Due Date Calculation Methods

An estimated due date (EDD) is a planning date used to describe pregnancy timing. Birth can occur before or after that date, and a website calculation does not confirm pregnancy or override the date documented during prenatal care.

Last-menstrual-period method

The conventional last-menstrual-period (LMP) method starts with the first day of the last menstrual period:

The method works best when the LMP date is known and reasonably reliable. Its conventional model assumes an approximately 28-day cycle with ovulation around cycle day 14, but real ovulation timing and cycle length vary. Irregular cycles, atypical ovulation timing, or uncertain recall can make an LMP-only estimate fit less closely.

Gestational age is not time since fertilisation

Conventional gestational age is counted from the first day of the LMP. In the standard 28-day model, that count begins roughly two weeks before fertilisation, so 40 weeks of gestational age does not mean 40 weeks since conception. A calendar calculator does not observe when ovulation or fertilisation occurred.

How ultrasound informs clinical dating

First-trimester ultrasound using appropriate early measurements is the most accurate ultrasound period for establishing or confirming gestational age. It can be especially relevant when the LMP is uncertain, cycles are irregular, or the LMP date and early measurements differ materially.

Clinical redating follows documented criteria and considers how early the ultrasound was performed. A later scan does not automatically keep replacing an already established best obstetric estimate: later measurements are also used to assess growth, which is related to but different from establishing pregnancy dates.

Assisted reproduction and embryo-transfer dates

Pregnancies conceived through assisted reproductive technology use known treatment dates and embryo age rather than substituting a generic LMP estimate. ACOG gives these examples:

• Day-5 embryo transfer: estimated due date = transfer date + 261 days.
• Day-3 embryo transfer: estimated due date = transfer date + 263 days.

These branches are separate: no LMP or cycle-length adjustment is added on top. The Calzivo calculator does not accept embryo-transfer details, so people using fertility treatment should use the date documented by their fertility or prenatal care team.

Use the best obstetric estimate for care

During prenatal care, a clinician can reconcile the reported LMP, early ultrasound, and assisted-reproduction information and document a best obstetric estimate. Once an appropriate estimate is established, it is not repeatedly changed without a clinical reason. Medical decisions should use that documented estimate rather than an independent website result.

Early-pregnancy safety

A due-date calculation cannot confirm pregnancy location or viability. Bleeding does not by itself diagnose pregnancy loss, but bleeding or pain during a possible or confirmed pregnancy warrants appropriate professional assessment.

Possible pregnancy with sudden severe abdominal or pelvic pain, shoulder pain, weakness, dizziness, or fainting can signal an emergency such as a ruptured ectopic pregnancy. In the United States, call 911 or emergency services for severe or life-threatening symptoms. Do not use a calculated due date to rule out ectopic pregnancy or pregnancy loss.

Common interpretation mistakes

• Treating an estimated due date as a guaranteed delivery date.
• Assuming 40 weeks means 40 weeks since fertilisation.
• Claiming that the current calculator adjusts for cycle length, ultrasound, or IVF dates.
• Replacing a documented clinical estimate with an independent calendar result.

Sources

• ACOG: Methods for Estimating the Due Date
• ACOG: When Pregnancy Goes Past Your Due Date
• ACOG: Ultrasound Exams
• ACOG: How Your Fetus Grows During Pregnancy
• ACOG: Estimated Due Date Data Definitions
• ACOG: Ectopic Pregnancy
• ACOG: Bleeding During Pregnancy
• ACOG: Early Pregnancy Loss