Pregnancy Due Date Calculator

How this due date calculator works

Estimated due dates are commonly calculated from the first day of the last menstrual period (LMP) using Naegele’s rule: add 280 days (40 weeks). That convention assumes a 28-day cycle with ovulation around day 14. Many pregnancies are also dated by ultrasound, which can revise the estimate when LMP is uncertain or cycles are irregular.

Why LMP is used

Conception timing is hard to pin exactly. LMP is a date most people can report, so clinics standardized on it. If cycles are irregular, longer, or shorter than 28 days, the calendar due date may be less accurate — another reason ultrasound dating matters. Known ovulation or assisted reproduction dating follows clinical protocols that this page does not attempt to replace.

What “40 weeks” actually means

Gestational age is commonly counted from LMP, not from conception. That is why a pregnancy is described as about two weeks further along than the time since fertilization under standard assumptions. Trimester labels in educational materials usually follow this LMP-based clock. Your clinician’s chart is the authority if dates are revised.

Worked example

If LMP was March 1, adding 280 days lands near December 6 (exact calendar math depends on month lengths). The page also shows how many weeks pregnant you would be from today relative to that LMP, which is useful for understanding trimester language in articles and class materials — not for self-managing prenatal care.

How to use it

1. Enter the first day of your last menstrual period.
2. Read the estimated due date and current gestational age estimate.
3. Bring the same LMP date to your clinician; follow their dating if ultrasound differs.
4. If your cycle length differs meaningfully from 28 days, say so at your appointment — dating may be adjusted.

Scenario: LMP and ultrasound disagree

Early ultrasound dating sometimes shifts the estimated due date by several days or more. Clinics have rules for when to keep LMP dating and when to adopt ultrasound dating. This calculator always follows the simple 280-day LMP rule so you can see the classic educational estimate; it will not automatically reconcile conflicting clinical dates.

Frequently asked questions

What if I do not know my LMP exactly?

Give your clinician the best estimate and any cycle notes. Ultrasound dating may replace LMP-based dates when dates are uncertain.

Does a due date mean the baby arrives that day?

No. It is an estimate around which birth often occurs within a window of weeks. Only a clinician should interpret timing for your pregnancy.

Can I use this after IVF?

IVF and related treatments usually date pregnancy from procedural milestones, not from a standard LMP assumption. Use your clinic’s dating; treat this page as a general explanation of Naegele’s rule only.

Why show weeks alongside the due date?

Gestational age in weeks is how many prenatal schedules and educational resources are organized. It is derived from the same LMP assumption as the due date on this page.

Limitations

This tool does not diagnose pregnancy, predict complications, or adjust for cycle length, lactation, or assisted reproduction. It is an educational implementation of Naegele’s rule only.

Not a substitute for professional medical care.