Pregnancy Due Date Calculator
Figuring out when your baby is due is usually the first thing on your mind after a positive test. This calculator estimates your due date using whichever information you have — your last menstrual period, a known conception date, or an IVF embryo transfer date — and breaks down your trimester dates and key pregnancy milestones. As with any due date estimate, treat this as a starting point for conversations with your healthcare provider, not a diagnosis.
Last updated: July 17, 2026
Choose your calculation method
This calculator provides an estimate only, not a medical diagnosis. Due dates calculated from LMP, conception, or IVF transfer dates are statistical averages — only about 1 in 20 babies is born on their exact due date, and a full-term pregnancy can range from 37 to 42 weeks. Your healthcare provider's estimate, especially if based on an early ultrasound, is more accurate than any online calculator and should be used for medical decisions. This tool does not replace prenatal care.
Our calculation rules and editorial review process are documented in our Methodology and Editorial Policy.
This calculator provides an estimate only, not a medical diagnosis. Due dates calculated from LMP, conception, or IVF transfer dates are statistical averages — only about 1 in 20 babies is born on their exact due date, and a full-term pregnancy can range from 37 to 42 weeks. Your healthcare provider's estimate, especially if based on an early ultrasound, is more accurate than any online calculator and should be used for medical decisions. This tool does not replace prenatal care.
How due dates are calculated
From your last menstrual period (LMP):The most common method, based on Naegele's Rule — adding 280 days (40 weeks) to the first day of your last period. This assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter than 28 days, the calculator adjusts by the difference.
From conception date: If you know roughly when you conceived (from ovulation tracking or a single-encounter cycle), your due date is 266 days (38 weeks) from that date — two weeks less than the LMP method, because LMP dating counts from before ovulation actually happens.
From IVF transfer date:For IVF pregnancies, due date is calculated from the transfer date adjusted for embryo age at transfer (Day 3 or Day 5), since the embryo's actual conception date is already known precisely — this tends to be the most accurate method available.
Why your due date might change
Ultrasound dating in the first trimester can be more accurate than LMP-based dating, especially if your cycle is irregular or you're unsure of your last period date. Doctors will often adjust your due date after your first ultrasound if it differs by more than about a week from the LMP estimate. This is normal and not cause for concern.
What "40 weeks" actually means
Pregnancy is measured as 40 weeks from the first day of your last period — but conception typically happens about 2 weeks after that, meaning you're actually “pregnant” for closer to 38 weeks by the time you reach your due date. This is a source of common confusion and worth explaining clearly, since it's one of the most-searched sub-questions on this topic.
Your trimester dates explained
Use the trimester breakdown in the calculator above to see your specific date ranges. In general:
- Trimester 1: weeks 1–13
- Trimester 2: weeks 14–27
- Trimester 3: weeks 28–40 (until birth)
“Full term” is generally considered 39–40 weeks, “early term” 37–38 weeks, and “late term” 41 weeks, per standard obstetric terminology — babies born 37–42 weeks are all considered within a normal range.
How accurate is a due date calculator?
Due date calculators, including this one, provide a statistical estimate based on population averages. Only about 5% of babies are born on their exact calculated due date. Most births happen within about a week before or after the estimate, and a full-term range spans 37 to 42 weeks. The accuracy depends heavily on the input method — a known IVF transfer date is precise, while LMP-based dating depends on regular cycles and accurate recall of your last period's start date. If you're unsure of your dates, an early ultrasound (ideally in the first trimester) is the most reliable way to confirm.
Our calculation rules and editorial review process are documented in our Methodology and Editorial Policy.
Frequently asked questions
Add 280 days (40 weeks) to the first day of your last menstrual period. This is known as Naegele's Rule and assumes a 28-day cycle with ovulation on day 14.
Yes. If an early ultrasound estimate differs from your LMP-based due date by more than about 5–7 days (in the first trimester), your provider may adjust your due date to match the ultrasound estimate, which is often more accurate.
They're estimates, not predictions. Only around 1 in 20 babies are born exactly on their calculated due date — most arrive within a week or two of the estimate, and anywhere from 37 to 42 weeks is considered a normal, full-term range.
LMP-based due dates count from the first day of your last period, before ovulation occurs — so they estimate a 40-week pregnancy. Conception-date-based calculations start about two weeks later, so they estimate 38 weeks from that date. Both point to roughly the same due date if your cycle is regular.
Because the embryo transfer date and embryo age (Day 3 or Day 5) are known precisely, IVF due dates are calculated directly from that date rather than relying on LMP or ovulation estimates — making this method generally more precise than LMP-based dating.
The standard 280-day calculation assumes a 28-day cycle. If your cycles run longer or shorter, ovulation happens later or earlier relative to your period, which shifts your actual conception date — adjusting for cycle length improves the estimate.
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Written & Reviewed by
DateCalx Team
Editorial Team
The DateCalx Team researches, builds, tests, and reviews every calculator to ensure accurate, reliable, and easy-to-understand date and time calculations. Our goal is to provide fast, trustworthy tools for everyday personal, educational, and professional use.
Last reviewed: July 17, 2026
Learn more in our About, Editorial Policy, and Methodology pages, or contact the DateCalx Team.