目的评价肝素对改善妊娠中晚期脐动脉收缩期最大血流速度（S）与舒张末期血流速度（D）的比值异常的临床效果。
ObjectiveTo evaluate the efficiency of heparin on the ratio of peak systolic to least diastolic flow velocity (S/D) of umbilical artery flow velocity waveforms at second-trimester pregnancies.
目的评价肝素对改善妊娠中晚期脐动脉收缩期最大血流速度（S）与舒张末期血流速度（D）的比值异常的临床效果。
ObjectiveTo evaluate the efficiency of heparin on the ratio of peak systolic to least diastolic flow velocity (S/D) of umbilical artery flow velocity waveforms at second-trimester pregnancies.
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