How accurate is a pregnancy due date calculator?
By Pregora Editorial Team · Updated 2026-04-26 · 7 min read
A pregnancy due date calculator gives you an estimated due date — but how accurate is that estimate? The honest answer: it's a useful target date, but only about 5% of babies are actually born on their estimated due date. Most healthy full-term pregnancies deliver between 37 and 42 weeks, with the peak around 40 weeks.
That doesn't mean due date calculators are useless. The estimate is medically meaningful — it's used for prenatal appointment scheduling, anatomy scans, and decisions about labor induction. This guide explains exactly how accurate due date calculators are, what affects accuracy, and when ultrasound dating should override the calculator.
The 5% rule: what it actually means
Research published in the journal Human Reproduction tracked thousands of pregnancies and found that the natural variation in pregnancy length is much larger than most people realise. Even after accounting for ovulation timing, embryo age at implantation, and individual differences, the gestational length varies by up to 5 weeks across healthy pregnancies.
Practically, this means:
- About 5% of babies arrive on the exact estimated due date
- About 50% arrive within 7 days before or after
- About 80% arrive within 14 days before or after
- About 95% arrive within 21 days before or after
Use our due date calculator to get your estimated due date — and our pregnancy week calculator to track how far along you are right now.
What affects due date calculator accuracy
1. Cycle length irregularity
Naegele's rule (the formula behind most due date calculators) assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculation needs adjustment. Our calculator handles this automatically when you set your cycle length, but irregular cycles introduce day-to-day uncertainty.
For cycles that vary by more than 7 days from one to the next, calendar-based dating becomes unreliable. In these cases, an early-pregnancy ultrasound is the more accurate option.
2. Uncertain LMP date
About 1 in 4 women cannot recall the exact first day of their last menstrual period when pregnancy is confirmed. Even guessing within a week introduces a meaningful error in the due date estimate. Spotting between periods can also confuse the LMP date.
3. Late or early ovulation
Even with regular cycles, ovulation can shift by 1-3 days due to stress, illness, travel, or hormonal fluctuations. If you ovulated later than expected in the cycle that resulted in pregnancy, the LMP-based due date will be slightly off.
4. Hormonal birth control history
For the first 1-3 cycles after stopping hormonal birth control, ovulation timing can be unpredictable. Calculating a due date based on LMP during this window is less accurate than usual.
When ultrasound dating beats the calculator
The American College of Obstetricians and Gynecologists (ACOG) recommends that an early-pregnancy ultrasound (typically between 7 and 13 weeks) takes priority over LMP-based dating when the two differ by more than 7 days. The first-trimester ultrasound measures the crown-rump length (CRL) and gives gestational age accurate to within ±5 days.
For IVF pregnancies, the situation is even cleaner: the embryo age and transfer date are known exactly, so dating is the most accurate of any method. If you had IVF, use our IVF due date calculator for Day 3, Day 5, Day 6 transfers and FET cycles.
Twin and multiple pregnancies
Twin pregnancies typically deliver earlier than singletons. The average gestational age at delivery for twins is 35-37 weeks, not 40. If you are expecting twins, the calculator's due date is a target but the actual delivery is much more likely to be 2-5 weeks earlier.
Should you trust the calculator?
Yes — within its limits. Use the calculator to:
- Schedule your first prenatal appointment
- Track which week of pregnancy you are in
- Estimate when major milestones (anatomy scan, viability, full term) will occur
- Plan maternity leave timing
- Get a target date for delivery
Don't treat the date as a deadline. Once your healthcare provider performs an early ultrasound, that becomes the definitive due date.
Frequently asked questions
How many days off can a due date calculator be?
For LMP-based calculations with regular cycles, the calculator is typically accurate within ±5-7 days. For irregular cycles or unknown LMP, accuracy can be off by up to 14 days. Early ultrasound dating reduces this to ±5 days.
Can my due date change during pregnancy?
Yes. If your first-trimester ultrasound shows a date more than 7 days different from the LMP-based calculation, your provider will update the due date. After 22 weeks, due date is generally not changed even if measurements suggest otherwise — too much individual variation in size by then.
Why does the calculator add 280 days?
Pregnancy lasts about 266 days from conception, plus an average 14 days from LMP to ovulation = 280 days from LMP. This convention (Naegele's rule) was developed in the 1800s and is still standard worldwide.
What happens if I go past my due date?
Going 1-2 weeks past the due date is normal. Most providers recommend induction around 41 weeks if labor hasn't started naturally. Pregnancies that pass 42 weeks are called "post-term" and warrant closer monitoring.
Medical disclaimer: This article is for general information only and does not constitute medical advice. Always consult your healthcare provider about your individual pregnancy.