Use the retrieval date, which is the date the egg(s) actually fertilzed, then go back 2 weeks from there (pregnancy is usually counted from the first day of your last menstrual period (LMP), but since you did IVF, then 2 weeks before retrieval will give you the date you should use as the 1st day of your LMP). So, if your retrieval was 2 weeks ago today and you count back another 2 weeks, you are 4 weeks pregnant today.