How the math works
A typical pregnancy is 280 days from the first day of the last menstrual period (LMP) — the convention used by obstetricians, midwives and Naegele’s rule since 1812. Conception itself happens around day 14 of the cycle, so the conception-to-birth window is 266 days. For IVF, we know the exact transfer day and embryo age, so the calculation is straightforward.
What the date doesn’t tell you
Only ~5% of babies arrive on the predicted day. About 80% are born between 37 and 42 weeks. First-trimester ultrasound dating (between 8 and 13 weeks gestation) is the most accurate method and will likely shift your due date by a few days from what this calculator shows — that’s expected.
Talk to your provider
A due-date estimate is a planning number — it isn’t a substitute for regular prenatal care. Canadian families can access pregnancy care through family physicians, obstetricians and registered midwives, all of which are covered by provincial health insurance. The Society of Obstetricians and Gynaecologists of Canada has a public-facing site, Pregnancy Info, with guidance on what to expect at each visit.