How is the pregnancy due date calculated?

The due date is 40 weeks (280 days) from the first day of your last menstrual period (LMP). Only about 5% of babies are born on their exact due date - most arrive within 2 weeks before or after.

What if I don't know my LMP?

If you don't know your LMP, an early ultrasound (8-12 weeks) can give an accurate due date based on fetal measurements. After 13 weeks, ultrasound dating becomes less precise.

How accurate are due dates?

Due dates are estimates, not deadlines. Only 4-5% of babies are born on their exact due date. About 80% arrive within 10 days of it, either before or after. First pregnancies often go past the due date.

Pregnancy has three trimesters, each about 13 weeks. The first trimester (weeks 1-13) is when most development happens. The second (14-27) is often called the 'golden period.' The third (28-40) is final growth and preparation for birth.

Pregnancy Due Date Calculator - Free

Enter your last menstrual period

Type the first day of your last period. This is the standard reference date used by most obstetricians.

We add 280 days

Naegele's rule: due date = LMP + 280 days (40 weeks). The calculator also shows the date range (38-42 weeks is full term).

See week-by-week milestones

Get your current week, trimester, and key dates: first prenatal visit, anatomy scan, viability (24w), and full term (37w).

Key things to know about our pregnancy due date calculator:

Due date accuracy: Only 5% of babies arrive on their exact due date. Most come within 2 weeks before or after. Treat the due date as a range, not a deadline.
Conception date: If you know your conception date (e.g. from IVF or tracking), add 266 days for an alternate due date. Conception typically occurs 14 days after LMP.
First vs subsequent pregnancies: First pregnancies average 40 weeks + 1 day. Subsequent pregnancies average 40 weeks + 0 days, but individual variation is large.

For related calculations, try our pregnancy calculator.

Frequently asked questions

Everything you need to know about this calculator. Can't find what you're looking for? Email our team.

A pregnancy due date calculator is a free tool that estimates when your baby will arrive based on your last menstrual period, conception date, or ultrasound dating. Our pregnancy due date calculator uses the Naegele rule plus adjustments for irregular cycles. The pregnancy due date calculator also shows a week-by-week timeline of fetal development milestones.

How to estimate your pregnancy due date

Our pregnancy due date calculator lets you estimate your baby's due date and current week of pregnancy using Naegele's Rule in three steps.

1
Enter the first day of your last period
Type the first day of your last menstrual period (LMP). This is the anchor date for Naegele's Rule.
2
Enter your average cycle length
Type the typical length of your menstrual cycle in days. The default 28 days works for most people.
3
Read your due date and current week
The calculator shows the estimated due date, your current week and trimester, and key milestone dates for each trimester.

Pregnancy Due Date Calculator

Pregnancy Due Date Calculator

Enter Your LMP Date

How Due Date is Estimated

How our pregnancy due date calculator works

Enter your last menstrual period

We add 280 days

See week-by-week milestones

Key things to know about our pregnancy due date calculator:

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Frequently asked questions

How to estimate your pregnancy due date

Enter the first day of your last period

Enter your average cycle length

Read your due date and current week