I've been searching for this info for a while.
The syncytiotrophoblast is the outermost layer of the fertilized cells and is thicker and in contact with uterine spaces. It becomes a syncitium (layer of nuclei and cytoplasm without lateral cell borders) and is termed the "syncytiotrophoblast".
Wikipedia confirms it (if you trust Wikipedia as a source)
The syncytiotrophoblast secretes progesterone in addition to human chorionic gonadotropin (hCG) and human placental lactogen (HPL); hCG prevents degeneration of the corpus luteum.
The trophoblast differentiates into two different cell masses, shortly before it comes into contact with the endometrium:
- the outer syncytiotrophoblast (ST)
- the inner cytotrophoblast (CT)