How this calculator works
Choose how you’re dating the pregnancy (last menstrual period, a known conception date, or an IVF transfer date), enter that date, and this calculator applies the corresponding standard offset to estimate your due date, plus your current gestational age and trimester.
The formula
LMP: dueDate = LMP + 280 days (Naegele's rule) Conception: dueDate = conceptionDate + 266 days IVF day-3 transfer: dueDate = transferDate + 263 days IVF day-5 transfer: dueDate = transferDate + 261 days Gestational age = today − (LMP-equivalent start date), in weeks + days Trimester: 1st (weeks 0–13) · 2nd (weeks 14–27) · 3rd (weeks 28+)
Worked example
A last menstrual period starting January 1, 2026:
- Due date = January 1, 2026 + 280 days = October 8, 2026
Frequently asked questions (FAQ)
How accurate is a due date estimate?
Only about 5% of babies are born on their exact estimated due date; most arrive within about two weeks before or after. These calculators (and your provider's estimate) give a target range, not a guarantee, and your provider may adjust the estimate based on an early ultrasound.
What's the difference between LMP dating and conception dating?
LMP (last menstrual period) dating adds 280 days to the first day of your last period, per Naegele's rule: the standard method, since LMP is usually known even when the exact conception date isn't. Conception dating adds 266 days directly from a known conception date, which is 14 days shorter since it skips the typical two-week period-to-ovulation gap.
How is an IVF due date calculated differently?
Since the embryo's exact age is known precisely at transfer, IVF due dates are calculated directly from the transfer date: 263 days for a day-3 embryo transfer, or 261 days for a day-5 (blastocyst) transfer. Both land on the same LMP-equivalent timeline as the standard 280-day rule.
What are the three trimesters?
The first trimester runs from conception through week 13, the second from week 14 through week 27, and the third from week 28 until birth (roughly week 40). Each trimester is associated with different typical developmental milestones.
Should I rely on this instead of my doctor's estimate?
No. This is an estimate based on standard dating conventions, for informational purposes only. Your healthcare provider may adjust your due date based on an ultrasound measurement, which is often more accurate than date-based calculations alone, especially if your cycle is irregular.
Sources
Naegele’s rule and standard IVF dating conventions are widely published obstetric standards; no single external citation is needed beyond the formula itself. This calculator is for informational purposes only; always confirm your due date with your healthcare provider.