Technically you're right. The age of the fetus is only 3 weeks, but in gestational age, which is what most doctor's use, she's 5 weeks. Basically, gestational age includes the two weeks in the beginning of the cycle, starting with the first day of her last menstrual period. Since your wife did IVF, however, they just add 2 weeks to the time from retrieval to get gestational age. Hope that makes sense.