Pregnancy Due Date Calculator

About the Pregnancy Due Date Calculator

The first practical question after a positive pregnancy test is usually when the baby is due. Doctors and midwives answer it with a standard convention: pregnancy is counted as 40 weeks, or 280 days, starting from the first day of your last menstrual period, even though conception happens about two weeks later. This calculator applies that convention. You choose whether you know the first day of your last period or a specific conception date, enter that date, and it returns an estimated due date along with how far along you are if the date is recent.

The arithmetic follows Naegele's rule, a long-standing method of dating pregnancy. From the last menstrual period, it adds 280 days. From a known conception date, it adds 266 days, because those two weeks of the cycle before conception have already passed. If your cycle was close to 28 days and ovulation fell around day 14, the two inputs should land on almost the same date, which you can test by entering both versions of the same pregnancy.

A due date is better thought of as the center of a window than as an appointment. Very few babies arrive on the exact day. Most births happen within a couple of weeks on either side, and full term is considered to span several weeks. Still, the date matters in practice, because your care team uses it to time screening tests, ultrasounds, and decisions about monitoring. Treat the calculator's answer as a good first estimate to bring to your first prenatal visit, where it will be confirmed or adjusted.

When to Use It

How to Use It, Step by Step

1. Under Calculate from, choose Last menstrual period (LMP) if you know the first day of your most recent period. This is the standard starting point for dating and is the best choice unless you know the exact day of conception.
2. Choose Conception date instead only if you have a reliable date, such as a single act of intercourse during a fertile window, an insemination, or an embryo transfer. Ovulation tracking results can also help.
3. In the Date field, enter the matching date. For the LMP option, use the first day of your last period, which is the day bleeding began, not the day it ended. Use the date picker or type it in.
4. Check the year carefully. The tool accepts any valid date, so a typo such as entering last year can produce a due date that is already in the past, and a future date will not show a current week count.
5. Select Calculate Due Date and read the Estimated due date. It is shown as a full date, for example May 10, 2027.
6. Look for the Current gestational age line. It appears when the date you entered is between zero and 300 days ago, and it shows weeks and days counted from the date you entered.
7. If your cycles are not close to 28 days, adjust your expectations. A cycle of 35 days, for example, usually means ovulation was later, so the true due date may be about a week after the one shown.
8. Share the date with your doctor or midwife at your first visit. They may revise it using an early ultrasound, which is generally the most reliable way to date a pregnancy.

Understanding Your Results

The estimated due date is the day that completes 40 weeks from the start of your last period, or 38 weeks from conception. It is not a deadline. Births are commonly grouped by timing: early term is 37 to 38 weeks and 6 days, full term is 39 weeks through 40 weeks and 6 days, late term is 41 weeks through 41 weeks and 6 days, and post-term is 42 weeks or more. So a baby arriving a week or two either side of the date is well within the usual range.

The gestational age line, when it appears, tells you how many completed weeks and extra days have passed since the date you entered. Clinicians quote gestational age counted from the last period. When you choose the conception option, the tool adds the roughly 14 days that elapse between the start of the last period and conception, so the number it shows follows the same clinical convention your doctor uses.

To sanity-check the output, count forward from your date in 40-week increments or compare with a calendar. The due date should fall in the ninth month, roughly nine months and one week after the last period. The next step is to confirm with a healthcare provider. An ultrasound in the first trimester measures the embryo and can refine the date, and your provider's date, rather than this estimate, is the one that will guide your care.

More Worked Examples

Tips for Better Results

• Write down the first day of your period each month, even when you are not trying to conceive. A reliable record of that date is the single most useful piece of information for dating a pregnancy later.
• If you tracked ovulation with test strips or a thermometer, use the conception option with the estimated ovulation date, then compare it with the period-based estimate. A large gap can be worth mentioning to your provider.
• If your cycle is regularly longer than 28 days, shift the due date later by the number of extra days. If it is shorter, shift it earlier. Your provider can help you decide which adjustment makes sense.
• Remember that the weeks pregnant figure includes the two weeks before conception. If you tell your doctor you are 6 weeks pregnant, you mean 6 weeks since your last period, not 6 weeks since conception.
• Plan leave, travel, and childcare around a window rather than one day. Many people schedule important commitments to end a couple of weeks before the due date, and keep flexibility for the weeks afterward.
• Bring a printout or note of the estimated date to your first visit, along with the date of your last period and cycle length. That helps your provider decide whether an early ultrasound is needed to confirm the timing.

Common Mistakes to Avoid

• Entering the last day of the period instead of the first day. The calculation counts from the start of the period, so the wrong day shifts the due date by several days.
• Using a conception date that is really a guess. If you are not sure when conception occurred, the period-based option is safer, and an ultrasound will provide a more reliable date.
• Assuming every cycle is 28 days. The simple method assumes ovulation on day 14, so people with long, short, or irregular cycles may find that their true due date differs from the one shown.
• Treating the date as exact. Only a small fraction of babies are born on the due date itself, so building plans that depend on it can cause unnecessary stress.
• Comparing weeks pregnant between the two options and expecting them to match exactly. The conception option assumes conception happened about two weeks after the period began, so its gestational age is only as accurate as that assumption.

Assumptions and Limits

• Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. For irregular cycles, recent hormonal contraception, breastfeeding, or uncertain dates, the estimate can be off by a week or more.
• The conception option adds 266 days to the date you enter. If your conception date is only an estimate, the resulting due date will be only as accurate as that estimate, and an ultrasound is better at dating.
• The displayed gestational age is counted in completed weeks and days from the date you entered and is shown only when that date is within 300 days of today. For the conception option, the tool adds 14 days so the figure follows the clinical convention.
• The estimate is for planning, not medical advice. Your provider will confirm your due date, usually with an early ultrasound, and will adjust it if measurements differ significantly. Call your provider about any concerns, bleeding, or severe pain at any stage.

Key Terms

Naegele's rule

A traditional method for estimating a due date by adding 280 days to the first day of the last menstrual period, assuming a 28-day cycle.

Last menstrual period (LMP)

The first day of your most recent menstrual bleeding. It is the standard reference point for counting gestational age, even though conception happens later.

Gestational age

How far along a pregnancy is, expressed in weeks and days. Clinicians count it from the first day of the last period, not from conception.

Estimated due date (EDD)

The date at which a pregnancy reaches 40 weeks of gestation. It is the center of a window of possible birth dates, not a prediction of the exact day.

Full term

A pregnancy that has reached 39 weeks through 40 weeks and 6 days, when the baby is considered fully developed for birth under the commonly used definitions.

Dating ultrasound

An ultrasound scan, ideally in the first trimester, that measures the embryo or fetus to estimate gestational age and confirm or adjust the due date.