I had a 1 d5FET and 1 6dFET on 7/25. BFP on hpg 4 days later. Confirmed on 8/3 via blood. US at 6w1d showed 2 sacs no fetal pole. Had emergency US at hospital on 8/22 which showed a sac with fetal pole and HB of 157. RE also saw another sac, but it looked empty. RE said that there was a lot of blood flow activity around 2nd empty sac. Has anyone else had this, could another baby develop or is this a vanishing twin?