However, if you have a larger than average baby, the ultrasound will apply the husky figures to the “normal” measurements.
This creates error in that the baby will compute out to be further along than he or she really is.
Therefore, measurements taken later on, when babies begin to grow at different rates among pregnancies, yield increasingly inaccurate dating of pregnancy.
Even when the last period is known, ultrasound is reassuring to demonstrate adequate growth, especially when there’s a risk of delayed growth, as in hypertension or smoking, or if there’s the risk of exaggerated growth, as in gestational diabetes.
It is not uncommon for babies that are labeled “Large for Gestational Age (LGA)” and “Intra Uterine Growth Restriction (IUGR)” to have monthly or even weekly ultrasounds during the pregnancy.
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).
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!
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.