I'd also be going with your most recent scan.
My OB told me Toby was a girl when he scanned me at 14 weeks. He looked again at 18 weeks and said he wasn't so sure, he could see something between baby's legs but wasn't sure if it was doodle or cord. Went for my 20 week scan at a specialist ultrasound clinic and they got much clearer pictures and we could finally see clearly enough to satisfy ourselves that we definitely had a boy.
Often the machines OB's use in their office aren't that great so that makes it even harder for them to tell.
Ultrasound scans are only as good as the machine and the skills of the operator. One may have a wonderful Ob but they are not necessarily trained specifically in performing ultrasounds. Anyone can buy a machine and put it in their office.
This is very true.