28歲婦女,G2P0,孕31周,從28周時(shí)反復(fù)陰道出血,共4次,出血量少于月經(jīng)量,不伴腹痛,近1天陰道出血量大于月經(jīng)量,收入院。BP100/80mmHg,P84次/分,子宮軟,宮縮不規(guī)律,胎頭高浮,胎心率144次/分。依據(jù)病史及查體主要診斷為()