Crown Rump Length Calculator

Crown rump length, commonly abbreviated CRL, is the distance from the top of an embryo or fetus to the lower end of the torso. A trained sonographer measures it on a properly aligned first-trimester ultrasound image while the fetus is in a neutral position. The result is reported in millimeters. Because early fetal growth follows a relatively consistent pattern, CRL is widely used to estimate gestational age and establish pregnancy dating during the first trimester. This calculator uses the Robinson and Fleming relationship: gestational age in days equals 8.052 times the square root of CRL in millimeters, plus 23.73. It is intended for CRL values from 5 through 84 mm, roughly the period from six to fourteen weeks. The calculated age is rounded to the nearest day and divided into completed weeks and remaining days. An estimated due date is then found by adding the number of days remaining in a 280-day pregnancy to the ultrasound measurement date. Measurement technique is essential. The image should show a true midsagittal plane, clear crown and rump landmarks, and neither excessive flexion nor extension. Calipers are placed at the outer borders. A clinician may use an average or best technically acceptable measurement according to local guidance. A value copied from an informal image, measured by an untrained observer, or taken outside the validated size range should not be treated as reliable pregnancy dating. The optional last menstrual period date provides a simple comparison. LMP dating assumes a cycle pattern and ovulation timing that may not apply to every pregnancy. A discrepancy can arise from uncertain recall, irregular cycles, delayed ovulation, measurement variation, or pregnancy development. Professional guidelines specify when an ultrasound difference is large enough to revise an established due date; the calculator only reports the absolute difference and does not make that decision. CRL dating has uncertainty, often several days even with good imaging. It does not assess fetal viability, anatomy, genetic conditions, or growth prognosis, and one measurement cannot explain symptoms. The official gestational age and due date should come from the obstetric record after review of ultrasound quality, menstrual history, assisted-reproduction dates, and prior scans. Pain, heavy bleeding, fainting, or other urgent pregnancy concerns require prompt medical assessment.