How long does pregnancy really last? How do doctors determine due dates?

Pregnancy is 10 months long. The due date starts the first day of a female's ovulation date. The ovulation date starts around 7 days after her period starts. If the doctor told her that her due date is less than 9 months away, she may be pregnant before the date she assumed she was pregnant. Usually, it takes 1-2 months before you can detect whether you're pregnant or not.