THANK YOU...WE ARE JUST SO ANXIOUS AND NERVOUS TO GET THROUGH THE FIRST FEW MONTHS...
I would just use the retrieval date as a starting point, then go back two weeks and use that as "last menstrual period". Then go online and find a pregnancy calculator, enter the LMP (last menstrual period) and you'll be able to get a due date. Although if it's twins, she may deliver before then ;-)
Best of luck!