Due Date Calculator

Estimate your due date and important pregnancy milestones based on your last period, conception date, ultrasound, or IVF transfer.

First day of last period

Average cycle length

Pregnancy timeline

How Does a Due Date Calculator Work?

A due date is only an estimate. The actual birth date can be earlier or later. A healthcare provider may confirm or adjust the due date using ultrasound measurements, especially during early pregnancy.

Cleveland Clinic notes that ultrasound is commonly used to confirm due dates and is considered a highly reliable method for pregnancy dating.

Calculation methods

Different Ways to Estimate

This calculator supports common pregnancy dating methods, so you can use the information you know best.

Last Menstrual Period (LMP): The most common method because many people know the first day of their last period. Standard formula: LMP + 280 days.
Conception Date: If the exact conception date is known, the calculation is simply Conception Date + 266 days.
Ultrasound Date: Calculated by taking the ultrasound date and gestational age, then predicting the remaining days to 40 weeks.
IVF Transfer: Depends on embryo age. A 5-day embryo transfer assumes 261 days remaining; a 3-day embryo transfer assumes 263 days.

Math and logic

The Math Behind the Date

Each method estimates the same 40-week pregnancy timeline from a different starting point.

Using last menstrual period

\mathrm{Due\ Date}=\mathrm{LMP}+280\ \mathrm{days}

This assumes a 28-day cycle.

Cycle-adjusted LMP

\mathrm{Adjusted\ Due\ Date}=\mathrm{LMP}+280+(\mathrm{Cycle\ Length}-28)

This adjusts the LMP method for shorter or longer cycles.

Using known conception

\mathrm{Due\ Date}=\mathrm{Conception\ Date}+266\ \mathrm{days}

This uses a known conception date instead of the last period date.

IVF formulas use the transfer date and embryo age: a day-3 embryo uses transfer date + 263 days, and a day-5 embryo uses transfer date + 261 days.

Pregnancy planning

Why Due Date Calculation Is Important

Knowing the estimated due date helps plan pregnancy care and gives a timeline for appointments, screenings, fetal growth checks, and birth preparation.

Milestones: A due date helps estimate current pregnancy week and trimester milestones.
Prenatal tests: It provides timing windows for important prenatal tests, screening appointments, fetal growth checks, and birth preparation.
Documentation: ACOG says that once LMP data, the first accurate ultrasound, or both are available, the estimated due date should be determined, discussed, and clearly documented.

Accuracy

Is the Due Date Always Accurate?

No. A due date calculator gives an estimate, not an exact birth date. It can be more helpful to think of the due date as part of a range.

Your healthcare provider may give a more accurate due date after an ultrasound. Cleveland Clinic also says the provider's due date should be treated as the official due date.

Due Date Calculator FAQ

Frequently Asked Questions

A due date calculator estimates the expected delivery date during pregnancy. It can use the last menstrual period, conception date, ultrasound date, or IVF transfer date.

The most common formula is: Due Date = First Day of Last Period + 280 Days.

Pregnancy weeks are usually counted from the first day of the last menstrual period, not from conception. This is because many people know their last period date more easily than the exact conception date.

Yes. A healthcare provider may adjust the due date based on ultrasound measurements or other pregnancy information. Cleveland Clinic notes that due dates can change, usually by a few days.

No. The due date is an estimate. Many babies are born before or after the estimated date.

For IVF, use the embryo transfer date and embryo age. Common formulas are transfer date + 263 days for a day-3 embryo and transfer date + 261 days for a day-5 embryo.

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