Ay, confusing talaga yan! The best person to ask about which dating method to follow is your OB, siya ang may full picture ng case mo and kung bakit may size difference between scans. Huwag mag-alala, common lang na may variation sa ultrasound, pero seryoso talaga ang dating para sa delivery plan, kaya definitely i-clarify mo with her kung aling reference point siya talaga gagamitin para sa follow-ups mo. Good na nag-ask ka!