About the Due Date Calculator
This due date calculator estimates your baby’s due date (EDD) using the same rules obstetric providers use. Pick the information you have — the first day of your last menstrual period (LMP), a known conception date, an IVF embryo transfer date, or a dating ultrasound — and it returns the estimated due date, how many weeks and days pregnant you are on any date you choose, your trimester and the key milestones ahead.
The LMP method is Naegele’s rule (LMP + 280 days) adjusted for cycle length: a longer cycle means you probably ovulated later, so the due date moves later by the same number of days. Conception and IVF dates are more precise because the day of fertilisation is known, and a first-trimester ultrasound is considered the most accurate way to date a pregnancy when it disagrees with the LMP.
Only about 4–5% of babies arrive exactly on their due date; most are born within two weeks either side. Treat the result as the centre of a window, and confirm dating with your midwife or doctor.
How to use the due date calculator
- 1Choose what you know: last period, conception date, IVF transfer or ultrasound.
- 2Enter the date — and your usual cycle length if you use your last period.
- 3Leave “Show progress as of” on today, or pick any date to see how far along you will be.
- 4Read your estimated due date, current weeks and trimester.
- 5Scroll the milestone table for scan windows, full term and the 42-week mark.
Formula and method
Pregnancy is counted in “gestational age” from the first day of the last menstrual period, about two weeks before conception in a 28-day cycle. A full-term pregnancy is 280 days (40 weeks) from that date, which is Naegele’s rule. If your cycle is longer or shorter than 28 days, ovulation probably happened later or earlier, so the calculator shifts the due date by the difference.
When conception is known, the due date is 266 days (38 weeks) later. For IVF, a day-3 embryo is already 17 days along in gestational terms on transfer day and a day-5 blastocyst 19 days, giving 263 and 261 days to the due date. With an ultrasound, the scan date minus the gestational age measured on the scan gives day 0, and 280 days are added.
- EDD
- Estimated date of delivery (due date)
- LMP
- First day of the last menstrual period
- cycle length
- Days from the start of one period to the start of the next
Worked examples
Last period March 1, 2026, 28-day cycle
Adding 280 days to March 1, 2026 gives December 6, 2026. On September 29, 2026 that is 212 days — 30 weeks 2 days — into the pregnancy, the third trimester, with 68 days to go. Conception most likely happened around March 15.
35-day cycle
Naegele’s rule gives October 17, but a 35-day cycle means ovulation was about 7 days later than average, so the due date moves to October 24, 2026. On April 1 the adjusted gestational age is 74 days, or 10 weeks 4 days.
Day-5 IVF transfer
A day-5 blastocyst transferred on February 10 is dated as 2 weeks 5 days pregnant that day, so the due date is 261 days later: October 29, 2026. By March 1 you are 5 weeks 3 days.
Ultrasound at 8 weeks 5 days
A scan on May 1 measuring 8 weeks 5 days (61 days) places day 0 on March 1, so the ultrasound due date is December 6, 2026 — the same as the LMP example, which confirms the dating.
Frequently asked questions
How accurate is a due date calculator?+
It gives the same estimate your provider starts with, but only about 4–5% of babies are born on the exact date. Around 80–90% arrive within two weeks either side, so think of the due date as the middle of a window.
Why is pregnancy 40 weeks if conception happens at week 2?+
By convention gestational age is counted from the first day of your last period, because that date is usually known and ovulation is not. Conception typically happens about two weeks later, so the baby’s actual age is roughly 38 weeks at the due date.
Which is more accurate: my period date or the ultrasound?+
A first-trimester ultrasound measuring crown-rump length is the most accurate dating method. ACOG guidance is to change the due date when the scan differs from the LMP date by more than 5–7 days, depending on how early the scan was done.
Does an irregular cycle change my due date?+
Yes. The standard rule assumes ovulation on day 14 of a 28-day cycle. With longer cycles you ovulate later, so the due date is later; with irregular cycles an early dating scan is the most reliable way to set it.
How is an IVF due date calculated?+
Add 263 days to a day-3 embryo transfer or 261 days to a day-5 blastocyst transfer. Because the embryo’s age is known exactly, IVF due dates are the most precise of all methods.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.