Doctors work out the how far along you are in weeks using amenorrhœa (missed periods) as a ‘marker’. Your pregnancy is recorded from the first day of your last period. The length of the pregnancy is 41 weeks, starting from the first day of your last period (this is the average and it can vary a couple of days).
In order to calculate your due date, add 9 months and 15 days from the start of your last period. So if your last period began on 1st January, ovulation took place around the 15th. Add 9 months and you have a rough due date: around 15th October.