Mucus discharge after 12 day embryo transfer is a good sign?

Possibly. 12 days after an embryo transfer is defined according to the embryo age on the day of transfer. Day 3 embryos should give a positive blood pregnancy test 12 days after transfer. If blastocysts were transferred ( 5 day-old embryos ) the pregnancy test will be positive 9 days after transfer. Have your fertility doc do a blood pregnancy test .