About Pregnancy Due Date Calculator
Pregnancy is an exciting journey, but it invariably begins with the question “When will my baby arrive?” Our free due‑date calculator offers a quick, evidence‑based estimate that you can use to plan prenatal visits, prepare your home, and simply day‑dream about meeting your little one. This page explains how the calculation works, what affects accuracy, and why you should always confirm timelines with your healthcare provider.
1. The science behind the calculation
We rely on Naegele’s rule, proposed in 1830 by German obstetrician Franz Karl Naegele. The rule adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). The formula assumes a 28‑day menstrual cycle with ovulation on day 14. Despite being nearly two centuries old, Naegele’s rule remains the global standard for estimating gestational age because it is simple, reproducible, and aligns with the way prenatal milestones are charted.
2. Why “estimated” really means estimated
- Cycle length varies. Only about 13 % of people have a perfect 28‑day cycle. Longer cycles push ovulation (and due date) later, while shorter cycles move it earlier.
- Ovulation timing differs. Even in the same person, ovulation can vary by several days from month to month.
- Implantation window. The fertilised egg can take 6–12 days to implant, altering the hormonal clock that pregnancy tests detect.
- Individual growth rates. Genetics and placental function influence fetal size, so ultrasound measurements shift slightly from the “average” curve.
Because of these factors, only ≈4 % of babies are born on their exact due date. Most arrive within a two‑week window before or after.
3. How ultrasound refines the date
A first‑trimester ultrasound measures crown–rump length (CRL). Research shows this method dates pregnancy within ±5 days for scans done between 7–13 weeks. If the ultrasound estimate differs from LMP by more than one week, clinicians usually adopt the scan date. Second‑trimester scans are less accurate for dating (±10 days) but are still useful for anatomy checks and growth assessment.
4. Special scenarios
IVF pregnancies: Because fertilisation is known precisely, the due date is calculated by adding 266 days to the fertilisation date (or 38 weeks to a day‑3 embryo transfer, 37 weeks to a day‑5 blastocyst transfer). Irregular cycles: If your cycles are longer/shorter than 28 days, add or subtract the difference from the Naegele estimate, or rely on an early ultrasound.
5. Safety disclaimer
This tool is for educational use only. It does not replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider to confirm your personal timeline and to monitor your health and your baby’s development.
6. Data privacy
The date you enter is processed only in your browser and never stored on our servers. If you choose to subscribe to our weekly e‑mails, we store your e‑mail address securely with our e‑mail provider and you can unsubscribe at any time. See our Privacy Policy for details.
7. Sources & further reading
- ACOG Committee Opinion #700 (2021) — Method for Estimating Due Date
- WHO (2016) — Recommendations on Antenatal Care
- Haberman S & Halaska MJ (2024) — Dating of Pregnancy, StatPearls
Last updated: 08 May 2025