How pregnancy due dates are calculated

Standard last-period dating adds 280 days, or 40 weeks, to the first day of the last menstrual period. A well-supported ovulation or documented fertilization date uses 266 days, or 38 weeks. IVF dating uses the embryo’s age and the actual transfer date.

Early ultrasound can establish or confirm gestational age; first-trimester ultrasound is the most accurate ultrasound method for dating. Once a due date is established in the pregnancy record, that documented date should take priority over repeated online recalculation.

Pregnancy is conventionally dated from the first day of the last menstrual period, not from the day fertilization occurred. In the standard 28-day-cycle model, ovulation is assumed to occur about 14 days after the period begins. That is why someone may be described as four weeks pregnant roughly two weeks after fertilization.

This is a dating convention. It does not mean that fertilization or implantation had already happened during the first two counted weeks. See the Gestational Age Calculator when a due date is already documented.

Formula: first day of LMP + 280 days, equivalent to 40 completed weeks. This common calendar method (often called Naegele’s rule) assumes an approximately 28-day cycle and ovulation around cycle day 14.

It can fit less well when the LMP is uncertain, bleeding was not a normal period, cycles are irregular, or ovulation occurred earlier or later. Pregnancy Metrics uses the fixed standard calendar convention. It does not apply a custom cycle-length adjustment or interpret ultrasound measurements.

Formula: well-supported ovulation date + 266 days. Formula: documented fertilization date + 266 days. These dates correspond to about 38 weeks after fertilization rather than 40 weeks from LMP.

A date predicted only by an app or symptoms may still be an estimate. An intercourse date is not a documented fertilization date: intercourse and fertilization can occur on different dates. Use the Due Date Calculator and open “Use a different date” when the record is well supported.

ACOG states that first-trimester ultrasound through 13 weeks, 6 days is the most accurate ultrasound method to establish or confirm gestational age. A care team compares ultrasound findings with available menstrual and treatment records, and may keep or revise the EDD when relevant clinical criteria are met. Subsequent EDD changes should be uncommon and documented.

An online calendar guide cannot interpret crown–rump length, ultrasound quality, fetal measurements, or redating thresholds.

Use the due date recorded by your care team when it differs from a date-only estimate.

Day-5 embryo transfer: transfer date + 261 days. Day-3 embryo transfer: transfer date + 263 days. The two-day difference reflects the embryo age recorded at transfer. Fresh and frozen transfers both use the actual transfer date; do not substitute an earlier egg-retrieval date. The fertility clinic’s recorded due date takes priority.

1. The first day of the last period was uncertain.
2. The menstrual cycle did not match the standard 28-day assumption.
3. Ovulation occurred earlier or later than the standard model assumes.
4. Two calculations started from different records.
5. Early ultrasound provided better dating information.
6. IVF or other ART records replaced ordinary last-period assumptions.
7. One date is an old estimate while another is the currently documented obstetric EDD.

Do not average several due dates. Use the current date documented in the pregnancy record.

• Due Date Calculator — use a last-period date, well-supported ovulation date, or documented fertilization date.
• IVF Due Date Calculator — use a Day-3 or Day-5 embryo transfer record.
• Gestational Age Calculator — use an already documented due date for pregnancy weeks.
• Conception Date Calculator — use one derived “around” estimate, not confirmation of a biological event.

• the exact day of birth
• whether a pregnancy is viable, its location, fetal growth, or wellbeing
• whether symptoms are urgent, clinical redating is appropriate, or treatment is needed
• the exact fertilization, intercourse, or implantation date

For pregnancy care, symptoms, ultrasound questions, or disagreement between records, use the pregnancy record and guidance from a qualified healthcare professional. Read the medical disclaimer and methodology.

• ACOG — Methods for Estimating the Due Date

  Reaffirmed 2025. Supports the 280-day LMP convention, first-trimester ultrasound dating through 13 weeks 6 days, documentation of the EDD, and Day-3/Day-5 IVF transfer dating.

• NHS — Pregnancy due date calculator

  Explains the estimated due date from the last period and that the pregnancy dating scan may provide a more accurate estimate of pregnancy weeks.

• MedlinePlus — Fetal development

  Explains why gestational age is counted from the first day of the last menstrual period and why the first two counted weeks precede conception.

These sources do not endorse Pregnancy Metrics.