Pregnancy MetricsPregnancy & preconception calculators

Gestational Age Calculator

Count completed pregnancy weeks and days for today or another date from a recorded due date, last period, or dated week record.

Which pregnancy dating record do you have?

Use the current recorded due date when you have one.

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The pregnancy dating guide explains the calendar those weeks use.

How completed weeks are counted

This page answers “how many weeks and days is this pregnancy on a chosen date?” It starts from a dating anchor you already have: a recorded due date, a last menstrual period, or pregnancy weeks written on an earlier date. Creating the due date itself is the job of the Due Date Calculator. An around-date fertilization estimate is the Conception Date Calculator. A Day-3 or Day-5 transfer is the IVF Due Date Calculator.

ACOG’s FAQ on how a fetus grows counts gestational age from the first day of the last menstrual period and describes pregnancy length on a 40-week calendar. Gestational age is expressed as completed weeks plus extra days. Eight weeks and four days means eight full weeks and four additional days. It does not mean 8.4 weeks, and the “.4” is not a decimal fraction of a week.

From a recorded due date, the calculator finds the pregnancy-calendar start by subtracting 280 days, then counts days from that start to the date you want to check. From a last period, it counts days forward from the period date you enter. From a dated week record, it steps back by the written weeks and days to the same kind of start, then counts forward to the date you chose. The arithmetic is date-only, so a timezone cannot move the result by a day. The methodology states each path.

Gestational age and conceptional age are different clocks. Conceptional age counts from fertilization and is about two weeks less than gestational age in the standard 28-day model, because that model places ovulation 14 days after the period starts. A result of 8 weeks 4 days is a pregnancy-calendar age. It is not a claim that fertilization happened exactly 6 weeks and 4 days earlier.

Worked example

Recorded due date: October 7, 2024. Date to check: March 1, 2024. The implied pregnancy-calendar start is October 7 minus 280 days, which is January 1, 2024. January 1 through March 1, 2024 is 60 days: January contributes 31 days and February 2024 contributes 29 because it is a leap year. Sixty divided by 7 is 8 weeks with a remainder of 4 days, so the pregnancy age on March 1 is 8 weeks 4 days.

The same 60 days from a last period of January 1, 2024 produces the same week count. If a record already said “8 weeks 4 days on March 1,” checking March 1 again returns 8 weeks 4 days, and checking a later date adds the days that have passed. A cycle-adjusted due date from the due-date calculator uses its adjusted date, not the raw period, as the 280-day anchor. The recorded period remains the menstrual date; the adjusted start is a derived reference.

How to read weeks and days

Read the result as completed weeks and a remainder from 0 through 6 days. Trimester labels on this site follow the same completed-week calendar used in the methodology: the first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days through 27 weeks 6 days, and the third from 28 weeks 0 days. Those labels organize the calendar. They do not grade the health of a pregnancy.

ACOG’s definition of term pregnancy divides the end of pregnancy into early term (37 weeks 0 days through 38 weeks 6 days), full term (39 weeks 0 days through 40 weeks 6 days), late term (41 weeks 0 days through 41 weeks 6 days), and postterm (42 weeks 0 days and beyond). When a result falls in one of those ranges, the label is a timing category from that opinion. It is not an instruction about induction or birth, and it is not a safety score.

If you are using a week count to look up weight-gain ranges, the time axis and the weight range are separate questions. The Pregnancy Weight Gain Calculator and the weight-gain guide use pre-pregnancy BMI for the range. This page only supplies the week count when you ask it to.

Why a scan can change the count

ACOG Committee Opinion 700 says first-trimester ultrasound, through 13 weeks 6 days, is the most accurate ultrasound method to establish or confirm gestational age, using crown–rump length. Later biometric measurements are less precise for dating. NICE antenatal care guideline NG201 says to offer an ultrasound between 11 weeks 2 days and 14 weeks 1 day to determine gestational age and detect a multiple pregnancy. The WHO antenatal ultrasound recommendation recommends one scan before 24 weeks to estimate gestational age, among other purposes.

If a scan led the care team to change the due date, the weeks you should count are the weeks that follow the documented due date, not a home recount from an earlier period. Committee Opinion 700’s discrepancy cutoffs start at more than 5 days before 9 weeks and widen to more than 21 days from 28 weeks. The due-date page lists each band. This calculator does not measure crown–rump length or apply those cutoffs. Enter the due date or the week record after the clinician has written it.

The pregnancy dating guide explains why a period count and a scan date can both be calculated correctly and still differ.

Limits and when to talk with a clinician

When the week count here disagrees with the week in the prenatal record, ask the clinician which due date the record is using. A second online count does not settle the conflict. This page cannot interpret ultrasound images, judge whether a fetus is growing as expected, or relabel a pregnancy as suboptimally dated.

Pregnancy weeks stop being the right clock at birth. A child’s age and growth afterward are measured differently. Child Growth Lab is a separate site for that next step. It does not calculate gestational age, and this page does not chart a child’s growth.

Worry about bleeding, pain, or how you feel belongs with your prenatal clinician or local urgent-care service. This calculator does not rank symptoms or decide whether care is urgent.

Frequently asked questions

What does 8 weeks 4 days mean?

It means 8 completed weeks and 4 additional days, or 60 days from the pregnancy-calendar start. Say “8 and 4,” not “8.4 weeks.” The leftover days are never 7 or more, because 7 days would already be another completed week.

Why is gestational age about two weeks ahead of conception?

The pregnancy calendar starts at the last period in the standard model, and ovulation is placed about 14 days later. Fertilization age is the shorter of the two. The conception calculator estimates that earlier calendar point; it still cannot name the exact day fertilization occurred.

Can I check a date in the past or future?

Yes. Pick the date you want to inspect. The count is the number of days between the dating anchor and that date, grouped into weeks and a remainder. It does not predict a measurement that a scan would show on that day.

Which date should I enter if my due date was changed?

Enter the current due date from the pregnancy record. Committee Opinion 700 says that once the due date is determined and documented, later changes should be uncommon and should be documented. Keep the older period date only when you are deliberately comparing it with the record, and label that comparison as a comparison.

Sources

These sources describe how pregnancy weeks and term categories are defined. They do not review or endorse Pregnancy Metrics.