Nak tanya. Actually kita start kira berapa weeks kita based on 1st day period or a week after?

dont worry sis.. bila baby da makin besar and doctor akan buat scan utk ukuran baby.. so if lewat or cepat due date tu, doc akan betulkan.. cthnya mcm sy.. bila ikt tarikh 1st day period, i'm already 26-27 weeks.. so my due date will be around middle april.. but after scan, doc ukur size baby, sy baru 24th week.. so doc akn change the due date..