Prior to having your first ultrasound, you can estimate your due date using the first day of your last menstrual period. Below is a link to a due date calculator:
http://www.materna.ca/pregnancy/due-date-calculator
The most accurate way to determine your due date is using an ultrasound done in the first trimester (ideally between 7-12 weeks). If more than one ultrasound is done in the first trimester, the earliest ultrasound (past 7 weeks) is the most accurate. Your due date will not change based on ultrasounds done later in the pregnancy, as they are less accurate for dating the farther you are along.
If you do not have a first trimester ultrasound, a second trimester ultrasound can be used to determine a due date, although it is less accurate. A third trimester ultrasound is less accurate still. Ultrasounds done before 23 weeks gestation are more accurate than last menstrual period dating in most cases.
It is important to determine as accurate a due date as possible in order to know if your pregnancy is progressing normally, and to know when a pregnancy becomes “post-dates” or “over due”. There are some risks associated with going well past your due date, and for these reasons we may counsel you about induction of labour when your pregnancy has gone 1-2 weeks past your due date. There are also risks associated with induction, and therefore we do not want to initiate inductions based on an inaccurate due date.
*SOGC guidelines February 2014