If the baby's femur length measures a week and 3 days shorter than the BPD which measurement is used to determine a due date?

They are both compared with your date calculated from your last period. If you don't know your dates the BPD willprobably be used. You will probably have another scan to check there is good growth. It also depends when the scan is done,the later the scan the less the measurements correlate. You might also have a very active baby making absolutely accurate measurements difficult. 10 days is well within the margin of error for scans. Don't forget tht babies can be born anytime from 3 weeks before to 2 weeks after the due date so it is not set in stone.