Due to spinal segmentation, the Crown-Rump Length (CRL) measurement loses accuracy after which week?

The Crown-Rump Length (CRL) measurement is most accurate for assessing fetal age during early pregnancy, particularly within the first trimester. After around 11 weeks of gestation, the accuracy of the CRL diminishes significantly due to the process of spinal segmentation. As the fetus develops, structures such as somites begin to form along the spinal column, and the increase in variability in body proportions affects the reliability of the CRL measurement for dating purposes.

At this stage, variations in fetal position, curvature of the spine, and the stage of development can result in less consistent and reliable CRL measurements. Consequently, relying on this parameter for estimating gestational age becomes increasingly problematic after this point, as it may not accurately reflect the true developmental age of the fetus. Instead, alternative measurements and imaging techniques are often employed as the pregnancy progresses beyond this critical timeframe.