If she had an early ultrasound, like in her 7th or 8th week
, it would be more helpful than one in her 22nd week
, since babies can grow at different rates
and this is more pronounced as time goes by. If she did have an early ultrasound, they should have given her an estimated due date based on the measurements in that ultrasound. If they did, and if the ultrasound was early, the estimated due date can be used to count backwards and see when conception might have occurred.