I may not be answering your question directly but I would advise don’t count 😂. Just follow the EDD on your first scans else you’ll stress yourself out cause in between before you give birth, your EDD will change depending on baby’s size then you will lost count again. If you want to know your rough date of conceiving, use the first EDD date and backtrack. (Plus minus 1-2 days)
Doctor normally use the first day of the last cycle to calculate the number of weeks. then as the baby grows, then they able to calculate the edd..