Foetus ultrasound scans and calendar showing the estimated date for delivery (EDD). Ultrasound is a diagnostic imaging technique that is routinely carried out during pregnancy. It allows the foetus' health to be assessed and enables the date of birth to be determined more accurately. Early in pregnancy, the baby's crown-to-rump length is measured, while later the length of the femur and size of the head are measured to give an indication of the stage of pregnancy. Babies are most often born within a week either side of the EDD.