Crown/rump length and gestational age are closely compared with each other until around the end of the first trimester.The ideal time for a sonographer to assess the gestational age in the first trimester is between 7-10 weeks of pregnancy.The limbs and the yolk sac, though obviously important, are not the primary means of measuring growth.An average length of the embryo at 7 weeks is anywhere between 5mm-12mm. Obviously, every pregnancy is unique and individual factors influence the size of the embryo at this early stage, and the embryo shows development week by week.This is the period of time which provides the most accurate assessment.Generally, ultrasounds which are performed in the first trimester are within 3-5 days of being accurate in terms of assessing gestational age.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.
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.
For transvaginal ultrasound you can have an empty bladder Dating scans are usually recommended if there is doubt about the validity of the last menstrual period.
For example if you a tiny sac can be seen in the uterus, but the baby and its heart beat may not be detected yet.
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co.
5 ½ weeks gestation means 5 ½ weeks from the first day of the last menstrual period, which is usually about 3 ½ weeks from the date of conception (confusing, isn’t it!