Accurate dating sonogram
2 weeks for second trimester & 3 wks difference in the third trimester to change the due date compared to an lmp due date.
Always keep the earliest us edc, it should never change.
In our practice, the EDC will be determined at the first prenatal visit whenever possible, and subsequent ultrasounds, which often will report a different (often earlier) EDC will not override the first EDC.
In fact, the only time that this doesn't apply is when the date of conception is known without doubt, as when an infertility patient conceives with timed intercourse or artificial insemination.
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.
If one baby is measuring say 67mm and one 76mm, and they date them both at 13w, how does that work??
Isn't that measurement what they do on the scan to work out your EDD?
I went in to the OBGYN today to go over blood tests that I'd submitted a few weeks ago, and for what I thought was an ...but, turns out I'm farther along than I thought, and I'm 10 weeks and 1 day (instead of 8 5)!
Other information used to calculate the gestational age, or the number of weeks and days from the first day of the last menstrual period (LMP), includes the size of the uterus on pelvic bimanual examination.
However, it is extremely common to encounter patients who have irregular or infrequent menstrual cycles, or have fibroid tumors that cause their uterus to feel enlarged on pelvic exam, or who conceive shortly after a pregnancy ends (without ever actually having had a period after the last pregnancy), or who got pregnant while taking birth control pills, and these situations often render the above methods useless and misleading when trying to figure out a reliable EDC.
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!