Due Date Calculator
Three Starting Points, One Due Date
Not every pregnancy is dated the same way. Most are dated from the last menstrual period, but IVF pregnancies have a known fertilization date that makes LMP dating unnecessary and less accurate, and some people simply know their conception date more precisely than their last period. This calculator handles all three starting points and converts each into a due date using the arithmetic appropriate to it.
The Formulas
From conception date: Due Date = Conception Date + 266 days
From IVF transfer: Due Date = Transfer Date + (266 − Embryo Age at Transfer)
280 days (40 weeks) is counted from the last period; 266 days (38 weeks) is counted from conception itself, which happens roughly two weeks into a standard cycle. The IVF version starts from a known transfer date and simply subtracts however many days old the embryo already was at transfer.
The Three Methods Agree, Given Consistent Dates
For a hypothetical last period start of January 5, 2026 on a 28-day cycle (implied conception January 19, implied blastocyst transfer January 24):
| Mode | Input date used | Computed due date |
|---|---|---|
| From last period | Jan 05, 2026 (cycle 28) | Oct 12, 2026 |
| From conception date | Jan 19, 2026 | Oct 12, 2026 |
| From IVF transfer (Day 5) | Jan 24, 2026 | Oct 12, 2026 |
| From IVF transfer (Day 3) | Jan 22, 2026 | Oct 12, 2026 |
All four rows land on the same date because the underlying inputs are equivalent — this is the arithmetic check, computed with the calculator's own formulas, not a claim that real cycles are always this tidy.
Which Mode to Use
- From last period — the standard method for a natural conception when you know your LMP and typical cycle length.
- From conception date — useful when ovulation timing is known more precisely than the last period, for instance from ovulation tracking.
- From IVF embryo transfer — the standard for IVF pregnancies, since the embryo's exact age is known at transfer, removing the guesswork inherent in LMP dating.
How to Use This Calculator
- Select a mode: From Last Period Date, From Conception Date, or From IVF Embryo Transfer Date.
- For last period mode, enter the Last Period Start Date and Average Cycle Length (days) (default 28).
- For conception mode, enter the Conception Date.
- For IVF mode, enter the Embryo Transfer Date and select Day 5 (Blastocyst) or Day 3 (Cleavage-stage).
- Select Calculate for the estimated due date.
Related Calculations
For an IVF-specific version with additional dating detail, see the IVF Due Date Calculator. To track gestational age and trimester once you have a due date, use the Pregnancy Week Calculator.