It should be the first day of your last menses. But I don’t think it matters now because gynae will always scan and measure based on the size of the bb to slightly determine the edd. On the first visit to the gynae. I told her the first day/date of last menses and that i was probably 8w pregnant, but when she scan.. bb was about 7weeks based on the size. On and off will change edd but they will use 1 date as rough gauge - don’t worry!
if doctor/nurses ask when was your last menses, you have to give the first day of your last menses. they will then estimate your EDD. during ultrasound , it shud state the number of weeks of your baby.
ema