Due Date Calculator

What this due date calculator does

A pregnancy due date is an estimate of when a baby will be born. This calculator uses the last menstrual period, conception date, an ultrasound estimate, or an IVF transfer date to calculate a due date.

Only a small share of babies are born on the exact due date. The estimate is a window, not an appointment.

How to use this calculator

1. Choose what you are basing the estimate on.
2. Enter the relevant date.
3. For a last-period estimate, enter the average cycle length.
4. For an ultrasound estimate, enter the gestational age measured at the scan.
5. For IVF, choose the embryo age at transfer.
6. Click Calculate.

The default example uses today's date as the last period and a 28-day cycle.

Due date formulas

Last period: Due date = LMP + 280 days + (cycle length − 28)
Conception:  Due date = conception date + 266 days
Ultrasound:  Due date = scan date + (280 − gestational age in days)
IVF:         Due date = transfer date + (266 − embryo age)

The 280-day rule is called Naegele's rule. It assumes a 28-day cycle and ovulation around day 14. The cycle-length adjustment moves the estimate when the cycle is shorter or longer.

What the results mean

The result is the estimated due date. A clinician may revise it after an early ultrasound, especially if the scan dating differs from the last-period estimate by more than a few days.

The due date is used to schedule prenatal visits and estimate gestational age. It is not a prediction of the exact birth date.

Assumptions and limitations

The calculator assumes a regular cycle for the last-period method and a valid scan date for the ultrasound method. It does not replace clinical dating, which considers ultrasound measurements, cycle history, and medical factors.

It does not estimate the probability of delivery on a specific day, and it does not account for twins, growth restriction, or planned early delivery.

Frequently asked questions

How accurate is the last-period method?

It is a good starting point for a regular 28-day cycle, but it can be off if ovulation occurred earlier or later. Ultrasound dating is often more accurate in early pregnancy.

Why is the due date 40 weeks from the last period?

Pregnancy is measured from the first day of the last period, not from conception. That adds about two weeks before ovulation.

Can I use this after IVF?

Yes. Choose the IVF mode and the embryo age at transfer. The transfer date and embryo age determine the estimated due date.

What if my cycle is not 28 days?

Enter your average cycle length. The calculator adjusts the due date by the difference from 28 days.

Source

References:

• ACOG — Calculating your due date
• NICHD — Pregnancy

Disclaimer

This calculator is for informational and educational purposes only. It is not prenatal care or medical advice. Confirm your due date with a qualified obstetric clinician.