How is the week of pregnancy calculated?
Pregnancy is dated from the first day of the last menstrual period, not from conception. Counted this way, a pregnancy lasts about 40 weeks (280 days). For a 28-day cycle, the due date is the last period plus 280 days (Naegele's rule).
With longer or shorter cycles, ovulation happens later or earlier. The calculator corrects for the cycle length you choose.
IVF pregnancies
In IVF the embryo age and transfer date are known exactly, so the calculation is based on the transfer date. A day-5 (blastocyst) transfer corresponds to 2 weeks and 5 days of pregnancy on the day of transfer, and a day-3 transfer to 2 weeks and 3 days.
How accurate is the result?
The date is an estimate. Only a small share of babies are born exactly on the due date; most are born between 38 and 42 weeks. The most accurate dating is by measuring the baby's crown-rump length on a first-trimester ultrasound.