Due Date Calculator
Enter the first day of your last period and your cycle length — the calculator shows your estimated due date, current pregnancy week, trimester, and how many days are left.
verified Last updated: May 2026 · Naegele's rule · ACOG guidelines
How the calculator works
The calculator uses Naegele's rule — the most commonly used clinical method for estimating a due date, in practice for over 200 years. Add 280 days (40 weeks) to the first day of your last period and that's the estimated due date.
The classic rule assumes a 28-day cycle with ovulation on day 14. If your cycle runs longer or shorter, the calculator adjusts the estimate — a longer cycle pushes the date slightly later, a shorter one pulls it earlier. The adjustment assumes the cycle length difference reflects later or earlier ovulation.
Gestational age is counted from the first day of your last period, not from conception — that's the medical standard. So at week 4 of pregnancy, the baby is actually about 2 weeks old; the offset stays constant throughout the pregnancy.
The most accurate estimate comes from an early ultrasound between weeks 8 and 12, when the baby is measured (CRL — crown-rump length) and the date is adjusted to biological age. If your LMP-based and ultrasound-based dates differ by more than a week, the doctor usually switches to the ultrasound date.
How accurate is the due date
The due date is a guide, not a fixed date. Long-term studies show:
- About 5% of babies are born on the exact calculated date.
- Around 80% are born within two weeks before or after.
- A full-term pregnancy is between 37 and 42 weeks — anything in that range is considered normal.
- Before 37 weeks is preterm; past 42 weeks calls for closer monitoring.
In other words: the date helps you plan and follow the pregnancy, but don't expect labor on the exact day. Most doctors plan check-ups around the due date and may intervene after 41+0 weeks.
Trimesters and pregnancy weeks
| Trimester | Weeks | What's happening |
|---|---|---|
| 1st | 1–13 | Organ formation, highest miscarriage risk. Nausea and fatigue are typical. |
| 2nd | 14–27 | Energy returns, baby starts to grow visibly. Often the most comfortable part of pregnancy. |
| 3rd | 28–40+ | Baby prepares for birth. Symptoms may return, but you're close to the end. |
Pregnancy week is counted from the first day of your last period, not from conception.
Methodology
The calculator uses Naegele's rule — the standard in clinical practice per ACOG (American College of Obstetricians and Gynecologists) and NHS guidelines. Formula: due date = LMP + 280 days + (cycle_length − 28).
Accuracy: ~5% of births fall on the exact date; ~80% within ±2 weeks; full-term pregnancy 37–42 weeks (Mongelli et al. 2007; ACOG Committee Opinion 700, 2017).
Last reviewed: May 2026. Naegele's rule has been unchanged since 1812 — the date marks when our implementation was last verified against the clinical guidelines.
About the tool: KidyGrow was built by a parent and software engineer who wanted a simpler way to track pregnancy and early parenting. The goal isn't to replace your OB-GYN, but to give you a reference you can cross-check with your provider.
help_outlineFAQ
How is a due date calculated?
Naegele's rule: add 280 days (40 weeks) to the first day of your last period. The calculator further adjusts for your cycle length — a longer cycle pushes the date later, a shorter one earlier.
The approach assumes a regular cycle and ovulation around day 14. For irregular cycles, an early ultrasound is more accurate.
How accurate is the due date?
The due date is a guide, not a fixed date. About 5% of babies are born on the exact calculated date; around 80% within two weeks of it. Full-term pregnancy is 37–42 weeks — anything in that range is considered normal.
What if my cycles are irregular?
Naegele's rule assumes a regular cycle. For irregular periods, it can be off by more than a week — the most reliable date comes from an early ultrasound (8–12 weeks).
If you have an ultrasound-based date, use that instead of the LMP calculation.
When does the due date change after an ultrasound?
If the ultrasound date differs from the LMP-based date by more than a week in the first trimester, the doctor usually switches the official date to the ultrasound one.
In the second and third trimesters, ultrasound estimates become less precise (±2 weeks), so the original date is usually kept.
What is a trimester and how long does it last?
1st: weeks 1–13 — organ formation, highest miscarriage risk. 2nd: weeks 14–27 — energy returns, baby visibly grows. 3rd: weeks 28–40+ — baby prepares for birth.
Can I use this calculator after my due date?
Yes — it will show how many days have passed. If you've gone past week 41, talk to your doctor: closer monitoring (NST, ultrasound) typically begins at 41+0 weeks due to increased risk.
Sources
- ACOG Committee Opinion 700 (2017). Methods for Estimating the Due Date.
- Mongelli M., Wilcox M., Gardosi J. (2007). Estimating the date of confinement.
- NHS — Pregnancy due date calculator
- American Pregnancy Association — Pregnancy week-by-week guides.
Informational only — not a substitute for prenatal care. All pregnancy decisions belong with your provider based on a clinical exam.