A method of assessing a pregnant woman's risk of having a fetus with a fetal abnormality by determining the fetus' BPD/OFD ratio and using the BPD/OFD ratio in conjunction with one or more other screening markers for the fetal abnormality. Also provided is a method of determining whether a pregnant woman is screen-positive or screen-negative by comparing the BPD/OFD ratio of the pregnant woman's fetus to a risk cut-off level.