Establishing an estimated due date early on in pregnancy is of vital importance, as this date is then used for the rest of the pregnancy to determine when the baby is likely be born. An ultrasound scan can be used to confirm menstrual dates or to accurately determine how old an embryo is if the last menstrual period is unknown. It is also important in determining the number of embryos, and in the case of twins, to determine what type of twins they are (identical vs non-identical, or monochorionic vs dichorionic). Ultrasound can also determine if a pregnancy is viable, or if there is a problem (for example, a miscarriage or ectopic pregnancy).
Crown-rump length (CRL) is the measurment of the length of embryos and fetuses from the top of the head (crown) to the bottom of the buttocks (rump). After 12 weeks, the accuracy of CRL in predicting gestational age diminishes and is replaced by measurment of the fetal biparietal diameter, head cirumference, abdominal circumference and other femur length.