Confuse 4weeks or 5weeks

Korang, boleh tolong kirakan jap tak? 1st day of my last period is 15/10. So skrg ni sy pregnant 4weeks atau 5weeks ye? Kalau letak tarikh kat apps, dia tulis 4weeks. Tapi bila kita kira guna jari kat calendar, patutnya 5 weeks la kan kalau start dari 1st day last period tu?