Why IVF due dates are calculated differently
Standard pregnancy due date calculations work from the last menstrual period, adding 280 days under the assumption of a typical cycle — an approximation, since the exact conception date isn't directly observed. IVF removes most of that uncertainty: the embryo's precise age is known at the moment of transfer, since it was fertilized and cultured under direct observation.
The day-3 vs day-5 difference
Embryos can be transferred at different developmental stages. A day-5 transfer (blastocyst stage) is more developed than a day-3 transfer (cleavage stage) at the moment it's placed in the uterus — so reaching the standard 40-week gestational milestone requires counting fewer additional days forward from a day-5 transfer than from a day-3 one.
Day 3 transfer: due date = transfer date + 263 days
Day 5 transfer: due date = transfer date + 261 days
Why this estimate tends to be more precise
Because the embryo's exact age is known rather than estimated from a last period date, IVF-based due date calculations are generally considered more individually accurate than typical LMP-based estimates — though as with any pregnancy, an early ultrasound and your fertility clinic's ongoing monitoring remain the most authoritative source for your specific case.