Aw, don't worry too much, this is actually super common and honestly, your OB is doing the right thing! So from what I've heard from other mums, kapag may 3-week gap like yours, the ultrasound measurement (CRL) is way more reliable than the LMP, especially kung ang LMP mo ay hindi ka 100% sure or kung nag-ovulate ka ng later than usual (which happens to a lot of us, di ba?). The progesterone is just extra support para sa baby at peace of mind para sa OB, and it's totally normal to prescribe it kahit walang spotting, better safe than sorry with progesterone! The good news is may heartbeat na si baby, so that's a really positive sign, the fact na maliit pa lang lang siya matches perfectly with the 6-week dating from the ultrasound, kaya everything seems aligned actually. My neighbour went through something similar and her OB explained na ang dating ay mag-adjust based sa ultrasound findings, at ang baby niya turned out perfectly fine! That said, definitely ask your OB to explain kung ano talaga ang dating na gagamitin throughout your pregnancy (usually they'll stick with the ultrasound one), at balik-usap ninyo kung comfortable ka sa progesterone plan, pero super worth it na i-follow siya for now. Sending you a hug, mama, alam ko nerve-wracking ito!