Pregnancy Due Date Calculator
Calculate your pregnancy due date and key milestone weeks.
By Syed Mubashir Ali · Last updated
For general information only — not medical advice. If you have a health concern, speak to a qualified professional.
The trimesters, in plain terms
Pregnancy is divided into three trimesters, but the boundaries feel arbitrary until you know what changes at each one. The first trimester (to the end of week 13) is when the organs form. It is also when most people feel worst, and when the miscarriage risk, while still low in absolute terms, is at its highest. The 12-week scan falls near its end.
The second trimester (weeks 14–27) is usually the easiest stretch: energy returns, the bump shows, and the 20-week anatomy scan checks development in detail. The third trimester (weeks 28–40) is the growth sprint (the baby gains about half its birth weight here) and appointments get more frequent as the due date approaches.
When the date moves
Don't be surprised if your due date changes once. It happens routinely: the dating scan at 8–12 weeks often shifts the estimate by a few days, and providers adopt the scan date when it disagrees with the period maths by more than about 5–7 days. A second shift is uncommon and worth asking about.
What matters more than the exact day is the week count it implies, because care decisions are made in weeks: steroids for lung maturity before 34 weeks, group B strep screening around 36, induction discussions from 41. Keep the calculator's date for your own planning, but always use your provider's date for anything medical.
How to Use Pregnancy Due Date Calculator
- 1Pick how you want to date the pregnancy: from the first day of your last period (the standard clinical method), from a known conception date, or from an IVF embryo transfer date.
- 2Enter the date. If you chose the last-period method and your cycles run longer or shorter than 28 days, set your average cycle length. It shifts the estimate by the difference.
- 3For IVF, enter the transfer date and the embryo's age in days (day-3 or day-5 are the common ones). The calculator converts this to the equivalent last-period date automatically.
- 4Read your estimated due date, how many weeks and days along you are right now, and a timeline of milestones with their dates: the first prenatal visit, the NT scan, the start of each trimester, screening tests, the anatomy scan and weekly check-ups.
- 5Use the milestone dates to plan: the NT scan around 12 weeks, the anatomy scan at 20, glucose screening between 24 and 28, and weekly check-ups from 36.
Frequently Asked Questions
How accurate is the due date?
The last-period method (Naegele's rule) lands within about two weeks of the actual birth for most pregnancies, which is why doctors treat it as an estimate, not an appointment. A first-trimester ultrasound dates the pregnancy more precisely (usually within 3 to 5 days) and your provider will use the scan date if it differs from the period-based date by more than a week.
What is Naegele's rule?
Add 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is longer than 28 days, add the extra days too: with a 32-day cycle, the due date moves 4 days later. The rule assumes ovulation on day 14, so it is least reliable with very irregular cycles.
Only about 5% of babies arrive on their due date, so why bother?
The date is a reference point, not a prediction. It anchors everything else: when each scan is scheduled, when screening tests are offered, and when your provider starts watching for post-term risks (usually from 41 weeks). Most births fall within two weeks either side of it.
How is the due date worked out after IVF?
From the transfer date. A day-5 embryo is already 5 days old, so the calculator treats you as 2 weeks + 5 days pregnant on transfer day (19 days). A day-3 transfer counts as 17 days. This matches how clinics date the pregnancy.
What if I don't remember my last period?
Use the conception date if you know it: add 266 days (38 weeks) instead of 280. If neither date is certain, an early ultrasound is the most reliable way to date the pregnancy; ask your provider to arrange one.
Does the calculator replace my doctor's dating?
No. It uses the same arithmetic your provider uses, but your provider may adjust the date after a scan or based on your history. Always go with their date for medical decisions. This tool is for planning and curiosity between appointments.
About Pregnancy Due Date Calculator
The due date is the single number every pregnancy revolves around, and working it out is simple arithmetic once you know which date to count from. This calculator handles the three situations people actually land in: a normal cycle where the last period is the anchor, a known conception date, and IVF where the embryo's age does part of the counting for you.
What makes the date useful is everything hung off it. The calculator lays out nine milestones as calendar dates: the first prenatal visit at 8 weeks, the NT scan at 12, the second trimester from 14, serum screening at 16, the anatomy scan at 20, glucose screening at 24, the third trimester from 28, weekly check-ups from 36 and the due date at 40. Seeing them as dates turns an abstract week count into a plan.
A note on what this is and isn't. The arithmetic here (Naegele's rule, the 266-day conception adjustment, the IVF day-count) is the same maths clinics use, and it is fine for scheduling your life around the pregnancy. It is not medical advice, it doesn't account for complications, and your provider's dated scan overrules it every time. If anything about your pregnancy feels off, call your midwife or doctor rather than recalculating.