I havent a clue...

I was just wondering if any of you would share how you worked out how many weeks along you are? I went to my doctor, thinking they would estimate it but they didnt? My last period was the 18th May and my cycle usually lasts about 31 days if thats any help? Sorry for sounding a little stupid, first baby so this is all a little new to me🤷🏼‍♀️