How to use it
Choose a method and enter the corresponding date. Changing the method clears the date field. The reference date starts with your device’s current date and can be changed.
- First day of last period: enter the first day and your average cycle length. The default is 28 days; the tool accepts 21–45 days. This input range is not a definition of a normal cycle.
- Conception date: enter a known conception date or estimated ovulation date.
- IVF embryo transfer: enter the actual transfer date and select a day-3 or day-5 embryo.
- Ultrasound result: enter the scan date and gestational age on that report. Weeks can be 4–42 and additional days 0–6; blank days count as 0. For 8 weeks 6 days, enter 8 and 6 separately.
The due date and gestational age are estimates based on the method you select. This tool cannot confirm a pregnancy.
Formulas and examples
| Method | Due date calculation | Example |
|---|---|---|
| Last period | First day + 280 days + (cycle length − 28) days | January 1, 2026, 28-day cycle → October 8, 2026 |
| Conception | Conception date + 266 days | January 15, 2026 → October 8, 2026 |
| Embryo transfer | Day-3 embryo + 263 days; day-5 embryo + 261 days | January 20, 2026, day-5 embryo → October 8, 2026 |
| Ultrasound | Scan date + (280 days − gestational age in days at scan) | March 5, 2026, 8 weeks 6 days → October 9, 2026 |
Last-period dating assumes a 28-day cycle with ovulation about 2 weeks after the period begins. Cycle adjustment simply shifts the assumed ovulation date by the difference in cycle length. With the same January 1 date and a 35-day cycle, the due date moves 7 days to October 15. This adjustment may not fit irregular cycles.
Reading gestational age and elapsed time
Gestational age is not time since conception. With a 28-day cycle, the first day of the last period is 0 weeks 0 days; conception-based dating adds about 2 weeks. IVF dates are converted to the same gestational-age scale.
The calculator’s dating origin is the due date minus 280 days. After cycle adjustment or ultrasound dating, it may differ from the actual last-period date. For January 1 with a 35-day cycle, the calculated origin is January 8. If the reference date is earlier, a dash and an explanation replace the gestational age.
The elapsed percentage divides calculated gestational days by 280. It shows up to one decimal place and stays at 100% after 40 weeks. It does not measure fetal development or readiness for birth.
Milestones and term categories
The 20-week row is the midpoint relative to 40 weeks. A milestone on the reference date is highlighted first; otherwise, the next milestone is highlighted. These dates do not recommend an examination or delivery schedule.
ACOG and the Society for Maternal-Fetal Medicine (SMFM) use these categories:
| Category | Gestational age |
|---|---|
| Early term | 37 weeks 0 days–38 weeks 6 days |
| Full term | 39 weeks 0 days–40 weeks 6 days |
| Late term | 41 weeks 0 days–41 weeks 6 days |
| Post-term | From 42 weeks 0 days |
WHO defines preterm birth as birth before 37 completed weeks. These categories alone do not determine a safe delivery date for an individual pregnancy.
When a clinician has set your due date
ACOG describes first-trimester ultrasound, up to 13 weeks 6 days, as the most accurate way to establish or confirm gestational age. This tool’s ultrasound method only converts the entered scan date and gestational age into dates. It does not interpret scans or decide whether an established due date should change.
IVF dating uses the embryo’s developmental age and transfer date. Do not reset your due date based only on a calculation using a scan with a different gestational age. Follow the date and care plan set by your clinician.