What Causes The Presence Of Sac In Uterus After Surgery For Ectopic Pregnancy?

Hi,My wife got pain in abdomen ,we admitted her to hospital after sonography and blood test,doctor said it is ectopic pregnancy. Dr operated on her .After operation they found some ml of pus and adhesion around her fallopian tube.So they removed pus and does milking process of tube.But after operation they agian did blood test and found test positive for pregnancy. Now they did sonography in which they found 4 mm sac in uterus. Now they are sayin we will watch and see growth of baby.So please tell should I go aheadwith baby or do MTP?