Insert first day of last menstrual period: Insert CRL (Crown Rump Length):*: mm: Insert date of the CRL measurement: * CRL (Crown Rump Length) is fetal length measured from the top of the head (crown) to the bottom of the buttocks (rump), obtained by ultrasonographic examination. If you know the CRL measurement, you can add it into calculator together with the date of your last menstrual period. This way of calculation is appropriate especially when the date of the last menstrual period is unknown. You can use both parameters (CRL and last menstrual period) and compare if the gestational ages counted according to the CRL and last menstrual period are concordant. If the difference between them is bigger than seven days, it is more precise to keep to the calculation of the gestational age according to the CRL measurement.