Pregnancy Due Date Calculator – Estimate Your Baby’s Due Date

How Does a Pregnancy Due Date Calculator Work?

This pregnancy calculator estimates your due date using one of three starting points: the first day of your last menstrual period, an estimated conception date, or an existing due date you already have from a doctor or ultrasound. Depending on the information you enter, the calculator can also estimate your conception date, current pregnancy week, and trimester timeline.

A common method for estimating a due date is Naegele’s rule, which uses 280 days (40 weeks) from the first day of the last menstrual period (ACOG). This is an estimate based on a typical 28-day cycle and does not account for every person’s actual ovulation timing. If your cycle length differs from 28 days, the estimated date may shift.

If you are not sure of your cycle dates, the period calculator can help you keep track of recent start dates and work out your usual cycle length.

Due Date vs. Conception Date vs. Last Period Date

These three dates are related but different, and mixing them up is a common source of confusion.

• Last period date: the first day of your most recent menstrual period. Pregnancy dating is commonly based on this date.
• Conception date: the date fertilization occurred or is estimated to have occurred. It is usually later than the first day of the last menstrual period.
• Due date: the estimated date of birth based on pregnancy dating.

Conception timing is not exact for every pregnancy. If you want to understand when ovulation may have occurred, you can estimate your ovulation date and fertile window. These tools give calendar-based estimates and cannot confirm when conception happened.

How Accurate Is a Due Date Calculator?

A pregnancy due date is an estimate, not a guaranteed delivery date. Pregnancy length varies, and the actual date of birth may be earlier or later than the estimated due date.

A calculation based on your last menstrual period depends on having an accurate period date and makes assumptions about cycle length and ovulation timing. If your cycles are irregular or you are unsure of your last period date, the estimate may be less reliable.

An ultrasound dating scan can provide a more accurate assessment of how far along the pregnancy is because pregnancy dating can be based on measurements made during the scan.

Understanding Your Trimesters

These week ranges are general guidelines, not strict boundaries. Individual pregnancies vary, and your healthcare provider will track your own progress.

How Many Weeks Pregnant Am I?

Pregnancy is commonly measured in weeks starting from the first day of your last menstrual period, rather than from the date of conception. This means pregnancy dating can include approximately two weeks before conception in a typical cycle. The exact timing can vary because ovulation and conception do not occur on exactly the same day in every pregnancy.

This is called gestational age. Fetal age, which counts from conception, is usually about two weeks less, so the two terms are not interchangeable.

This pregnancy due date calculator handles the week calculation automatically based on the information you enter.

How Is a Due Date Counted for Each Method?

• Last menstrual period (LMP): by convention, the estimated due date (EDD) is 280 days after the first day of the LMP (ACOG).
• Conception date: because conception usually happens about two weeks after the LMP, the count from conception is commonly about 266 days (38 weeks).
• IVF: if you conceived with IVF, the due date is usually assigned from the age of the embryo and the date of transfer rather than from a last period. Your clinic will confirm the date that applies to you.
• Ultrasound: the date is based on measurements made during the scan. A first-trimester ultrasound is generally the most accurate ultrasound method for establishing or confirming gestational age (ACOG).

Comparing Due Date Calculation Methods

If your calculator-based estimate and a healthcare professional’s ultrasound-based estimate differ, the healthcare professional may recommend which date should be used based on the timing and findings of the scan.

Can My Due Date Change?

Yes, it can. Healthcare providers usually set the estimated due date once they have your last period date, a first accurate ultrasound, or both. According to ACOG, if an early scan differs from the last-period date by more than 5 days before 9 weeks, or by more than 7 days before 14 weeks, the due date is generally changed to match the scan. After that, changes are reserved for rare circumstances.

If your provider gives you a different date from this calculator, follow your provider’s date and use the calculator’s result as a general guide.

Early Term, Full Term and Late Term: What the Weeks Mean

A due date falls at 40 weeks, but a healthy pregnancy can end before or after it. ACOG describes the weeks around the due date as follows (ACOG).

A birth before 37 weeks is called preterm. Your healthcare provider will advise you on the timing that is right for your pregnancy.