September 19, 2026

Conception date calculator: when did I conceive

Calculadora de fecha de concepción

Introduce tu fecha de última regla, tu fecha probable de parto o tu semana de gestación

Primer día de la última reglaFecha probable de partoSemana de gestación actual

Primer día de la última regla

“When exactly did I get pregnant?” It is one of the questions I get asked the most, and I get it: as soon as you see a positive test, your head starts doing the maths. The good news is that there is a reliable way to estimate your conception date, though there is always a margin. In this article I explain how to work it out yourself (and I leave you my free calculator), which methods are only an estimate, and why the exact date is never 100% certain. ❤️

Calendar with a heart marking the conception date

What is the conception date?

Your conception date is the moment a sperm fertilised the egg, and it is almost always the same day as your ovulation (or at most a few hours later, because the egg lives 12 to 24 hours after release).

Here is the key point that almost nobody gets: conception does not match the start of the pregnancy, because gestation is counted from the first day of your last period, which happened around two weeks before ovulation. That is why at week 4 of pregnancy, the baby is really only about 2 weeks old. If that sounds confusing, no worries: I break it down with examples below.

Because it depends so much on ovulation, the conception date is estimated, not known: it depends on how regular your cycle is and on the exact day you ovulated. With that in mind, let’s look at the two most common methods.

How to calculate your conception date

There are two main ways to estimate it, and I have included both in my calculator above. Choose whichever suits you best:

Adding 14 days to the first day of your last period (LMP). If your cycle is regular and you ovulate around day 14, it is straightforward: conception usually falls about 14 days after the first day of your period.
Subtracting 266 days from your due date. Your due date is calculated by adding 280 days (40 weeks) to your last period; since conception happens about 2 weeks earlier, subtracting 266 days from that due date gives you, roughly, the conception day.

In both cases the result is the same, because both methods start from the same point. Mine also gives you a window of a few days around the estimated date, because ovulation does not always fall exactly on day 14. If you want to revisit how the due date is worked out, my article on the gestogram and how to calculate your due date will help.