My due date was based on ivf transfer: scan due date showed day ahead but they still went for ivf due date as it more accurate.
Same thing happened to me! Due date was 2/22 but baby measured a couple days ahead at the 12 week scan and they changed the date to 2/19. The hospital computer likes the 2/19 date better because it lines up more with my last menstrual cycle. My midwife said it's difficult to change in the system and that due dates are just best guesses anyways for most people so we left it as is. If it had been a week or more we would have changed it.
For mine they determined due date at early US scan when I was around 8 weeks. It was only a 2 day difference between it and the estimated due date based on transfer.
When was your transfer? The short answer is: start at your last menstrual period with the IVF pregnancy and count 9 months + 1 week to achieve your estimated date (example: my period started 9/18/18, weeks transferred my daughter 10/7/18, and estimated due date was 6/25/19 even though she ended up arriving about 10 days early on 6/14/19). Hope this helps!
My due date hasn't changed even if baby has been measuring bigger. At my 12 weeks scan, the sonographer was adamant that regardless of size my due date wouldn't change!