Due Date Calculator

How the Due Date Calculator Works

This calculator estimates your due date using one of two standard methods depending on the input you provide. The most common approach is based on your last menstrual period (LMP).

Last Menstrual Period (LMP) Method

If you enter your LMP date, the calculator applies Naegele's rule: it adds 280 days (40 weeks) to the first day of your last period. This 40-week count is the standard duration of a full-term pregnancy, counting from the LMP rather than from conception.

Conception Date Method

If you enter a conception date, the calculator adds 266 days (38 weeks) to that date. This accounts for the fact that conception typically occurs about two weeks after the start of your last menstrual period.

Both methods produce the same estimated due date when the inputs are consistent with a typical 28-day cycle and ovulation occurring around day 14.

How to Use the Calculator

1. Select whether you want to calculate based on your last menstrual period or your conception date.
2. Enter the relevant date using the date picker.
3. The estimated due date will be displayed immediately.

No additional inputs are required. The calculator assumes a standard 28-day menstrual cycle with ovulation at day 14. If your cycle is significantly longer or shorter, the estimate may differ from what your healthcare provider calculates.

Understanding Your Results

The due date shown is an estimate, not a guarantee. Only about 4% of babies are born exactly on their estimated due date. Most full-term births occur between 37 weeks and 42 weeks of pregnancy.

Your healthcare provider may adjust your due date based on early ultrasound measurements, particularly if your cycle length differs from the standard 28 days or if you are unsure of your LMP date.

What the Due Date Represents

• 40 weeks from the first day of your last menstrual period
• 38 weeks from the date of conception
• A target date for the end of a full-term pregnancy

Common Questions About Due Dates

Why is pregnancy counted from the last period instead of conception?

Historically, the LMP is easier to identify than the exact date of conception. Most women can recall when their last period started, but ovulation and fertilization dates are harder to pinpoint without tracking. Using the LMP provides a consistent starting point for calculating gestational age.

Can my due date change?

Yes. If an early ultrasound shows a significant difference between the gestational age estimated by the ultrasound and the age calculated from your LMP, your provider may revise the due date. This is most common in the first trimester.

What if my cycle is longer than 28 days?

If your cycle is consistently longer than 28 days, ovulation occurs later than day 14. In that case, your actual due date may be later than the estimate from this calculator. Your healthcare provider can adjust the due date accordingly.

Limitations of This Calculator

• Assumes a standard 28-day cycle with ovulation at day 14
• Does not account for irregular cycles or unknown LMP dates
• Does not replace clinical assessment or ultrasound dating
• Provides an estimate only; individual pregnancies vary