Working Out Your Conception Date From a Due Date or Period

The date a conception calculator hands you rests on two assumptions: that you ovulated in the middle of your cycle, and that your cycle is as long as you told it. Both are worth checking before you rely on the answer.

What conception date actually means

Two different events get called conception. Fertilisation happens within about 24 hours of ovulation, while the egg is still viable. Implantation happens six to ten days later, when the embryo settles into the lining of the uterus. Calculators report the first one.

Why it is a window and not a day

Sperm can survive in the reproductive tract for up to five days. So if you had sex on 11 March and ovulated on 15 March, the 11th is when the opportunity arose but the 15th is when the pregnancy began. The fertile window runs about six days: the five before ovulation plus the day itself. A calculator that gives you a range rather than a single date is being honest.

The arithmetic behind it

Starting from your last period

Pregnancy is dated from the first day of your last period, not from conception, which is why you count as two weeks pregnant before anything has happened. On a 28-day cycle, ovulation lands about 14 days in. A period starting 1 March puts conception around 15 March, and the due date 280 days from 1 March, which is 6 December.

When your cycle is not 28 days

The second half of the cycle is fairly fixed at around 14 days. The first half is the part that varies. On a 32-day cycle ovulation moves to roughly day 18, conception shifts to about 19 March, and the due date moves four days later to 10 December. Four days matters a great deal at 39 weeks, so cycle length is the single input most worth getting right. If you track ovulation directly with test strips or waking temperature, enter that date instead, because it removes the only real guess in the sum.

Using the conception calculator

You need one of two things: the first day of your last period, or a due date from a scan. Enter whichever you have in the date field, then set your average cycle length, which is the field most people leave on the default without thinking. The tool returns an estimated conception date, the fertile window around it, and the matching due date. Working from a due date, it subtracts 266 days, which is the usual 280 minus the two weeks before conception. Run that on a due date of 6 December and you land back on 15 March, the same answer approached from the other end.

If you have already had a dating scan, use the scan due date rather than your period, because a first-trimester measurement beats any calendar sum. For how the two dates connect across the whole pregnancy, see our ovulation to due date guide, and there is a separate walkthrough of the conception date calculator if you want the formula spelled out.

Common questions

Can it tell me which night it happened? No, and be careful with this one. The calculator points at the day of ovulation, but sperm from any of the previous five days could be responsible. If the exact date matters legally or personally, a scan estimate plus the fertile window is the only honest answer.

Why does my scan date not match the calculator? Because a scan measures the baby and the calculator measures your calendar. A first-trimester measurement is the more reliable of the two, so clinicians usually override the period-based date when the two disagree by more than a week.

Does it work for IVF? Not the standard version. With IVF you already know the transfer date, so the timeline counts from there: subtract three days for a day-3 transfer or five for a blastocyst to get the equivalent conception date.

My cycles are irregular, so is it useless? Not useless, but treat the output as a wide band. If your cycles run anywhere from 26 to 38 days, the conception date could sit across a fortnight, and only a scan will narrow it down.