How is my pregnancy due date calculated?

This calculator estimates your due date by adding about 280 days (40 weeks) to the first day of your last menstrual period for a 28-day cycle. If your cycle is longer or shorter, the estimate is adjusted slightly.

What if my cycles are irregular?

If your cycles are irregular, the due date estimate may be less accurate. Your doctor may use ultrasound measurements to give you a more precise due date.

Are online pregnancy calculators accurate?

Online pregnancy calculators provide helpful estimates but are not medical tools. They should not replace prenatal care or the advice of your healthcare provider.

Pregnancy Due Date Calculator (USA)

Estimate your baby’s due date using the first day of your last menstrual period. This free USA pregnancy calculator also shows your current week of pregnancy, trimester, and an approximate conception window. Always confirm results with your doctor or midwife.

Calculate Your Due Date

First day of your last period (LMP)

If you’re not sure, use your best estimate or check your period tracking app.

Average cycle length (days)

Leave blank if unsure. We’ll use 28 days, which is average for many people.

How to Use This Pregnancy Due Date Calculator

Enter the first day of your last menstrual period (LMP).
Enter your average cycle length in days (or leave it as 28 if you’re not sure).
Click “Calculate Due Date”.
Review your estimated due date, current week of pregnancy, and trimester.

How Is Your Due Date Calculated?

This calculator uses a standard method called Naegele’s rule. It adds 280 days (40 weeks) to the first day of your last period for a 28-day cycle. If your cycle is longer or shorter than 28 days, the estimate is adjusted slightly.

Remember, this is only an estimate. Fewer than 5% of babies are born exactly on their due date. Most arrive within two weeks before or after the estimated date.

Pregnancy Weeks and Trimesters

Pregnancy is usually divided into three trimesters:

First trimester: Week 1 to 12
Second trimester: Week 13 to 27
Third trimester: Week 28 to birth (around week 40)

The calculator estimates which week and trimester you are currently in, based on today’s date and your last period.

Limitations of Due Date Calculators

Online due date calculators are helpful for planning, but they are not medical tools. Your actual due date may change based on:

Irregular cycles or uncertain period dates
Ultrasound measurements taken by your doctor
Carrying twins or multiples
Certain medical conditions or complications

Always follow your healthcare provider’s advice and schedule for the most accurate information about your pregnancy.

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Frequently Asked Questions

Is this pregnancy calculator for USA users?

Yes. While pregnancy weeks work the same everywhere, this tool is written for USA users and uses US-style dates and terms.

What if I don’t know the exact date of my last period?

Try using your best estimate based on your memory or a period tracking app. Your doctor can give a more accurate due date using ultrasound.

Do all pregnancies last 40 weeks?

No. Forty weeks is an average. Many healthy babies are born between 37 and 42 weeks. Only a small percentage arrive on the exact due date.

Is my data saved on this website?

No. All calculations happen right in your browser. Your dates and results are not stored or sent to a server.