Crown rump length (CRL) is the length of the embryo or fetus from the top of its head to bottom of torso.
It is the most accurate estimation of gestational age in early pregnancy, because there is little biological variability at that time.
CRL is measured as the largest dimension of embryo, excluding the yolk sac and extremities.
After 13 weeks, head circumference, biparietal diameter, and femur length measurements become more useful measurements for assessing fetal growth.
The earlier in pregnancy a scan is performed, the more accurate the age assignment from crown rump length .
If the original CRL measurement was adequate, the measurement is considered the baseline for all subsequent age measurements.
Overall, the accuracy of sonographic dating in the first trimester is +/-5 days (95% confidence range).
The measurement of CRL is useful in determining the gestational age (menstrual age starting from the first day of the last menstrual period) and thus the expected date of delivery (EDD).
Different babies do grow at different rates and thus the gestational age is an approximation.
Recent evidence has indicated that CRL growth (and thus the approximation of gestational age) may be influenced by maternal factors such as age, smoking, and folic acid intake.
Early in pregnancy it is accurate within +/- 4 days but later in pregnancy due to different growth rates, the accuracy is less.
Cardiac activity should be present in an embryo with a CRL ≥7 mm .
If it not detected at this size on transvaginal scanning performed by an experienced operator, it is an indicator of failed early pregnancy (missed miscarriage).
It has been reported that patients in whom MSD (mean sac diameter) is less than 5 mm greater than crown rump length (i.e.