A pregnancy due date calculator estimates when your baby may arrive. Enter the first day of your last menstrual period and your usual menstrual cycle length to receive an estimated due date. The calculator is designed for people whose pregnancy is dated from a menstrual period rather than an IVF transfer or clinical ultrasound.

Pregnancy lasts about 40 weeks when counted from the first day of the last normal menstrual period, but an online result can’t predict the exact day labor will begin. Your prenatal care provider may confirm or revise the date using your health history and ultrasound findings.

How to Use the Pregnancy Due Date Calculator

1. Select the first day of your last period. Use the date your most recent normal menstrual period began, not the date it ended or the date you first received a positive pregnancy test.
2. Enter your usual cycle length in days. Count from the first day of one period to the first day of the next. If your cycles are generally 28 days long, leave the default value at 28.
3. Select “Calculate due date.” The tool will display one estimated calendar date for your baby’s arrival.
4. Review the estimate. Treat the result as a planning date, not a prediction of the exact day labor will begin.
5. Recalculate if necessary. You can change the period date or cycle length and run the calculation again. If your cycles vary considerably, discuss pregnancy dating with your healthcare provider instead of relying on several online estimates.

How the Pregnancy Due Date Calculator Works

Pregnancy is conventionally dated from the first day of the last menstrual period, even though conception usually occurs later. This system is known as gestational age. The Office on Women’s Health explains that pregnancy lasts about 40 weeks when counted from the first day of the last normal period.

This calculator uses the following method:

Estimated due date = first day of the last menstrual period + 280 days + the cycle-length adjustment

The cycle-length adjustment is the difference between the entered cycle length and 28 days. For example, a 35-day cycle moves the calendar estimate seven days later than the standard 28-day calculation. A 25-day cycle moves it three days earlier.

This method assumes that:

• The entered period was a normal menstrual period.
• The date is remembered correctly.
• The entered cycle length reasonably represents the user’s usual cycle.
• Ovulation timing generally shifts with cycle length.
• The pregnancy wasn’t conceived through IVF, which requires a different clinical dating method.

The calculation does not use an ultrasound date, embryo-transfer date, conception date, pregnancy-test date, symptoms, fetal measurements, or medical history. It also does not calculate current gestational age. Its only displayed result is an estimated due date.

Why pregnancy begins at the last menstrual period?

Gestational age starts on the first day of the last menstrual period because the exact date of fertilization often isn’t known. This means the first roughly two weeks of a typical 28-day pregnancy timeline occur before conception.

Gestational age and fetal age therefore aren’t the same. Gestational age is the standard measurement used for prenatal scheduling and pregnancy records. Fetal or conception age is usually about two weeks shorter, although ovulation timing can vary.

Understanding Your Estimated Due Date

Your result identifies the date on which the pregnancy would reach 40 weeks under the calculator’s assumptions. It is not an appointment date, a deadline, or a guarantee that labor will begin that day.

A due date gives your healthcare team a common reference point for discussing pregnancy progress, fetal growth, prenatal testing, and the timing of care. During an initial prenatal visit, a provider may calculate and document an estimated due date using menstrual history, ultrasound information, or assisted-reproduction records.

How pregnancy timing relates to your due date?

These definitions come from the American College of Obstetricians and Gynecologists and the National Institute of Child Health and Human Development.

The classifications describe gestational age, not whether an individual pregnancy is healthy or whether delivery should be scheduled. Decisions about monitoring, induction, or delivery require individualized medical guidance.

Why Your Clinical Due Date May Be Different

A calendar estimate works best when the last menstrual period is known, cycles are fairly predictable, and the pregnancy began without assisted reproductive technology. Several factors can make period-based dating less reliable:

• Irregular or widely varying menstrual cycles
• Uncertainty about when the last period began
• Bleeding that was mistaken for a menstrual period
• Recent pregnancy or breastfeeding
• Recent use or discontinuation of hormonal birth control
• Ovulation occurring earlier or later than expected
• IVF or another assisted-reproduction procedure
• A difference between menstrual dating and early ultrasound measurements

According to ACOG, first-trimester ultrasound measurement is the most accurate ultrasound method for establishing or confirming gestational age. Your provider will decide whether an ultrasound difference is large enough to revise the official date.

Once a reliable estimated due date has been established, it generally shouldn’t be changed repeatedly in response to later measurements. Fetuses grow at different rates, particularly later in pregnancy, so a size difference doesn’t automatically mean the pregnancy dates are wrong.

Tips for Entering More Accurate Information

Use the first day, not the final day, of your period

The calculation begins with the first day menstrual bleeding started. Entering the last day of bleeding can shift the estimate several days later.

Use a typical cycle length

Cycle length is counted from the first day of one menstrual period to the first day of the next. If your recent cycles were 27, 29, and 28 days, using 28 days is reasonable. If they were 24, 35, and 48 days, a single average may not represent your ovulation timing well.

Check a calendar or tracking app

If you recorded your period before learning you were pregnant, use that original record rather than relying entirely on memory. Confirm that the entry represents a normal period rather than spotting.

Don’t use the date of a positive pregnancy test

A pregnancy test can confirm that pregnancy hormones are present, but it does not establish the exact day of conception. Testing dates vary depending on ovulation, implantation, cycle length, test sensitivity, and when a person chooses to test.

How to Use the Result for Pregnancy Planning

Use the calculated date as a starting point for conversations and broad planning. It can help you estimate the general season of birth, consider when you may need parental leave, begin a baby budget, and organize questions for your first prenatal appointment.

Avoid using the result to independently schedule medical tests or decide when an induction or cesarean delivery should occur. Prenatal care schedules depend on established gestational age, health history, test results, pregnancy complications, and whether the pregnancy involves multiples.

At an initial visit, the provider may review menstrual history, calculate the due date, perform an examination, order laboratory tests, and discuss appropriate care.

Consider recording the following information for your appointment:

• The first day of your last normal period
• Your usual cycle length and whether cycles are regular
• The date of your positive pregnancy test
• Any bleeding since the last period
• Previous pregnancy history
• Fertility treatment or embryo-transfer information
• Current medications and health conditions

When to Recalculate and When to Ask Your Provider

Recalculate if you notice that you entered the wrong date or used the duration of menstrual bleeding instead of the full cycle length. You can also rerun the tool if a reliable tracking record shows a different typical cycle length.

Contact your prenatal care provider if you don’t know your last menstrual period, have highly irregular cycles, recently gave birth, were breastfeeding when you conceived, or became pregnant through fertility treatment. The provider can select a dating method appropriate to your circumstances.

If your clinician gives you an official due date, use that date for prenatal scheduling and medical decisions, even when it differs from this calculator’s result.

Safety and Limitations

This calculator provides an educational estimate. It cannot confirm pregnancy, diagnose a complication, measure fetal growth, determine whether a pregnancy is progressing normally, or replace prenatal care. It also cannot predict the day labor will begin.

Pregnancy symptoms should be assessed according to their severity, not according to how close you are to the calculated due date. Seek medical care immediately for an urgent warning sign such as heavy vaginal bleeding, leaking fluid, severe abdominal pain that doesn’t go away, chest pain, trouble breathing, fainting, a severe headache, vision changes, a fever of 100.4°F or higher, or a noticeable reduction in your baby’s usual movement. The CDC provides a complete list of urgent maternal warning signs. Call 911 when symptoms appear life-threatening.

Frequently Asked Questions

How accurate is a pregnancy due date calculator?

It provides a useful estimate, but it cannot predict the exact day labor will begin. Irregular cycles or uncertain period dates can reduce accuracy.

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

Ask your prenatal care provider about clinical dating. An early ultrasound may provide a more reliable estimate.

Do irregular periods affect the result?

Yes. Irregular cycles make ovulation harder to estimate, so the calculated due date may be less reliable.

Can I calculate my due date from the day I had sex?

Not precisely, because intercourse and conception may occur on different days. This calculator uses your last period and usual cycle length.

Should I use this calculator after IVF?

No. IVF pregnancies should be dated using the embryo-transfer date and embryo age provided by the fertility clinic.

Can an ultrasound change my due date?

Yes. Your provider may revise the date when an early ultrasound differs significantly from menstrual dating.

Does the calculator work for twins?

It can provide an initial estimate, but it doesn’t account for the different monitoring and delivery planning associated with multiple pregnancies.

What if my due date passes without labor?

Contact your prenatal care provider and follow their monitoring plan. Seek prompt care for reduced fetal movement or other concerning symptoms.