Crown-Rump Length corresponds to the distance located between the top of the cephalic pole of the embryo and the buttocks. This distance is measured by ultrasound. It makes it possible to estimate the age of pregnancy to within 3 days, that between 7 and 13 weeks of amenorrhea. This is the reference measurement for estimating gestational age.
Accurate knowledge of the gestational age from the start of pregnancy is very important. It takes on particular importance in the following situations:
Declaration of pregnancy (before 15 w);
Prescription of three obstetric ultrasounds (12, 22 and 32 w);
Screening for trisomy 21 and chromosomal abnormalities by serum markers (between 11 and 13 w + 6 days);
Implementation of end-of-pregnancy monitoring in the event of a missed term (40 weeks).
The values indicated are reliable to within 3 days if the measurement was taken between the 7th and 14th week of amenorrhea. Outside of these values, the error may be larger.