How accurate is my due date?

Only about 5% of babies arrive exactly on their due date. Due dates are best understood as a 5-week window: most healthy births happen between 37 and 42 weeks of pregnancy. An early ultrasound (before 12 weeks) is the most accurate way to confirm your due date — it can be within 3–5 days. Later ultrasounds are less precise.

Can my due date change?

Yes. Your healthcare provider may adjust your due date based on your first-trimester ultrasound measurements. If your ultrasound date differs from your LMP-based date by more than 7 days (before 9 weeks) or more than 10 days (9–13 weeks), providers typically use the ultrasound date.

What is the LMP method?

LMP stands for Last Menstrual Period. Most due date calculations use this date because conception is difficult to know precisely. The standard formula (Naegele's Rule) adds 280 days to your LMP date, assuming a 28-day cycle. Our calculator adjusts for your actual cycle length — important if yours differs from 28 days.

What if I have irregular periods?

If your cycles are irregular, the LMP method is less reliable. You can use your conception date instead if you know it (e.g. from IVF, ovulation tracking, or a known single encounter). Your provider will use an early ultrasound to establish a more accurate due date.

What does "full term" mean?

Pregnancy is considered full term at 39–40 weeks. Babies born between 37–38 weeks 6 days are "early term"; 39–40 weeks 6 days is "full term"; 41–41 weeks 6 days is "late term"; 42+ weeks is "post-term." Babies born before 37 weeks are premature (preterm).

What are early signs of labour I should watch for?

As you approach your due date, watch for: regular contractions that get closer together and more intense, lower back pain that radiates to the front, your water breaking (a gush or slow leak of fluid), or your mucus plug being released (a thick, sometimes bloody discharge). When in doubt, call your healthcare provider.

Can I calculate my due date after IVF?

Yes. For IVF pregnancies, due dates are calculated differently. For a 3-day embryo transfer, the due date is 263 days after transfer. For a 5-day blastocyst transfer, it's 261 days. Your fertility clinic will typically provide your due date directly, and it's very precise because the exact fertilisation date is known.

Due Date Calculator

Find out when your baby is due — and see your current pregnancy week, trimester, and fetal development milestones.

Understanding Your Due Date

Your due date — officially called the estimated due date (EDD) — is calculated using Naegele's Rule, the standard formula used by healthcare providers worldwide since the 19th century. It adds 280 days (40 weeks) to the first day of your last menstrual period (LMP).

If your cycle length differs from the standard 28 days, our calculator adjusts accordingly. A 32-day cycle shifts your due date 4 days later; a 24-day cycle shifts it 4 days earlier. This is the same adjustment your midwife or OB would make.

Important: Due dates are estimates, not deadlines. Healthy, full-term pregnancies can end anywhere between 37 and 42 weeks. An early first-trimester ultrasound is the most accurate way to confirm your due date — it can pinpoint your baby's gestational age within 3–5 days.

Due Date Calculator — Frequently Asked Questions

Sources & Methodology

Last reviewed: October 17, 2026

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Understanding Your Due Date

Due Date Calculator — Frequently Asked Questions

Sources & Methodology

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