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.
This is where our wonderful ultrasound machines can make a crucial difference.
With today's modern equipment, we can obtain very reliable images and measurements of even very early pregnancies, sometimes even seeing a heartbeat as early as 5-6 weeks!
The accurate determination of a patient's "due" date, referred to by doctors and midwives as the EDC (Estimated Date of Confinement) or EDD (Estimated Date of Delivery), is very important for a variety of reasons.
The timing of certain tests, the monitoring of the baby's growth, and the correct diagnosis of premature labor, or being truly "overdue," (postdates), as well as many other situations that arise in the course of a typical pregnancy, all depend on a correct determination of the EDC for appropriate management.
In the past, the EDC was calculated by using Naegele's Rule, which determined the date by subtracting 3 months from the 1st day of the last period and then adding 7 days.
In fact, when a woman who has very regular, "textbook" 28 day cycles presents for prenatal care, this often turns out to be the correct EDC more often than not.
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.
The simple rule in ultrasound is that when the due date based on ultrasound doesn’t vary from the mother’s dates by more than a week, stick with the mother’s dates; if the ultrasound disagrees by more than a week to ten days, it becomes wiser to rely on the ultrasound.