Extracting top & bottom fields from a YUV 4:2:0 Frame
Assuming the encoder properly low-pass filtered the chroma before sampling (which may not always be true), chroma samples from the adjacent macroblocks might need to be used to properly reconstruct (interpolate) any per pixel color.  Those samples may exist due to motion compensation from previous fields, or might need to be inferred by reflecting, duplicating or extrapolating edge data.