How is the due date calculated?

Using Naegele's rule: your estimated due date (EDD) is 280 days (40 weeks) from the first day of your last menstrual period. If your average cycle is not 28 days, the difference is added or subtracted. From a known conception date, the EDD is 266 days (38 weeks) later.

How accurate is the estimated due date?

Only about 5% of babies are born on their exact due date. Most births occur within two weeks before or after the EDD. An early ultrasound (before 14 weeks) typically provides the most accurate dating.

What is the difference between LMP and conception date methods?

The LMP method counts from the first day of your last period and assumes ovulation occurs around day 14 of a 28-day cycle. The conception method counts from the actual date of conception (fertilization), which is typically 14 days after LMP in a standard cycle.

When does each trimester begin?

The first trimester runs from week 1 to the end of week 12. The second trimester covers weeks 13 through 26. The third trimester begins at week 27 and continues until delivery.

What does "term" mean in pregnancy?

A pregnancy is considered full term at 39 to 40 weeks. Early term is 37 to 38 weeks. Late term is 41 weeks. Post-term is 42 weeks and beyond.

Can I use this calculator after an ultrasound?

Yes. Select "Ultrasound Dating" and enter the date your ultrasound was performed along with the gestational age confirmed by the sonographer. The calculator will project your due date from that reference point.

Pregnancy Due Date Calculator

Name: Pregnancy Due Date Calculator
Availability: InStock
Author: Nham Vu

Enter your last menstrual period or conception date to instantly calculate your estimated due date, current week, trimester, and upcoming pregnancy milestones.

Pregnancy Information

Calculation Method

Last Menstrual Period (LMP) Conception Date Ultrasound Dating

First Day of Last Menstrual Period

Average Cycle Length (days)

Standard is 28 days. Adjust if your cycle is shorter or longer (21–45 days).

Enter a date and click Calculate Due Date

Your results will appear here

Please enter a valid date.

Summary

Enter your last menstrual period or conception date to instantly calculate your estimated due date, current week, trimester, and upcoming pregnancy milestones.

How it works

Choose your input method: Last Menstrual Period (LMP), Conception Date, or Ultrasound Dating.
Enter the relevant date using the date picker.
Optionally adjust your average cycle length if it differs from 28 days.
Click "Calculate Due Date" to see your estimated due date, current week, trimester, days remaining, and milestone timeline.

Use cases

Estimate your baby's due date before your first prenatal appointment.
Determine which trimester you are currently in.
Plan around key pregnancy milestones such as anatomy scan and viability.
Calculate due date from conception date when LMP is uncertain.

Frequently Asked Questions

Last updated: 2026-06-09 · Reviewed by Nham Vu