The length of the embryo is measured from the head to the tail end, and is designated under the term CTP (Crown-Rump Length). It should be noted that the coccygeal-parietal size of the embryo is less subject to individual fluctuations than the average internal diameter of the ovum, and therefore, its use to determine the gestational age gives better results. The error usually does not exceed ±3 days. With clear visualization of the embryo, the gestational age is determined depending on its length, and not on the size of the average internal diameter of the ovum (SVD).
To correctly measure the coccygeal-parietal size of the embryo, its clear visualization is necessary. In this case, one should strive to measure the maximum length of the embryo from its head end to the tailbone.
During normal pregnancy, the diameter of the ovum increases by 1 mm per day. Lower growth rates are a poor prognostic sign. With a gestation period of 6-7 weeks, the diameter of the ovum should be about 30 mm.
It should be emphasized that determining the gestational age by the length of the CTE is best done before 12 weeks of pregnancy. At a later date, measurements of biparietal diameter, head and abdominal circumference should be used.