Can a three week pregnancy be detected accurately by ultrasound?

my friend plans to marry his girlfriend and they met personally last nov. 11 only (they had known each other only through internet) and now the girl is pregnant. they tested it using the home pregnancy kit and it was positive. the girl explained that she have irregular menstrual cycle and her last period was october 20. they went to the obstetrician and after the ultrasound the doctor said that it was estimated to be 5weeks! is it possible that it was not my friend's baby? they been living together for only 3weeks. we are confused because they already set their wedding next week.