Accurate weeks

Ano po ba ang accurate weeks? Yung first tvs base on sac diameter or yung first pelvic na base on CRL? nag iiba iba akso weeks ko pag inuultrasound pero 2-3days lang naman pagitan. 25 weeks ako today base on CRL tvs 25 weeks and 4 days base on Pelvic utz June 5 25weeks base on april 28 fetal biometry pelvic utz naguguluhan ako #1stimemom po.