The Complete Guide to Estimating a Pregnancy Due Date
By Arshi Khan, Founder of Just4Marketing · Last reviewed October 2026
A due date is one of the first questions every expecting parent asks, but it's always an estimate — not a prediction of the exact day. This guide explains the three dating methods this calculator supports, how each one works, and why the actual birth date so often falls on a different day.
What is a due date and why does it matter?
A due date is an estimate of when a full-term pregnancy (40 weeks) will reach its end, calculated from a known reference point — the last menstrual period, conception, or an IVF transfer date. It gives parents and doctors a planning target for prenatal appointments, screenings, and preparation, even though actual delivery dates vary widely around it.
This calculator also reports your current week and day of pregnancy and which trimester you're in, based on the number of days elapsed since the LMP-equivalent date.
How trimesters and dating methods work
| Stage / Method | Definition |
|---|---|
| First trimester | Weeks 0–12 (0–13 weeks pregnant) |
| Second trimester | Weeks 13–26 |
| Third trimester | Week 27 onward |
| LMP method | Due date = last period start date + 280 days |
| Conception method | Due date = conception date + 266 days |
| IVF method | Due date = transfer date − embryo age + 266 days |
Limitations to keep in mind
It assumes a standard 28-day cycle
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. Shorter or longer cycles shift the actual conception date, which shifts the true due date.
Ultrasound dating can be more precise
Early first-trimester ultrasounds measure fetal size and can refine the estimated due date, especially when the LMP date is uncertain.
Full-term babies arrive across a 5-week window
Only about 5% of babies are born exactly on their due date — most arrive somewhere between 3 weeks early and 2 weeks late.
This tool doesn't replace prenatal care
Always confirm dating and monitor pregnancy progress with a doctor or midwife — this calculator is for general planning only.
Frequently asked questions
What is Naegele's rule?
Naegele's rule is the standard method for estimating a due date from the last menstrual period (LMP): add 280 days (40 weeks) to the first day of the LMP. It assumes a 28-day cycle with ovulation on day 14, which is why it's an estimate rather than exact for people with different cycle lengths.
How does the conception-date method differ from LMP?
If you know your conception date instead of your LMP, the calculator adds 266 days (38 weeks) directly, since conception typically happens about 2 weeks after the LMP — the two methods are designed to reach the same due date when both dates are known accurately.
How is an IVF due date calculated?
For IVF, the due date is calculated from the transfer date minus the embryo's age at transfer (typically 3 or 5 days), then treated like a conception date 14 days after LMP — giving one of the most precise due date estimates since the exact embryo age is known.
How often do babies arrive on their exact due date?
Only about 5% of babies are born on their calculated due date. Most arrive within a 'term' window of anywhere from 3 weeks before to 2 weeks after — the due date is best treated as the middle of a range, not a deadline.