Pregnancy Due Date Calculator

Introduction:

An estimated due date is a clinical calendar anchor, not a prediction of the exact day birth will occur. It gives gestational age a common reference for prenatal visits, screening windows, fetal-growth assessment, and decisions near term. Many healthy births happen before or after that single date.

Pregnancy dating uses obstetric age, which traditionally begins on the first day of the last menstrual period (LMP). That convention places week zero about two weeks before conception in a typical 28-day cycle. A conception date, embryo-transfer record, or ultrasound measurement can be converted to the same LMP-equivalent timeline so weeks and days remain comparable.

Gestational age

Completed weeks and days counted from the obstetric dating anchor.

Estimated due date (EDD)

The date corresponding to 40 weeks, or 280 days, on that timeline.

Fetal or conception age

Time since conception, usually about two weeks less than gestational age.

The best anchor depends on the record. A certain LMP can be useful when cycles are regular. Known conception narrows the calendar estimate. In vitro fertilization (IVF) records add the embryo's documented age to the transfer date. Ultrasound estimates age from measurements, with first-trimester dating generally providing the most accurate scan-based estimate for establishing or confirming gestational age.

Cycle length affects an LMP estimate because the standard rule assumes a 28-day cycle and ovulation near day 14. A consistently longer cycle shifts the estimate later; a shorter cycle shifts it earlier. Irregular cycles, uncertain bleeding, recent hormonal changes, or uncertain dates make this adjustment less dependable.

Once a maternity care team establishes the official EDD, that documented date should guide clinical decisions. Later scans assess growth and wellbeing but should not automatically reset a well-established due date. A pregnancy first dated late in gestation carries more uncertainty than one dated from a reliable early record.

This information is for education and planning, not diagnosis or treatment. Contact a qualified maternity-care professional about bleeding, pain, reduced fetal movement, labor symptoms, uncertain dates, or any concern about the pregnancy.

How to Use This Tool:

Choose the dating record that best matches the documentation you have.

1. Select LMP, Known conception, IVF transfer, or Ultrasound, then enter the corresponding recorded date.
2. For LMP, enter the usual cycle length from 20 to 40 days. For IVF, choose the documented day 3, 5, or 6 embryo age. For ultrasound, enter the reported gestational weeks and extra days at the scan.
3. Set Timeline as of to the date for which you want gestational age. The dating record cannot be later than this date.
4. Choose the preferred Date display, then review the EDD, current gestational age, method basis, and term dates.
5. Compare the estimate with the EDD in the clinical record. If they differ, use the documented clinical date and ask the care team which dating evidence established it.

Interpreting Results:

Estimated due date marks 40 weeks on the selected dating timeline. Gestational age reports elapsed days from the reconstructed obstetric anchor to the as-of date. Days until due may become negative after the EDD without implying that something is wrong.

Term labels describe completed gestational weeks, not a prediction of spontaneous labor. Early term begins at 37 weeks, full term at 39 weeks, late term at 41 weeks, and postterm at 42 weeks. The exact boundary belongs to the later band.

Method notes explain the record's main limitation. Treat an LMP-only result cautiously when cycles are irregular, and treat later scan dating as less precise than early scan dating. The clinically documented EDD remains the result to use for appointments and care decisions.

Technical Details:

All date arithmetic uses civil calendar days rather than elapsed milliseconds, which avoids daylight-saving changes shifting a date. Each method first reconstructs an LMP-equivalent anchor and then places the EDD 280 days from that anchor.

Formula Core

For LMP dating, cycle length L adjusts the standard 28-day rule:

Known conception adds 266 days. IVF subtracts documented embryo age e from that interval:

For a scan performed at w completed weeks plus d days, the remaining interval to 40 weeks is:

Rule Core

LMP cycle length must be a whole number from 20 through 40 days. IVF embryo age is limited to 3, 5, or 6 days. Ultrasound dating accepts 5 through 30 completed weeks plus 0 through 6 days. An as-of date more than 48 gestational weeks from the anchor is rejected as a likely date error.

Term boundaries use exact completed-day comparisons: 259 days begins early term, 273 begins full term, 287 begins late term, and 294 begins postterm.

Limitations and Privacy:

A calendar estimate cannot account for uncertainty in LMP recall, variable ovulation, embryo records, scan measurement, or a clinician's complete dating criteria. It does not assess fetal growth, pregnancy viability, labor timing, or maternal and fetal health.

Dates are calculated in the browser and are not sent to a calculation service. Shared screenshots, copied summaries, or downloaded documents may still contain sensitive pregnancy dates, so store and share them with appropriate care.

Worked Examples:

References:

• ACOG Committee Opinion #700: Methods for Estimating the Due Date, American College of Obstetricians and Gynecologists, reaffirmed 2022.
• Definition of Term Pregnancy, American College of Obstetricians and Gynecologists, reaffirmed 2025.