free due date calculator
Due Date Calculator — by Last Period, Conception or IVF Transfer
Estimate your due date and how many weeks pregnant you are from your last period, conception date or IVF embryo transfer date. Free, private, no sign-up.
How the calculator works
Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. In a textbook 28-day cycle, ovulation and conception happen about two weeks after the period starts, which is why you are already counted as “2 weeks pregnant” on the day you conceive. A full-length pregnancy is 40 weeks, or 280 days, from the LMP.
| Method | Due date = | Notes |
|---|---|---|
| Last period | LMP + 280 days + (cycle length − 28) | Naegele’s rule, adjusted for your usual cycle length |
| Conception date | Conception + 266 days | Useful if you know the date of ovulation or conception |
| IVF, day-3 embryo | Transfer date + 263 days | ACOG method: 266 days minus embryo age |
| IVF, day-5 embryo | Transfer date + 261 days | |
| IVF, day-6 embryo | Transfer date + 260 days |
Example: if your last period started on 1 January and your cycle is 28 days, your estimated due date is 8 October (in a non-leap year). With a 32-day cycle it moves four days later, to 12 October.
Which method is most accurate?
For a pregnancy conceived with IVF, the transfer date is the most precise because the embryo’s age is known exactly. For a natural conception, the American College of Obstetricians and Gynecologists (ACOG) says an ultrasound measurement in the first trimester is the most accurate way to confirm or set the due date. A period-based date is a good starting point, but it is less reliable if your cycles are irregular, you recently stopped hormonal contraception, you conceived while breastfeeding, or you are not sure when your last period began.
If the scan-based date differs from your period date by more than a set margin (ACOG gives thresholds that depend on how far along you are), your care team will usually go with the scan. After that, the date normally stays fixed for the rest of the pregnancy, even if later scans suggest your baby is larger or smaller than average.
What “term” means around your due date
Only a small share of babies arrive on the exact day. ACOG describes a range of full-term timing around the due date:
- Early term: 37 weeks 0 days to 38 weeks 6 days
- Full term: 39 weeks 0 days to 40 weeks 6 days
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Post-term: 42 weeks 0 days and beyond
Birth before 37 weeks is preterm. If you are unsure whether a symptom means labour is starting, read our guide to the signs of labour and call your maternity unit or labor and delivery unit for advice.
After you know your due date
- Open your week in the week-by-week guide to see what is happening this week.
- Book or confirm your first antenatal (prenatal) appointment and dating scan if you have not done so yet.
- Use the weeks-to-months converter if family members keep asking “how many months?”.
- In the Elara app, the due date drives the countdown, the weekly baby view and the partner’s weekly updates.
This calculator runs entirely in your browser. The dates you enter are not sent to us or stored.
Common questions
How is a due date calculated from the last period?
Add 280 days (40 weeks) to the first day of your last menstrual period. This is Naegele’s rule and assumes a 28-day cycle with ovulation around day 14. If your cycles are regularly longer or shorter, the estimate shifts by the same number of days.
How is a due date calculated from the conception date?
Add 266 days (38 weeks) to the date of conception. Pregnancy is still described in weeks from the last period, so on the day of conception you are counted as about 2 weeks pregnant.
How do you calculate an IVF due date?
Use the embryo transfer date and the embryo’s age. ACOG’s method adds 266 days minus the embryo’s age: 263 days after a day-3 transfer, 261 days after a day-5 blastocyst transfer and 260 days after a day-6 transfer.
Can my due date change after an ultrasound?
Yes. An early ultrasound that measures the baby (crown–rump length) is the most accurate way to date a pregnancy. If it differs enough from the period-based date, your care team may change your due date. IVF due dates are normally not changed by a scan.
Will my baby arrive on the due date?
Probably not exactly. The due date is the midpoint of a normal window, and a full-term birth can happen in the weeks either side of it. Your maternity team will explain what they would do if you go past your due date.
Sources & editorial process
- ACOG Committee Opinion 700 — Methods for estimating the due date
- ACOG Committee Opinion 579 — Definition of term pregnancy
- NHS — Pregnancy due date calculator
- NHS — 12-week scan
- HFEA — In vitro fertilisation (IVF)
Written by the Elara editorial team from the sources above. This page has not been reviewed by an independent clinician. Read how we write and update health pages. Last updated .
Medical disclaimer: this page is general education, not medical advice, diagnosis or treatment. If you are worried about yourself or your baby, contact your midwife, doctor or maternity unit straight away. In an emergency call your local emergency number.