Cara pengiraan due date bersalin

Assalamualaikum and Hi to all lovely mommies here, ada sesiapa tahu macam mana nak kira due date bersalin? Ikut first day of last period ke? Kalau ikut yg itu, sy dah 11 weeks tapi jadi confuse sebab bila check size baby. Baru 9 weeks. #firsttimemom #firstbaby