How accurate is the calculated due date?

The LMP-based calculation assumes a 28-day cycle. Only about 5% of babies are born on their exact due date. Most are born within two weeks either side. Ultrasound measurements (especially in the first trimester) provide a more precise estimate.

What if I have irregular cycles?

Naegele's Rule is less accurate for irregular cycles. If your cycle is longer than 28 days, your actual due date may be later than the calculation suggests. A first-trimester ultrasound is the most reliable way to confirm gestational age with irregular cycles.

What is the difference between gestational age and fetal age?

Gestational age counts from the first day of the last menstrual period — the standard clinical measure. Fetal age counts from conception, approximately two weeks later. When doctors say "you are 10 weeks pregnant," they mean 10 weeks gestational age.

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How to Calculate Pregnancy Due Date Free — EDD Calculator (2026)

By Rui Barreira · Last updated: 18 June 2026

Calculate your estimated due date and current week of pregnancy with the brevio Pregnancy Due Date Calculator — free, no signup, runs entirely in your browser. Enter your last menstrual period (LMP) or conception date and get your due date, trimester, and days remaining instantly.

Knowing your estimated due date helps you plan prenatal appointments, prepare for birth, and understand which trimester you are in. Always confirm your due date with your healthcare provider, who may adjust it based on ultrasound measurements.

How to Use the Tool

Choose your input method. Select "Last Menstrual Period" if you know the first day of your last period, or "Conception Date" if you know when conception occurred.
Enter the date. Use the date picker to select the appropriate date.
Click Calculate Due Date. The tool shows your estimated due date, current weeks and days pregnant, trimester, and days remaining.

How the Due Date Is Calculated

The standard method, known as Naegele's Rule, adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a regular 28-day cycle with ovulation on day 14. Most pregnancies are carried to term between 38 and 42 weeks.

If you know your conception date instead of your LMP, the tool adds 266 days (38 weeks), which accounts for the approximately two weeks between the LMP and ovulation.

Trimester breakdown: the first trimester covers weeks 1–13, the second trimester covers weeks 14–26, and the third trimester covers weeks 27–40.

Frequently Asked Questions

How accurate is the calculated due date?: The LMP-based calculation assumes a 28-day cycle. Only about 5% of babies are born on their exact due date. Most are born within two weeks either side. Ultrasound measurements (especially in the first trimester) provide a more precise estimate and may adjust your due date.
What if I have irregular cycles?: Naegele's Rule is less accurate for irregular cycles. If your cycle is longer than 28 days, your actual due date may be later than the calculation suggests. A first-trimester ultrasound is the most reliable way to confirm gestational age with irregular cycles.
What is the difference between gestational age and fetal age?: Gestational age counts from the first day of the last menstrual period — the standard clinical measure. Fetal age (or embryonic age) counts from conception, which is approximately two weeks later. When doctors say "you are 10 weeks pregnant," they mean 10 weeks gestational age.

Frequently Asked Questions

How accurate is the calculated due date?: The LMP-based calculation assumes a 28-day cycle. Only about 5% of babies are born on their exact due date. Most are born within two weeks either side. Ultrasound measurements (especially in the first trimester) provide a more precise estimate.
What if I have irregular cycles?: Naegele's Rule is less accurate for irregular cycles. If your cycle is longer than 28 days, your actual due date may be later than the calculation suggests. A first-trimester ultrasound is the most reliable way to confirm gestational age with irregular cycles.
What is the difference between gestational age and fetal age?: Gestational age counts from the first day of the last menstrual period — the standard clinical measure. Fetal age counts from conception, approximately two weeks later. When doctors say "you are 10 weeks pregnant," they mean 10 weeks gestational age.