Due Date Calculator
Estimate your baby’s due date from your last menstrual period, conception, or ultrasound.
Enter the first day of your most recent period.
Average length of your menstrual cycle (used for the LMP method only).
Estimated Due Date
Days Remaining
Gestational Age Today
Current Trimester
How the Due Date Calculator Works
Your estimated due date (EDD) is the date around which your baby is likely to be born — about 40 weeks (280 days) from the first day of your last menstrual period, or 38 weeks (266 days) from conception. Only about 4% of babies arrive on their exact due date; most are born within two weeks on either side. The due date is a clinical estimate used to track the growth of your baby, schedule prenatal tests, and gauge whether labour is early, on time, or overdue.
Pregnancy Milestones at a Glance
| Stage | Gestational Age | Notes |
|---|---|---|
| First Trimester | 0w 0d – 13w 6d | Organs form; highest risk of early loss. |
| Second Trimester | 14w 0d – 27w 6d | Anatomy scan; movements usually felt. |
| Third Trimester | 28w 0d onward | Rapid growth; preparation for birth. |
| Full Term | 39w 0d – 40w 6d | Optimal window for delivery. |
| Due Date (EDD) | 40w 0d | 280 days from LMP / 266 days from conception. |
Calculation Methods Explained
Last Menstrual Period (Naegele's Rule)
The classic method, introduced by Franz Naegele in the 19th century, adds 280 days (40 weeks) to the first day of your last menstrual period. It assumes a regular 28-day cycle with ovulation on day 14. Because conception actually depends on when you ovulate, this calculator adjusts the result for your cycle length: a longer cycle pushes the due date later, a shorter cycle earlier.
Due Date = LMP + 280 days + (cycle length − 28) days
Conception / Ovulation Date
If you know the date of conception — for example from a tracked ovulation date or a known intercourse date — the due date is simply 266 days (38 weeks) later. This bypasses the 14-day assumption built into Naegele's rule and is often more accurate for women with irregular or non-standard cycles.
Due Date = Conception Date + 266 days
IVF Embryo Transfer
With IVF the embryo's age at transfer is known precisely, so the due date can be pinned to the transfer date. From the 266-day conception interval we subtract the embryo's age in days: 5 days for a day-5 blastocyst transfer, or 3 days for a day-3 cleavage-stage transfer.
Day-5 transfer: Due Date = Transfer Date + 261 days
Day-3 transfer: Due Date = Transfer Date + 263 days
Frequently Asked Questions
How accurate is my due date?
A due date is an estimate, not a deadline. Only about 1 in 20 babies arrives on the exact date. A first-trimester ultrasound is the most accurate way to date a pregnancy and may shift the estimate by a few days.
What is gestational age?
Gestational age counts pregnancy from the first day of your last period, which is roughly two weeks before conception. It is expressed in completed weeks and days, such as "12w 3d", and is the standard measure used by clinicians.
Why does cycle length matter?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, you ovulate and conceive on a different day, so the calculator shifts the due date by the difference between your cycle length and 28 days.
Can the due date change during pregnancy?
Yes. Your healthcare provider may revise it after an early dating ultrasound. Once an accurate date is established, it is usually not changed by later scans.
References
The conventions and gestational milestones used by this calculator are based on established obstetric guidelines:
- American College of Obstetricians and Gynecologists (ACOG). "Methods for Estimating the Due Date." Committee Opinion No. 700. ACOG Committee Opinion 700
- Naegele, F.K. (1830). Lehrbuch der Geburtshilfe. The original basis of the 280-day rule still used in modern obstetrics.
- Jukic, A.M. et al. (2013). "Length of human pregnancy and contributors to its natural variation." Human Reproduction, 28(10), 2848-2855. DOI: 10.1093/humrep/det297
- NHS. "Working out your due date." NHS Pregnancy Due Date
Note: This calculator provides an estimated due date based on standard obstetric formulas and should not replace professional medical advice. A dating ultrasound performed by your healthcare provider is the most reliable way to confirm your due date. Individual pregnancies vary, and the actual date of birth may differ from this estimate. Always consult your doctor or midwife for personalised guidance.
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