The computer in the ultrasound (ignorantly) lumps your bigger baby into the dates of babies that big in the “normal” population.This may then indicate that your 37-week baby is two weeks overdue!Therefore, measurements taken later on, when babies begin to grow at different rates among pregnancies, yield increasingly inaccurate dating of pregnancy.
This creates error in that the baby will compute out to be further along than he or she really is.
For instance, say you normally have eight and nine pound babies.
Then your baby at 37 weeks will have bigger measurements than babies destined to weigh seven or eight pounds at birth.
When an ultrasound is performed, measurements of the head, abdomen, thigh, and amount of amniotic fluid are done.
These measurements are computed automatically in the ultrasound machine’s software.
The software has certain measurement scales based on data from large populations, and your baby’s measurements are put into this scale.
In other words, by comparing your baby’s measurements to the data from this large collection of measurements, the ultrasound can then tell how far along your baby is.
Ultrasound has become so helpful that obstetricians now refer to the time before it was used routinely as “the olden days.” We use it to diagnose twins early on; we use it to document appropriate growth as the pregnancy progresses; we use it to determine fetal health; and we use it to guide conversion of breech to vertex (head-first) position and to guide amniocentesis.
Of all of these uses, dating the pregnancy is the most common reason to use ultrasound, particularly when the expectant mother cannot remember the date of her last period (as in breast-feeding or irregular cycles).