Due date calculator
Your estimated due date (EDD) is the day your baby is most likely to arrive — calculated as 40 weeks from the first day of your last period, or 38 weeks from conception. Enter either date below and you'll also see your current pregnancy week and trimester. Remember that a due date is an estimate, not a deadline: only about 1 in 20 babies actually arrives on the exact date, and an early ultrasound is the most accurate way to confirm it.
How the due date is calculated
Most due dates come from Naegele's rule, the standard method clinicians have used for over a century: take the first day of your last menstrual period (LMP) and add 280 days, or 40 weeks. If you know your conception date instead, the due date is 266 days later — conception happens about two weeks after your period starts, which is why the two methods land roughly two weeks apart. Both assume a textbook 28-day cycle with ovulation around day 14.
An early ultrasound is the most accurate dating method of all. Before 14 weeks, measuring the baby's crown-rump length pins down gestational age within a few days, which is why providers often revise an LMP-based date after the first scan (ACOG, Methods for Estimating the Due Date). However it's calculated, the EDD is a best estimate of a 37-to-42-week window, not a fixed appointment. This tool is for general information and is not medical advice — always confirm your due date with your healthcare provider.
Common questions
A due date marks the single most likely day, but only about 4–5% of babies are born on it. Roughly 9 in 10 arrive within two weeks either side. The estimate is most reliable when your cycle is regular and confirmed by an early ultrasound.
Yes, and it commonly does. If a first-trimester ultrasound shows the baby measuring more than about a week off the LMP-based date, your provider will usually update the due date to the scan. After 14 weeks, ultrasound dating becomes less precise, so early changes are the ones that stick.
An early ultrasound (before 14 weeks) is more accurate than counting from your last period, because it measures the baby directly rather than assuming a 28-day cycle. ACOG recommends using ultrasound dating when it differs meaningfully from the LMP estimate. Both are good starting points; your provider reconciles them.
Naegele's rule assumes ovulation on day 14, so a longer cycle pushes ovulation — and your true due date — later, while a shorter cycle pulls it earlier. As a rough guide, add the number of days your cycle exceeds 28 (or subtract if it's shorter). An ultrasound removes this guesswork entirely.
From your last period. Gestational age is measured from the first day of your LMP, which is about two weeks before you actually conceive. That means at the moment of conception you're already considered "two weeks pregnant," and the baby's actual age (fetal age) is roughly 38 weeks at birth, not 40.
Full term is 39 to 40 weeks and 6 days, when babies have the best outcomes. The wider healthy range runs 37 to 42 weeks: 37–38 weeks is "early term," 41 weeks is "late term," and past 42 weeks is "post-term." Birth before 37 weeks is considered preterm.
Medical Disclaimer
This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or qualified health provider with any questions you may have regarding a medical condition.
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