How due dates are estimated
A pregnancy due date is an estimate of when a baby may be born. It is not a guaranteed delivery date. Many pregnancies naturally deliver before or after the estimated date. The due date is mainly used for planning antenatal care, growth checks, screening windows, and pregnancy milestones.
Different methods can produce different estimates. The most common starting point is the first day of the last menstrual period, but conception date, IVF transfer date, and ultrasound measurements can also be used.
Last menstrual period method
The last menstrual period method usually adds 280 days, or 40 weeks, to the first day of the last period. This assumes a typical cycle and ovulation around the middle of the cycle. It is simple, but it can be less accurate for irregular cycles or uncertain period dates.
For someone with a longer or shorter cycle, ovulation may not have happened on the standard day assumed by the calculation. That is one reason an early scan can adjust the estimated date.
Worked example
If the first day of the last menstrual period was 1 May 2026, the standard calculation adds 280 days.
- LMP date: 1 May 2026
- Add 280 days
- Estimated due date: early February 2027
The exact displayed date depends on the calendar calculation, but the principle is 40 weeks from the first day of the last period.
Conception date method
If conception date is known, the due date can be estimated by adding about 266 days. This is because pregnancy dating from LMP includes roughly two weeks before ovulation and conception in a typical cycle.
Conception date can still be uncertain unless ovulation tracking, fertility treatment, or very clear timing is involved. Sperm can survive for several days, so intercourse date and conception date are not always the same.
IVF transfer method
IVF dating can be more precise because embryo age and transfer date are known. The calculation depends on whether the embryo was transferred at day 3, day 5, or another stage. Clinics usually provide the official dating method for the treatment cycle.
Because IVF pregnancies are closely monitored, the clinic or maternity team should be the source of truth for clinical dating.
Ultrasound dating
Early ultrasound can estimate gestational age from measurements. In many cases, the dating scan may be used to confirm or adjust the due date, especially when period dates are uncertain or cycles are irregular.
Later scans are less often used to redetermine the due date because babies naturally vary in size as pregnancy progresses.
Why due dates can change
- The last period date may be uncertain.
- Cycle length may be shorter or longer than average.
- Ovulation may have happened earlier or later than expected.
- An ultrasound may provide a different gestational age estimate.
- IVF dating may use embryo age rather than a standard cycle assumption.
Pregnancy dating scenarios
| Scenario | Best starting point | Why |
|---|---|---|
| Regular cycle and known LMP | Last menstrual period | The standard 40-week method usually fits best |
| Irregular cycles | Early ultrasound | Ovulation may not match the standard assumption |
| Known conception timing | Conception date estimate | Useful when timing is unusually clear |
| IVF pregnancy | Transfer date and embryo age | The clinic knows the embryo stage and treatment timeline |
| Period date uncertain | Dating scan | Clinical measurement can provide a better estimate |
Scenario example: IVF transfer
An IVF pregnancy may be dated from embryo transfer rather than from a natural cycle assumption. A day-5 embryo transfer already includes five days of embryo development, so the calculation is different from simply adding 280 days to a period date.
- Known transfer date: used as the anchor
- Known embryo age: day-3, day-5, or another stage
- Clinic dating: combines transfer date with embryo age
For IVF pregnancies, the fertility clinic or maternity team should confirm the official due date because the transfer details matter.
FAQ
Is a due date exact?
No. It is an estimate used for planning and care.
Why does LMP dating use 40 weeks?
It counts from the first day of the last period, which is usually about two weeks before conception in a typical cycle.
Can ultrasound change my due date?
Yes. A clinical team may update dating based on scan measurements.
Is conception date the same as intercourse date?
Not always. Sperm can survive for several days, so conception may occur later.
Who should confirm my due date?
A qualified healthcare professional or maternity team should confirm pregnancy dating.
This guide is general information only and is not medical advice.