I managed a doctor’s office that specialized in high risk pregnancies. We did a lot of sonograms. A sonogram shows a great deal and they do measure head circumference and the length of the femur. This is all put into a computer along with the mother’s date of her last period. All this is factored in and the due date is determined. If the mother is vague about the last date of her period then the results can be compromised. In that case the technician will rely heavily on the measurements compared to national averages, again factored by computer. The mother’s date of her last period is also important in figuring alpha fetal protein ratios which can sometimes give a false positive for downs syndrome. That can necessitate more expensive testing. Usually everything is fine and the baby is normal. Women of child bearing age should keep tract of the dates of their menstrual cycles.. This can be important for a number of health concerns.
