Pregnancy Due Date Calculator — Estimate Your Baby's Arrival

Here’s how due dates are calculated.

Most pregnancy due date calculators add 280 days (40 weeks) to the first day of your last menstrual period. That 280-day number comes from the average length of human pregnancy from LMP to birth. It sounds simple, but here's the nuance: pregnancy is dated from the first day of your period, not from conception, because most people know when their period started and few know the exact moment of ovulation.

The 280-day rule assumes a 28-day cycle with ovulation around day 14. If your cycle is longer, like 32 days, ovulation tends to happen later. A good calculator adjusts the due date by adding the difference between your cycle length and 28. Shorter cycles work the same way in reverse.

Naegele's Rule and the Standard Formula

Naegele's rule is the traditional method still taught in obstetrics. It’s named after the 19th-century obstetrician Franz Naegele. Take the first day of your last period, add 7 days, then subtract 3 months. The result is your estimated due date. For example, if your LMP was January 1, add 7 days to get January 8.

That formula comes out to roughly 280 days from LMP. Modern calculators do the same math digitally and often add a cycle-length adjustment. Naegele's rule does not account for cycle length.

What each trimester covers.

Pregnancy is divided into three trimesters. Each trimester is about 13 weeks. The first trimester (weeks 1–12) includes implantation, early organ formation, and the end of the embryonic period. Morning sickness, fatigue, and breast tenderness are common. By the end of the first trimester, the risk of miscarriage drops significantly.

The second trimester (weeks 13–26) is often called the "honeymoon" phase. Energy usually returns, nausea often fades, and you may feel the baby move for the first time. Anatomy scans usually happen around 18–20 weeks. The third trimester (weeks 27–40) brings growth, final organ maturation, and preparation for birth. Braxton Hicks contractions are common as the due date approaches. Shortness of breath is common as the due date approaches. Discomfort is common as the due date approaches.

Your due date calculator tells you which trimester you're in based on how many weeks have passed since your LMP. That can help you know what to expect and when to schedule key appointments.

How accurate are due dates?

Only about 5% of babies are born on their exact due date. Most arrive within a window of about two weeks before or after. Due dates are estimates, not deadlines. Natural variation in gestation length affects due date accuracy. Imprecise LMP recall affects due date accuracy. Irregular cycles affect due date accuracy.

An early ultrasound can provide a more accurate gestational age. This is especially true between 8 and 12 weeks. Many providers use the ultrasound date to adjust the due date if it differs from the LMP calculation by more than a certain threshold. If you've had an early scan, your provider's date typically overrides what a calculator shows.

Limitations: Calculators vs. Ultrasound

LMP-based calculators have clear limits. They assume you remember the first day of your last period correctly. They assume a regular cycle or that your stated cycle length is accurate. They don't account for late ovulation in a given month, which can happen even with usually regular cycles.

Ultrasound measures fetal size. Fetal size correlates with gestational age in the first trimester because growth is relatively uniform. After the first trimester, fetal size varies more between fetuses. That makes ultrasound less reliable for dating. If you have irregular periods, didn't track your cycle, or aren't sure about your LMP, an early ultrasound is the most reliable way to establish your due date.

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Limitations and Safety

This due date calculator is an informational tool only. It is not a medical device and does not replace professional medical advice. The estimate is based on the information you provide and standard obstetric formulas. Your healthcare provider may use ultrasound to set a different due date. Your healthcare provider may use other methods to set a different due date. That due date may be more accurate.

If you have irregular periods, rely on your provider's assessment. If you don't remember your LMP, rely on your provider's assessment. If you have a high-risk pregnancy, rely on your provider's assessment. Never use this calculator to make decisions about medication. Never use this calculator to make decisions about induction. Don't use this calculator to make decisions about medical care. Confirm your due date with your doctor or midwife.