Is loss of breast fullness during the 6th week of pregnancy is a sign of missed abortion ?

No: You could have lost weight. One cannot draw any conclusion about pregnancy health from your findings.

PossibleHormoneLoss: A missed abortion is a miscarriage that has been diagnosed by ultrasound that your body has not yet expelled and therefore it's a failed pregnancy that has been "missed." Usually your body will push the pregnancy on its own but this can take a few weeks dependent on when the pregnancy actually expired. Some women receive misoprostol to help or undergo D&C to complete the pregnancy.