A method comprising: encoding video data in a video encoder, wherein the encoding includes: receiving luma motion vector information for one or more luma field motion vectors for a macroblock of an interlaced video frame, each of the one or more luma field motion vectors being associated with at least part of the macroblock, wherein each of the one or more luma field motion vectors comprises x and y components; and deriving a chroma field motion vector associated with at least part of the macroblock for each of the one or more luma field motion vectors, wherein the chroma field motion vector comprises x and y components, wherein the macroblock is a 4:2:0 macroblock, and wherein the derivation of the chroma field motion vector comprises: calculating a horizontal component CMV x of the chroma field motion vector, wherein CMVx is equal to (LMV x +s_RndTbl [LMV x & 3])>>1, LMV x is a horizontal component of the luma field motion vector and s_RndTbl[] is a first rounding table, wherein the first rounding table is an integer array consisting of the following set of values: {0, 0, 0, 1}; and calculating a vertical component CMV y of the chroma field motion vector, wherein CMV y is equal to (LMV y >>4)*8 + s_RndTblField[LMV y &0xF], LMV y is a vertical component of the luma field motion vector and s_RndTblField[] is a second rounding table, wherein the second rounding table is a field-based rounding table to maintain correct field offset while rounding so that the luma field motion vector and the chroma field motion vector map to consistent field offsets, and wherein the second rounding table is an integer array consisting of the following set of values: {0, 0, 1, 2, 4, 4, 5, 6, 2, 2, 3, 8, 6, 6, 7, 12}; and outputting the encoded video data in a bitstream. receiving luma motion vector information for one or more luma field motion vectors for a macroblock of an interlaced video frame, each of the one or more luma field motion vectors being associated with at least part of the macroblock, wherein each of the one or more luma field motion vectors comprises x and y components; and deriving a chroma field motion vector associated with at least part of the macroblock for each of the one or more luma field motion vectors, wherein the chroma field motion vector comprises x and y components, wherein the macroblock is a 4:2:0 macroblock, and wherein the derivation of the chroma field motion vector comprises: calculating a horizontal component CMV x of the chroma field motion vector, wherein CMVx is equal to (LMV x +s_RndTbl [LMV x & 3])>>1, LMV x is a horizontal component of the luma field motion vector and s_RndTbl[] is a first rounding table, wherein the first rounding table is an integer array consisting of the following set of values: {0, 0, 0, 1}; and calculating a vertical component CMV y of the chroma field motion vector, wherein CMV y is equal to (LMV y >>4)*8 + s_RndTblField[LMV y &0xF], LMV y is a vertical component of the luma field motion vector and s_RndTblField[] is a second rounding table, wherein the second rounding table is a field-based rounding table to maintain correct field offset while rounding so that the luma field motion vector and the chroma field motion vector map to consistent field offsets, and wherein the second rounding table is an integer array consisting of the following set of values: {0, 0, 1, 2, 4, 4, 5, 6, 2, 2, 3, 8, 6, 6, 7, 12}; and calculating a horizontal component CMV x of the chroma field motion vector, wherein CMVx is equal to (LMV x +s_RndTbl [LMV x & 3])>>1, LMV x is a horizontal component of the luma field motion vector and s_RndTbl[] is a first rounding table, wherein the first rounding table is an integer array consisting of the following set of values: {0, 0, 0, 1}; and calculating a vertical component CMV y of the chroma field motion vector, wherein CMV y is equal to (LMV y >>4)*8 + s_RndTblField[LMV y &0xF], LMV y is a vertical component of the luma field motion vector and s_RndTblField[] is a second rounding table, wherein the second rounding table is a field-based rounding table to maintain correct field offset while rounding so that the luma field motion vector and the chroma field motion vector map to consistent field offsets, and wherein the second rounding table is an integer array consisting of the following set of values: {0, 0, 1, 2, 4, 4, 5, 6, 2, 2, 3, 8, 6, 6, 7, 12}; and outputting the encoded video data in a bitstream.