A natural pregnancy is measured from the beginning of the last menstrual period because the date of ovulation and egg fertilization is not always known. An IVF pregnancy is similar, except the process of conception is followed more precisely.
In IVF we know exactly when ovulation and conception occurs. Two weeks after conception and embryo transfer a pregnancy test (beta) is administered. If the pregnancy test is positive the mother-to-be is traditionally 4 weeks pregnant (includes two weeks pre-fertilization - similar to natural pregnancy).
As some have lightheartedly suggested, in IVF you are "pregnant until proven otherwise".