When you are pregnant, you may have a number of ultrasounds. During the procedure, the technician or nurse takes a number of different measurements. These measurements are then used by the ultrasound machine's software to compute the age of the fetus. This is done by comparing the length of the measurement of certain areas of the developing fetus against the database included in the machine's software. This database compares the measurement taken to the range of sizes common at different fetal ages. Using this method, the ultrasound machine determines the age of your pregnancy. There is a margin for error in this process; for example, if you have big babies or if you are expecting multiples.
First Ultrasound
Most typically, at your first ultrasound, the only measurement that will be feasible is the crown-to-rump length. This is because most women have their first ultrasound quite early in their pregnancy, between six to 10 weeks, and the fetus isn't developed enough for further measurement.
Next Ultrasound
For most women, the next ultrasound occurs between 18 and 22 weeks. At this ultrasound, a wide variety of measurements are taken. The crown-to-rump length is again measured, as are the femur (top bone of the thigh) and the distance across the head (called the biparietal diameter, or BPD).
Further Ultrasounds
Most women won't require further ultrasounds. However, if there is a question as to amniotic fluid levels or if there is a potential for gestational diabetes, for example, more ultrasounds may be required. Not all measurements are taken for these ultrasounds, unless medically necessary, as detailed ultrasounds are quite expensive and most regular obstetricians have equipment that provides only basic ultrasound imaging.
Ultrasound Accuracy
Because ultrasounds use a general population for their database of measurements, there is potential for making an error on the gestational age. Dr. Marjorie Greenfield, on the Dr. Spock website, notes that the first trimester ultrasounds are most accurate for dating the age of the pregnancy, and are typically accurate within a week of actual age. Ultrasounds taken in the second trimester are accurate to within two weeks, due to the individual growth of each baby. Third trimester ultrasounds are the least accurate as each baby grows at its own pace; they can be accurate only within a three-week range.
