A method in a computing device that implements a video encoder, the method comprising: computing a first motion vector predictor for a motion vector of a current portion of a current interlaced P-field, wherein the current portion is a macroblock or block of the current interlaced P-field, wherein the current portion has plural neighbors including a first neighbor and a second neighbor, and wherein the computing the first motion vector predictor includes: identifying a first candidate motion vector of the first neighbor of the current portion, the first candidate motion vector being for a first reference field having a first polarity; identifying a second candidate motion vector of the second neighbor of the current portion, the second candidate motion vector being for a second reference field having a second polarity different than the first polarity; scaling the second candidate motion vector to derive a scaled version of the second candidate motion vector, the scaled version of the second candidate motion vector being a candidate motion vector for the first reference field having the first polarity; determining the first motion vector predictor using at least the first candidate motion vector of the first neighbor and the scaled version of the second candidate motion vector of the second neighbor; computing a second motion vector predictor for the motion vector of the current portion of the current interlaced P-field; determining, based on information indicating which of the first reference field polarity and the second reference field polarity has been used predominately among the plural neighbors around the current portion, which of the first motion vector predictor and the second motion vector predictor is the motion vector predictor with dominant polarity; selecting between using the first motion vector predictor or the second motion vector predictor; computing a motion vector differential between the motion vector and the selected motion vector predictor; and signaling, in a bitstream, information indicating the motion vector differential and a predictor flag indicating whether the motion vector predictor with the dominant polarity is used. computing a first motion vector predictor for a motion vector of a current portion of a current interlaced P-field, wherein the current portion is a macroblock or block of the current interlaced P-field, wherein the current portion has plural neighbors including a first neighbor and a second neighbor, and wherein the computing the first motion vector predictor includes: identifying a first candidate motion vector of the first neighbor of the current portion, the first candidate motion vector being for a first reference field having a first polarity; identifying a second candidate motion vector of the second neighbor of the current portion, the second candidate motion vector being for a second reference field having a second polarity different than the first polarity; scaling the second candidate motion vector to derive a scaled version of the second candidate motion vector, the scaled version of the second candidate motion vector being a candidate motion vector for the first reference field having the first polarity; determining the first motion vector predictor using at least the first candidate motion vector of the first neighbor and the scaled version of the second candidate motion vector of the second neighbor; identifying a first candidate motion vector of the first neighbor of the current portion, the first candidate motion vector being for a first reference field having a first polarity; identifying a second candidate motion vector of the second neighbor of the current portion, the second candidate motion vector being for a second reference field having a second polarity different than the first polarity; scaling the second candidate motion vector to derive a scaled version of the second candidate motion vector, the scaled version of the second candidate motion vector being a candidate motion vector for the first reference field having the first polarity; determining the first motion vector predictor using at least the first candidate motion vector of the first neighbor and the scaled version of the second candidate motion vector of the second neighbor; computing a second motion vector predictor for the motion vector of the current portion of the current interlaced P-field; determining, based on information indicating which of the first reference field polarity and the second reference field polarity has been used predominately among the plural neighbors around the current portion, which of the first motion vector predictor and the second motion vector predictor is the motion vector predictor with dominant polarity; selecting between using the first motion vector predictor or the second motion vector predictor; computing a motion vector differential between the motion vector and the selected motion vector predictor; and signaling, in a bitstream, information indicating the motion vector differential and a predictor flag indicating whether the motion vector predictor with the dominant polarity is used.