The placenta was an organ that connected the developing fetus to the uterine wall to allow nutrient uptake via the mother's blood supply.
In 2370, the natural defense mechanisms in Spot's placenta protected her unborn kittens from the virus that had afflicted the USS Enterprise-D crew. (TNG: "Genesis")
In 2372, Ekoria's placenta absorbed an antigen Julian Bashir had administered as a possible cure to the Teplan blight, and thus protected her child from the illness. (DS9: "The Quickening")
In 2373, Kira Nerys suffered a placental laceration after being felled by the exertion of attacking two Bajoran deputies, but both she and the unborn baby were fine. (DS9: "The Darkness and the Light")