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Due Date Calculator: From Last Period, Conception or IVF Transfer
Pick the way your pregnancy is dated, enter the key date, and get an estimated due date, gestational age on a chosen day and a short list of milestones from the same dating.
Four ways to date a pregnancy
Every method ends at the same place: a 280-day (40-week) pregnancy counted from a notional first day of the last period. What changes is how that start date is found.
- Last period: add 280 days to the first day of the period, plus or minus the gap between your cycle length and 28 days.
- Conception date: ovulation and conception happen roughly 14 days after a 28-day cycle begins, so the due date is the conception date plus 266 days.
- Day-3 transfer: the embryo is 3 days old at transfer, so the due date is the transfer date plus 263 days (266 minus 3).
- Day-5 transfer: a blastocyst is 5 days old, so the due date is the transfer date plus 261 days.
Why IVF dates are usually more precise
After in vitro fertilisation the fertilisation date is known, which removes the guesswork about when ovulation happened. That is why clinics date IVF pregnancies from the embryo age rather than from a menstrual date. The uncertainty is mainly the length of the individual pregnancy, not the start.
Frozen embryo transfers are dated the same way, from the transfer day and the embryo's age on that day. If your clinic quotes a different date, follow the clinic: it may count the embryo's development day or the transfer protocol differently.
How to read the estimate
An estimated due date is a midpoint, not a prediction. Many births occur in the weeks either side, and the point at which a pregnancy is considered full term is defined as 39 weeks to 40 weeks 6 days. The table lists dates for 20 weeks, 28 weeks and the term boundaries because appointments, scans and leave from work are often planned around them.
If an early ultrasound measurement disagrees with these dates by more than the margin your care team allows for that stage, the scan date is normally adopted. For a calendar-style view of a period-based date, see the pregnancy calculator.
Limits and safety
Dating from the last period assumes regular cycles and a known start date. Irregular cycles, recent hormonal contraception and breastfeeding make it less accurate. A self-reported conception date is only as reliable as the record of intercourse, because sperm can survive for several days. Contact your midwife, doctor or emergency service for bleeding, severe pain or concerns about movement; a web page cannot judge that. These results are for general information and are not clinical care.
Worked example
A day-5 blastocyst is transferred on 10 February 2026, and the patient checks her dates on 21 April 2026.
Inputs
| Dating method | IVF transfer, day-5 blastocyst |
|---|---|
| Key date for that method | 2026-02-10 |
| Show gestational age on | 2026-04-21 |
Result
Estimated due date from your day-5 blastocyst transfer (embryo age 5 days).
| Gestational age on 21 April 2026 | 12 weeks 5 days |
|---|---|
| Days until due date | 191 |
| Equivalent first day of last period | 22 January 2026 |
Milestones from this dating
| Milestone | Date |
|---|---|
| End of first trimester (13 weeks 6 days) | 29 April 2026 |
| Halfway (20 weeks) | 11 June 2026 |
| Third trimester begins (28 weeks) | 6 August 2026 |
| Early term begins (37 weeks) | 8 October 2026 |
| Full term begins (39 weeks) | 22 October 2026 |
| Estimated due date (40 weeks) | 29 October 2026 |
- Gestational age counts from the equivalent first day of the last period, so it runs about two weeks ahead of the embryo's true age.
Medical disclaimer: This tool is for education and general planning. It is not medical advice and does not diagnose any condition. If you are pregnant, under 18, have an eating-disorder history or a medical condition, speak to a doctor or registered dietitian before changing your diet or exercise.
Frequently asked questions
How do I calculate my due date from my last period?
Add 280 days to the first day of your last period. If your cycle is not 28 days, shift the result by the difference. The calculator applies that cycle correction automatically.
How is an IVF due date calculated?
Add 266 days to the transfer date and subtract the embryo age. That is 263 days after a day-3 transfer and 261 days after a day-5 blastocyst transfer.
What is the due date if I know the conception date?
Add 266 days, which is 38 weeks, to the date of conception. This assumes conception happened about two weeks into a 28-day dating cycle.
Can the due date change?
Yes. Scan measurements can lead your care team to revise the date, especially when the last period date is uncertain. Use the date your clinic gives you.
How accurate is a due date?
Only a minority of babies are born on the estimated day. Treat it as the middle of a window of several weeks.
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References and sources
- American College of Obstetricians and Gynecologists (2017). Committee Opinion No. 700: Methods for Estimating the Due Date. Link
- ACOG, SMFM and AIUM. Definition of term pregnancy (early term, full term, late term, post-term), Committee Opinion No. 579.
- Naegele FK. The 280-day rule for estimating the date of confinement.