Crown-rump length
Abbreviation: CRL
Crown-rump length (CRL) is the measurement taken on ultrasound from the top of the embryo's or fetus's head (crown) to the bottom of its bottom (rump), deliberately excluding the legs because they are curled up. This measurement is the most accurate way to estimate gestational age in the first trimester, typically between 6 and 14 weeks. A small change in CRL corresponds to a precise number of days of development, making it a reliable tool for confirming your due date early on. After 14 weeks, other measurements such as head circumference and femur length become more useful for tracking growth.
What This Means for You
Understanding Crown-rump length is an important part of your first trimester journey. During pregnancy, your care team will use this term regularly. Being familiar with it means you can ask better questions and feel more confident at your antenatal appointments.
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If your CRL measurement at your dating scan differs from your expected dates by more than 5 to 7 days, your doctor may adjust your official due date, which can also shift the timing of your first trimester screening blood tests.
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Always consult your healthcare provider for personalised advice.
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