What Is a Due Date Calculator?
A due date calculator estimates when a baby is likely to arrive, based on the date of your last menstrual period, a known conception date, or an IVF transfer date. It's the same math obstetricians run at your first prenatal visit — nothing more exotic than counting forward a set number of weeks from a known starting point.
Say the first day of your last period was March 1. Running that through the standard formula puts your estimated due date at December 6 — 40 weeks later. 134 days in, that same pregnancy would sit at 19 weeks and 1 day, just crossing into the second trimester. Every number on this page traces back to that one example, so you can follow the math end to end.
Only about 4% of babies are actually born on their calculated due date, according to research published by the American College of Obstetricians and Gynecologists (ACOG). The date is best treated as the center of a window, not a deadline.
How to Use This Calculator
- Calculate Based On — choose whichever starting point you actually know: last period, a confirmed conception date, or an IVF transfer date
- Date — enter that date using the date picker
- Average Cycle Length — only shown for the last-period method; if your cycles run longer or shorter than 28 days, this adjusts the estimate accordingly
- Click "Calculate My Due Date" to see your estimated due date, current week, trimester, and the dates for full term and viability
The Due Date Formula
The standard method is called Naegele's Rule, named after the 19th-century obstetrician who formalized it. It assumes a 28-day cycle with ovulation around day 14, and a pregnancy lasting 280 days from the last period:
How Cycle Length Shifts Your Due Date
Naegele's Rule assumes ovulation happens on day 14 of a 28-day cycle. If your cycles run longer, ovulation typically happens later too — which pushes conception, and the due date, later. Using the same March 1 last-period date, here's how the estimate moves at different cycle lengths:
| Average Cycle Length | Adjustment from 28-day baseline | Estimated Due Date |
|---|---|---|
| 24 days | −4 days | December 2 |
| 26 days | −2 days | December 4 |
| 28 days (average) | — | December 6 |
| 30 days | +2 days | December 8 |
| 32 days | +4 days | December 10 |
| 35 days | +7 days | December 13 |
The gap between a 24-day cycle and a 35-day cycle, working from the identical last-period date, is a full 11 days of difference in the estimated due date. That's a meaningful swing, which is exactly why an early ultrasound — which measures the embryo directly rather than working backward from a period date — tends to override this calculation once one is available.
Trimesters and Key Milestones
Pregnancy is divided into three trimesters, and a handful of dates matter more than the rest for planning appointments and understanding what stage you're in. Continuing the same March 1 example:
| Milestone | Gestational Week | Date (from March 1 LMP) |
|---|---|---|
| 1st trimester | Weeks 1–13 | March 1 – May 31 |
| End of 1st trimester | Week 13 | May 31 |
| 2nd trimester | Weeks 14–27 | May 31 – Sept 6 |
| Viability milestone | Week 24 | August 16 |
| 3rd trimester begins | Week 28 | September 6 |
| Early term | Week 37 | November 15 |
| Full term | Week 39 | November 29 |
| Due date | Week 40 | December 6 |
"Viability" at 24 weeks refers to the earliest point where a baby born prematurely has a meaningful chance of survival with intensive care — it isn't a target, just a clinical reference point care providers use. "Full term" doesn't start until week 39, three full weeks after a baby is technically considered viable. Source: CDC — Preterm Birth.
Dating by Conception or IVF Transfer
If you know your conception date — common after ovulation tracking or a monitored IVF cycle — the math shifts slightly, since conception is roughly 14 days later than the last period in a standard 28-day cycle. For IVF specifically, fertility clinics count from the transfer date instead, adjusting for how many days the embryo had already developed before transfer:
All three methods land on the identical due date because they're describing the same pregnancy from different known starting points — the formulas are just Naegele's Rule shifted to account for how many days into development each method's "day zero" actually is.
How Accurate Is a Due Date, Really?
Not very, in the narrow sense of predicting an exact day — and that's normal, not a sign anything is wrong. ACOG defines a full-term delivery as anywhere from 39 weeks 0 days to 40 weeks 6 days, and "term" more broadly spans 37 to 42 weeks. A first pregnancy, in particular, runs slightly longer than average as a group.
| Delivery Window | Classification | Approx. Share of Births |
|---|---|---|
| Before 37 weeks | Preterm | ~10% |
| 37w 0d – 38w 6d | Early term | ~26% |
| 39w 0d – 40w 6d | Full term | ~44% |
| 41w 0d – 41w 6d | Late term | ~15% |
| 42w 0d and beyond | Post-term | ~5% |
Source: CDC National Vital Statistics Reports — Births: Final Data. Roughly 60% of babies arrive somewhere in a three-week window centered on the calculated due date, not on the date itself — which is why care providers talk about a due "window" more than a due "day."
What to Do With This Date
- Bring it to your first prenatal visit. Your provider will likely confirm or adjust it using an early ultrasound, which is generally considered more accurate than a period-based estimate, especially if your cycles are irregular.
- Use it to time key screenings. First-trimester screening typically happens between weeks 11 and 14; the anatomy scan usually falls between weeks 18 and 22 — both timed off this due date.
- Don't treat it as a deadline. With roughly 44% of births landing in the 39–40 week full-term window and meaningful numbers on either side, planning flexibility around the date matters more than hitting it exactly.
- Re-check it after an ultrasound. If an early scan shifts your date by more than about 5–7 days from the calculation here, providers typically use the ultrasound date going forward.
- Track weeks, not just the final date. Appointment schedules, testing windows, and leave planning all key off the current week number more than the due date itself.
Reference: ACOG — Fetal Development FAQs | CDC — Maternal and Infant Health | NICHD — Pregnancy