Pregnancy Due Date Calculator

Enter the first day of your last menstrual period to get your estimated due date, your trimester start and end dates, your likely conception date and how many weeks pregnant you are today.

How to Use the Pregnancy Due Date Calculator

Think back to the first day of your most recent menstrual period — not the day it ended, the day it began — and select that date. This single date is all obstetricians need to date a pregnancy, because ovulation is invisible but periods are memorable. Press Calculate Due Date.

You will get your estimated due date (40 weeks on), your likely conception date (about two weeks after the period started), the exact calendar span of each trimester, and — if the date is in the past — how many weeks and days pregnant you are right now. Book your first antenatal check-up as soon as you know; the calculator plans dates, it does not provide medical care.

The 280-Day Formula (Naegele's Rule)

Since the 1800s, doctors have dated pregnancy with Naegele's rule: add 280 days — 40 weeks — to the first day of the last menstrual period. The period is used instead of the conception date simply because women usually know it and ovulation day is rarely certain. The 40 weeks include roughly two weeks before conception, which is why pregnancy is already "two weeks along" at the moment of fertilisation.

Trimesters divide those 40 weeks into three: weeks 0–13+6 (first), 14–27+6 (second) and 28–40 (third). The rule assumes a textbook 28-day cycle with ovulation on day 14 — real cycles vary, which is why an early ultrasound can shift the official date by days.

Worked Example

Last menstrual period began 1 January 2026:

Only about 5% of babies arrive on the due date itself — consider it the middle of a four-week window (37 to 42 weeks) rather than an appointment.

First 12 Weeks: Milestones Most Calculators Don't Show

The first trimester is when the most dramatic development happens, yet most due-date tools say nothing about it. Using the same example dates, here is what is unfolding week by week:

This is the window when antenatal care matters most: start folic acid immediately if you have not, avoid alcohol and smoking, and schedule your first check-up and dating scan. This information is educational — your gynaecologist's guidance comes first.

Screening and appointment windows most calculators don't show

These are typical timing windows only — your clinician's dates come first, and local protocols vary.

This timeline is educational only — it does not replace antenatal care, and an early ultrasound may move your official dates.

Limitations

The 280-day rule assumes ovulation on cycle day 14; with irregular, very long or very short cycles the estimate can be off by a week or more. It cannot account for assisted reproduction (where the exact conception date is known and different dating is used), multiple pregnancies (which often deliver earlier), or medical conditions affecting cycle length. The milestone table describes typical development — every pregnancy varies. This calculator does not diagnose, date for medical purposes, or replace antenatal care; confirm everything with your doctor, preferably via a first-trimester ultrasound.

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