Accuracy of dating pregnancy by ultrasound
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!In early pregnancy, the measurement from the top of the fetal head to the bottom of the pelvis is called the “Crown-Rump” length.Before twelve weeks, it’s difficult to measure much more. In the second trimester, (past the first 12 weeks), the other measurements can be added.
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).Most babies, except in cases of early IUGR and deformities, grow about the same until 20 weeks or so.
After that, the variations among babies’ measurements cause increasing inaccuracies.
Your doctor might want to repeat the ultrasound to make sure that your pregnancy is developing as it should.