Due Date Calculator

Naegele's rule and conception dating

The most common way to estimate a due date is Naegele's rule, which counts a typical pregnancy as 280 days, or 40 weeks, from the first day of the last menstrual period (LMP). This method assumes a 28-day cycle with ovulation around day 14, so this calculator adjusts for your actual cycle length by adding the difference between your cycle and 28 days. If you instead know the conception date, pregnancy is dated as roughly 266 days from that point, because conception happens about two weeks after the last period begins.

In short, due date = LMP + 280 days + (cycle − 28), or conception + 266 days. Both approaches land on the same answer for an average 28-day cycle, and the page recalculates instantly when you change the method or the date.

Current week and trimester

Gestational age is measured from the LMP, not from conception, which is why providers say you are "two weeks pregnant" before conception technically occurs. The calculator counts the days since your LMP and reports how many weeks and days along you are today. It then places you in a trimester: the first trimester runs through week 13, the second from weeks 14 to 27, and the third from week 28 until birth. A full-term pregnancy is generally considered 37 to 42 weeks.

Accuracy and a health note

A calculated due date is an estimate, not an appointment. Only about five percent of babies arrive on the exact date, and most births happen in the weeks around it. Irregular cycles, uncertain dates and individual variation all affect the result, which is why an early ultrasound dating scan is the clinical gold standard. This page is for general information only and is not medical advice. Always follow guidance from your midwife or doctor. All calculations run locally in your browser and nothing is uploaded.