Dating a pregnancy accurately at the earliest possible time is important. In a normal pregnancy it does not matter what the dates are, but if anything should go wrong it becomes very important as to what treatments may be prescribed. Traditionally the last menstrual period has been used to date a pregnancy, but this may be highly inaccurate.
A simple formula is to take the first day of the menstrual period, subtract three months and add one week. For instance, if your period starts on the 5th April, then your estimated due date for delivery is the 12th January.
The dating scan is an ultrasound scan that can be done as early as six weeks after the last period, but the dating scan will be most accurate when done between 8 and 12 weeks gestation.
At this time, the fetus is a little larger and easier to measure. Even dating scans can be inaccurate for the due date, but they are never as inaccurate as using the menstrual period.
For instance a dating scan done at 6 weeks could get the date wrong five days either way. A scan at 12 weeks could get the date wrong seven days either way and a scan done at 20 weeks could get the date wrong up to ten days either way.
I prefer to do the dating scan via the transvaginal approach at approximately 8 weeks gestation. This is normally done in the consulting room for a small out of pocket cost.
This cost is nowhere near the out of pocket cost if the scan were to be performed at a radiology practice.
The safety of ultrasound scans in early pregnancy has been documented for over three decades. There are no known adverse effects from early pregnancy ultrasound scans recorded in the literature.