Hi Kimmi
When I did my first ever IVF cycle my FS wanted to be cautious and do an early scan (day 5 of stims if I remember rightly?), as I have polycystic appearing ovaries (no PCOS, just ovaries that look a little polycystic, which apparently up to 50% of women do). If you have polycystic ovaries, or particulary if you have PCOS you are at a greater risk of OHSS and so they will often want to monitor you more closely when doing your first cycle.

Not sure if you too have PCO or PCOS, but if you don't your FS might just be being extra cautious anyways, wanting to check things earlier to see how you're responding.

Good luck with it all
Arielle