Medical professionals use a standard set of up to three methods to date pregnancies: last menstrual period, ultrasound, and a physical exam.
That way, regardless of where they trained or where they practice, any two doctors dating a pregnancy will predict the same due date or gestational age.
Instead, we most commonly date pregnancies from the first day of the last menstrual period (LMP).
An image of your fetus will appear on an accompanying computer screen and while looking at this image, the technician will take some standard measurements from different angles and listen for a heartbeat.
However, the timing of the actual delivery is not so predictable.
Some women will deliver on their due date, others before, and others after.
However, if you did not get a first trimester ultrasound for dating and an ultrasound done to look at the baby's anatomy (usually at 18-20 weeks of pregnancy) differs from your LMP date by 10 to 14 days, we would change your date to the ultrasound estimate.
Every pregnant woman wants to know her due date, and a due date that is calculated from her last menstrual period often doesn't match the due date that is estimated by her first ultrasound (also known as a sonogram).