Hi! From what I know, it really depends sa clinic or hospital, some allow walk-in ultrasounds kahit walang doctor's request, pero iba ang policy eh. Best thing to do is call ahead sa OB clinic or diagnostic center na gusto mo para malaman kung pwede ka lang bumisita without referral, para hindi ka mapagod na bumyahe for nothing. Maraming clinics na okay lang talaga, lalo na kung private, so ask them directly, mas mabilis yan than guessing!