Due Date Calculator
Estimate your pregnancy due date from your last period, conception, or IVF transfer — with your current gestational age and trimester.
A due date is a best estimate of when your baby is likely to arrive, counted as 40 weeks from the start of your last period. It sets the schedule for prenatal care, screening tests, and tracking your baby’s growth — but it marks the middle of a normal range, not a deadline.
How the due date is estimated
The classic method is Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes ovulation on about day 14 of a 28-day cycle, with conception roughly two weeks after the period starts.
- From LMP: due date = last period + 280 days
- From conception or ovulation: due date = that date + 266 days (38 weeks)
Pregnancy is dated from the LMP by convention, which is why “40 weeks” already includes the two weeks before you actually conceived.
Adjusting for your cycle length
Naegele’s rule assumes a 28-day cycle. If your cycles run longer or shorter, you ovulate later or earlier, and an LMP-based due date shifts by the same amount. A 35-day cycle, for example, moves the due date about a week later. Enter your average cycle length and the calculator applies the shift for you.
Dating from conception or IVF
If you know your conception or ovulation date — from ovulation tests or tracking — dating from it (266 days) avoids the cycle-length guesswork.
IVF is the most precise of all because the embryo’s age is known exactly:
- Day-5 (blastocyst) transfer: due date = transfer date + 261 days
- Day-3 (cleavage) transfer: due date = transfer date + 263 days
Gestational age and trimesters
Gestational age is how far along you are, measured in weeks and days from the dating LMP. This calculator shows your gestational age today and which trimester you’re in:
- First trimester: weeks 1–13
- Second trimester: weeks 14–27
- Third trimester: weeks 28 to birth
A full-term pregnancy is 39–40 weeks; 37–38 weeks is early term and 41+ is late term.
How accurate is a due date?
Treat the due date as the centre of a window. Only around 4–5% of babies are born on it, and roughly 8 in 10 arrive within the two weeks before or after. A first-trimester ultrasound that measures the baby’s crown-rump length is the most reliable way to date a pregnancy, and clinicians will use it to confirm or adjust an LMP-based date — especially when periods are irregular or the LMP is uncertain.
When to check with your clinician
This tool is for information, not diagnosis. See your midwife or doctor to confirm your dates with a scan, and seek care promptly for bleeding, severe pain, or any concern about your pregnancy.
Frequently asked questions
How is a due date calculated?
The standard method (Naegele's rule) counts 280 days — 40 weeks — from the first day of your last menstrual period, assuming a 28-day cycle. From a known conception or ovulation date, it's 266 days (38 weeks). This calculator uses those rules and adjusts for your cycle length.
How accurate is a due date?
It's an estimate of a range, not a fixed day. Only about 1 in 20 babies is born on the exact due date; most arrive within two weeks either side. A first-trimester dating ultrasound (crown-rump length) is the most accurate method and may revise an LMP-based date.
Can I calculate a due date from an IVF transfer?
Yes. IVF dating is precise because the embryo's age is known. For a day-5 (blastocyst) transfer the due date is 261 days after transfer; for a day-3 transfer it's 263 days. Select the IVF method and enter your transfer date.
What if my cycle isn't 28 days?
A longer or shorter cycle shifts ovulation, so it shifts an LMP-based due date by the same number of days. If your cycle is 32 days, the due date moves about 4 days later. Enter your average cycle length and the calculator adjusts automatically.