Due date calculation
Usually calculated by doctors and midwives based on the first day of the last menstrual period, the delivery date is set after that date and then adjusted at the first ultrasound.
A pregnancy is normally 40 weeks long from the first day of last menstruation. The day you became pregnant is usually 2 weeks after the day of conception.
This means that you are not actually pregnant for the first two weeks as you have not yet ovulated.
If you choose to count from the conception date, your pregnancy will be 38 weeks long instead.
However, most people do not count the pregnancy from the conception date as it is impossible to know exactly when conception took place.
If your periods are very irregular, it will be even more difficult to set a due date using your period. But even for women with a more regular period, it is very unlikely that the birth will take place on the expected date as only just over 5% of babies are born as expected, usually going into labour 2 weeks before or after the expected date.