Yes, PLACENTAS is a Scrabble word!

Letter values in Scrabble US: P 3, L 1, A 1, C 3, E 1, N 1, T 1, A 1, S 1.

Is it a valid word?

PLACENTAS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

that part of the ovary of a flowering plant where the ovules form
the vascular structure in the uterus of most mammals providing oxygen and nutrients for and transferring wastes from the developing fetus