Estimated due date and dating basis
The due date is a dating estimate, not a predicted delivery appointment. The modeled dating start is exactly 280 days before the estimated due date; outside last-period mode it is an arithmetic anchor, not an observed period date.
To compare two known calendar dates in elapsed days, use the Date Difference Calculator .
The formula
All offsets use Gregorian calendar days. Last-period mode adjusts the 28-day convention by your average cycle length. Ultrasound mode uses gestational age already supplied by the scan; it does not infer age from measurements. IVF mode accepts the two embryo ages documented in the ACOG examples.
Last-period and IVF examples
For a first period day of January 1, 2026 and a 28-day cycle, adding 280 days gives October 8, 2026. A day-five embryo transferred on January 20, 2026 gives October 8 by adding 261 days; a day-three embryo transferred January 18 gives the same date by adding 263 days.
How to use this calculator
- Choose the dating method that matches the information you have.
- Enter the reference date and the fields shown for that method. Use gestational weeks, not weeks since conception, in ultrasound mode.
- Calculate and check the reference-date offset. Use your clinician’s established pregnancy dates for care.
Choose the reference date carefully
The four methods expect different events. Last-period mode starts from the first bleeding day, conception mode requires a known conception date, ultrasound mode starts from the date of a scan, and IVF mode uses the embryo transfer date. A date when intercourse occurred does not necessarily establish conception. Changing the method changes how the reference is interpreted, so confirm the date field’s label before submitting.
Check the arithmetic behind the result
The offset output shows exactly how many days the selected method adds. For a 30-day cycle in period mode, the offset is 282 days; for 28 days it is 280. A scan entered as eight weeks and zero days leaves 224 days to the 40-week dating anchor. These are whole-day calculations. Month lengths and leap days are handled by the calendar rather than by adding nine approximate months.
The modeled start has a specific meaning
Subtracting 280 days from the result gives a common starting anchor for gestational dating. It can help check that two methods align, but it does not establish an actual menstrual date, fertilization event, or pregnancy test result. This calculator does not compare scans, measure fetal size, or apply clinical rules for changing a previously established due date. Delivery timing can differ from the estimate; clinical dating is interpreted with the pregnancy history.
Assumptions & limitations
What this calculation assumes
- Last-period mode applies a fixed cycle-length adjustment to the 280-day convention.
- Ultrasound age is an already determined gestational age. IVF supports day-three and day-five transfer only.
What to keep in mind
- Reference and resulting dates must stay within 1900–2100. Cycle length accepts 21–90 whole days; scan age accepts 0 weeks through 42 weeks 6 days.
- No diagnosis, scan measurement interpretation, clinical redating decision, or prediction of the actual delivery day.
Common questions
Does the period mode use the first or last bleeding day?
Use the first day. The final bleeding day is not the starting point for the 280-day dating convention.
What do I enter from an ultrasound report?
Enter the scan date and the gestational weeks plus additional days reported for that date. Do not enter fetal measurements or weeks since conception.
How are day-three and day-five IVF transfers different?
A day-three transfer adds 263 days. A day-five transfer adds 261 days. The embryo’s age accounts for the two-day difference.