Pregnancy Due Date Calculator
Estimate baby due date based on last menstrual period or conception.
This pregnancy due date calculator (a pregnancy calculator) estimates your expected delivery date and current gestational age from your last menstrual period or a known conception date, using the standard 40-week pregnancy timeline. It also works for a pregnancy due date calculator based on conception when that date is known.
Enter your last menstrual period date (or conception date), and the calculator returns your estimated due date and pregnancy week by week milestones.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| Last menstrual period: January 1 | Estimated due date: October 8, 2026 | Naegele's rule: Jan 1 − 3 months = Oct 1 (previous year); + 7 days = Oct 8; + 1 year = Oct 8, 2026. This matches adding 280 days directly to January 1. |
| Last menstrual period: June 15 | Estimated due date: March 22, 2027 | Naegele's rule: Jun 15 − 3 months = Mar 15; + 7 days = Mar 22; + 1 year = Mar 22, 2027. |
How Due Date Is Calculated from Last Menstrual Period
The standard method (Naegele's rule) adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle. This is why due dates are calculated from the last period, not the conception date, even though conception typically occurs about two weeks after the period starts. This method, commonly known as Naegele's Rule after 19th-century German obstetrician Franz Naegele, simply adds 280 days (40 weeks) to the first day of the last menstrual period - a convention still used in clinical practice today even though later research has found actual gestation length varies naturally by more than a week in either direction for a normal, healthy pregnancy.
Due Date Is an Estimate, Not a Guarantee
Only a small percentage of babies are actually born on their exact calculated due date - most arrive within a window of about two weeks before or after, which is why due dates are best understood as a target range rather than a precise prediction.
Common Mistakes When Estimating Due Date
The most common mistake is entering the date a pregnancy test came back positive instead of the first day of the last menstrual period - the two can easily be several weeks apart. Another is assuming a 28-day cycle when yours runs meaningfully longer or shorter, which shifts the actual ovulation and conception timing relative to the standard assumption this calculator (and Naegele's rule generally) is built on. A clinical ultrasound dating estimate, when available, is generally considered more accurate than an LMP-based calculation, especially for anyone with irregular cycles.
Sources & References
Naegele's rule is named for 19th-century German obstetrician Franz Karl Naegele and remains the standard LMP-based due date estimation method taught in obstetric care and referenced by bodies such as the American College of Obstetricians and Gynecologists (ACOG), alongside first-trimester ultrasound dating as a more precise alternative when available.
How to Use the Pregnancy Due Date Calculator
- Open the Pregnancy Due Date Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the pregnancy due date calculator works.
Frequently Asked Questions
How is my pregnancy calculator due date calculated?
It adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle - this is the standard Naegele's rule method.
How many weeks pregnant am I based on my last period?
Count the number of full weeks since the first day of your last menstrual period - this is how gestational age is conventionally measured, even though conception occurs about two weeks later.
Will my baby actually be born on the calculated due date?
Probably not exactly - most babies are born within about two weeks before or after the calculated due date, so it functions as a target range rather than a precise prediction.
How do I use this pregnancy due date calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Pregnancy Due Date Calculator calculate?
Estimate baby due date based on last menstrual period or conception. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this pregnancy due date use?
The formula is: Due Date = LMP + 280 days (Naegele's rule shortcut: LMP − 3 months + 7 days + 1 year). LMP is the first day of your last menstrual period. Adding 280 days (40 weeks) gives the estimated due date directly, but the same result can be found faster by hand using Naegele's rule's practical shortcut: subtract 3 months from the LMP date, add 7 days, then add 1 year. If conception date is known instead, the estimate uses 266 days (38 weeks) from conception, since conception typically occurs about two weeks after the LMP.
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Last updated: September 29, 2026.