How the due date is estimated
The standard method — Naegele's rule — counts 280 days (40 weeks) from the first day of your last menstrual period (LMP). This calculator also adjusts for your cycle length: if your cycles run longer or shorter than the typical 28 days, ovulation shifts, and so does the estimate.
Pregnancy is measured in 'gestational weeks' from the LMP, not from conception — which is why you're counted as roughly 2 weeks pregnant at the moment of conception. It's a convention that confuses everyone at first, but it's what every doctor, scan and app uses, so the weeks here match what your gynaecologist will say.
What the trimesters mean
Pregnancy is divided into three stages, each with its own character:
- First trimester (weeks 1–13): the foundational stage — most early symptoms appear now, and the first scan and blood tests happen. This is when folic acid matters most.
- Second trimester (weeks 14–27): often the most comfortable stretch, with the anomaly scan around week 20 and the first movements felt.
- Third trimester (weeks 28–40): the baby gains weight rapidly, check-ups become more frequent, and preparation for delivery begins.
- 'Full term' is 37–42 weeks — a wide, normal window. The single due date is just the midpoint of when babies typically arrive.
Why the exact date is only a guide
Only about 1 in 20 babies arrive on their exact due date. Most arrive within the two weeks either side, and first babies often come a little late. The due date is a planning anchor, not a deadline.
An early ultrasound (especially in the first trimester) dates a pregnancy more accurately than the LMP, particularly if your periods are irregular or you're unsure of the date — your doctor may adjust your due date after the dating scan. Always treat this calculator as a friendly estimate and follow your doctor's dates and advice.
Frequently asked questions
›How is a pregnancy due date calculated?
By Naegele's rule: 280 days (40 weeks) added to the first day of your last menstrual period, adjusted for cycle length. This calculator does that and also shows your current week, trimester and days remaining.
›How many weeks pregnant am I?
Counted from the first day of your last period — so you're about 2 weeks 'pregnant' at conception. Enter your LMP above and the calculator shows your current gestational age in weeks and days, matching how your doctor counts.
›Can I calculate the due date if my periods are irregular?
The LMP method is less reliable with irregular cycles — enter your average cycle length for a better estimate, but rely on your early dating ultrasound, which is more accurate. Your doctor may revise the due date after that scan.
›What if I know my conception date instead?
Conception is roughly 2 weeks after the LMP, so if you know the conception date, the due date is about 266 days (38 weeks) from it. This tool uses the LMP method; enter the LMP as about two weeks before conception for a close match.
›Is the due date exact?
No — it's an estimate. Only around 5% of babies are born on the exact due date; most arrive within two weeks either side, and first babies often run a little late. 'Full term' spans 37 to 42 weeks. Follow your doctor's guidance over any calculator.