Conception Date Calculator

Back-calculating a conception date from your due date assumes standard cycle timing — specifically, that ovulation occurred around day 14. For women with irregular cycles, this assumption can shift the estimated conception date by days or even weeks.

Here's why: a due date is typically assigned based on a 40-week pregnancy starting from the last menstrual period (LMP). If your LMP-to-ovulation gap is longer than 14 days — as is common with longer cycles or PCOS — then conception actually occurred later than the standard calculation suggests. Conversely, if your cycles are short and you ovulate early, conception may have been earlier than the formula predicts.

For women with irregular cycles, ultrasound dating is significantly more reliable for both due date and conception date estimation. A first-trimester ultrasound measures the crown-rump length (CRL) directly and can pinpoint gestational age within 5–7 days — independent of your cycle history. If you have an ultrasound measurement available, use the "From Ultrasound" tab for the most accurate result.

IVF is the only method that gives a truly precise conception date, because the fertilization day is known exactly. If you conceived through IVF, use the "From IVF Transfer" tab for a result with no uncertainty range.