Pregnancy Due Date Calculator
Enter your pregnancy dating details to estimate your due date and see your gestational age on a chosen reference date.
General information only — this is an estimate, not a diagnosis or a promised birth date. Confirm dating with your maternity care team.
Use the calculatorEstimate Your Pregnancy Due Date
Choose your dating method. Calculations run in your browser.
Your estimate will appear here
Choose a dating method and enter the relevant date. Results are calendar estimates only.
Estimated due date
A calendar estimate based on the dating method you select.
Pregnancy weeks
Gestational age and stage on your chosen reference date.
Calculation basis
See the assumptions and calendar milestones behind the estimate.
Your pregnancy estimate
Medical disclaimer: This is an educational calendar estimate only. It is not intended for medical diagnosis and must not replace clinical dating by your maternity or fertility care team. It does not confirm pregnancy or measure fetal growth. Use the due date recorded in your medical notes, and discuss any difference with your care team.
Calendar milestones
| Point in gestational age | Calendar date | Meaning in this tool |
|---|
Which date should you use?
How each dating method is calculated
A pregnancy due date is not the same calculation as an age or date-difference result. The starting point depends on how your pregnancy was dated.
| Your information | Calculation | Important limitation |
|---|---|---|
| Last period + regular cycle | LMP + 280 days + (cycle length − 28) | Cycle adjustment is an approximation; it does not measure ovulation. |
| Known conception | Conception + 266 days | An intercourse date is not necessarily a conception date. |
| IVF transfer | Transfer + 266 days − embryo age | Day 3 adds 263 days; day 5 adds 261. Use your clinic’s recorded embryo age. |
| Clinician’s due date | Gestational-age baseline = due date − 280 days | This calendar baseline is not an inferred actual period or conception date. |
Sources: NHS due date calculator for period and cycle dating; ACOG dating guidance for gestational-age conventions and IVF dating.
A 28-day cycle
Last period: 1 January 2026. Adding 280 days gives an estimated due date of 8 October 2026.
At 1 April 2026: 12 weeks, 6 days using this calculation.
A 30-day cycle
The same last-period date with a regular 30-day cycle adds two more days: 10 October 2026.
This tool derives weeks from the adjusted due date, so the age is 12 weeks, 4 days on 1 April.
A day-5 transfer
Transfer: 20 January 2026. Adding 261 days gives an estimated due date of 8 October 2026.
Adding 266 days directly to this transfer date would overlook embryo age.
Gestational age is a calendar count, not a growth measurement
What the weeks actually mean
Why the count starts earlier
Gestational age conventionally starts from the last-period baseline. Known conception is treated as about two weeks into that calendar count. A result of “12 weeks” therefore does not mean conception happened 12 weeks ago.
The progress bar measures elapsed calendar days against 280 days. It does not measure development, viability or health.
When dates need clinical review
If periods are irregular, the period date is uncertain, or a scan gives a different due date, discuss the estimate with your care team. A dating scan can refine the estimate.
For an established clinical due date, use the clinician-provided option. The calculator should not override your medical record.
Reference: NHS guidance on dating scans and ACOG methods for estimating the due date.
Trimester labels used here
For consistency with the NHS week-by-week guide, this tool labels the second trimester from 13 weeks and the third from 28 weeks. Some clinical references use different boundaries; follow your clinician’s convention.
Due date ≠ full-term range. The estimate is a 40-week calendar point. NICHD defines full term as 39 weeks through 40 weeks and 6 days; this is not a recommended delivery window for an individual pregnancy. Verify the term definitions at NICHD.
Helpful resources
Related calculators, pregnancy dating references and guidance to discuss with your care team.
Useful resources
References are for verification; these organisations do not endorse this calculator. For individual pregnancy dating or care, speak to your maternity or fertility team.
Before you rely on an estimate
Questions about pregnancy dating
Can this calculator confirm that I am pregnant?
No. It estimates dates from the information you enter. It cannot confirm pregnancy or assess the health of a pregnancy.
Should I enter the first or last day of my period?
Use the first day of your last menstrual period, not the day bleeding ended. If you are unsure, discuss dating with your maternity care team.
Is a conception date the same as a sex date?
Not necessarily. A date of intercourse does not identify the exact day of fertilisation. Use this method only when the conception date is known; otherwise discuss pregnancy dating with your clinician.
Can I use an IVF transfer date?
Yes. Choose IVF transfer and select the embryo age recorded by your clinic. A day-3 transfer adds 263 days; a day-5 transfer adds 261 days. Confirm the result with your fertility team.
Why might my scan give a different due date?
A period-based estimate relies on menstrual dates and assumptions about ovulation. A dating scan can give a different estimate. Follow the due date agreed with your clinician rather than changing it yourself.
What happens to the dates I enter?
This calculator script uses your entries in browser memory to calculate the result. It does not upload them or save them to browser storage. Other website scripts and services are outside this calculator’s scope.