exception to all the uncertainty. The date of egg collection and fertilisation is recorded, and the embryo's age at transfer is known exactly, so the conception date is known to the day. For an IVF pregnancy, the clinic's records give a more precise answer than any calculation.

Understanding Your Result

Likely conception date is the central estimate.

Range gives the realistic span around it, which is narrow from a due date and wide from a birth date without gestational age.

When intercourse likely happened extends the range five days earlier, reflecting how long sperm survive.

Worth knowing explains how to narrow the estimate, or what it can and cannot tell you.

When Should You Use This Calculator?

Out of curiosity, or for a baby book or family record.

To understand a due date from a scan in terms of when the pregnancy began.

To narrow down timing where more than one date is possible, while remembering the limits.

Not to decide paternity. Use a DNA test.

Common Mistakes

Treating conception as the day of sex. Sperm survive up to five days.

Counting 280 days back from the due date. That reaches the period date, not conception; the right figure is 266.

Using a birth date without the weeks at birth. The range is then about five weeks.

Ignoring cycle length. A long cycle moves ovulation, and conception, later.

Expecting a single exact day. The biology does not allow one.

Questions about a pregnancy's dates are best answered by your midwife or doctor, especially where a scan is available. Nothing here is medical advice.