Knowing how many weeks pregnant

nagpacheck up at TVS po ako sa dalawang OBs. Iba iba po ang binigay nila kung ilang weeks na ako pregnant, 1 week po ang difference. Pero sa Asianparent app, same ang estimate sa former OB ko. - Last period was Oct 1 - On Dec 22, 11 weeks & 6 days (according to AsianParent app Importante po ba to know exactly kng ilang weeks ka preggy?