Ian Troi's embryo in Deanna Troi's uterus.
The uterus or womb was a reproductive organ. It was the main female reproductive organ found in the anatomy of most mammal species. It was the organ in which a child would grow after conception until childbirth. Various immune systems protected the developing child from infection. (TNG: "Genesis")
Klingons had the ability to have an unborn child gestated ex utero. (DIS: "Point of Light")
A rare complication in Ktarian/Human pregnancies was, if the mother was Human, when the baby shifted position in the uterus during the birthing process and the exo-cranial ridges lodged in the uterine wall. If not resolved quickly, the uterus could rupture and result in internal bleeding and possibly even death of both mother and child. It was very painful for the mother and prevents the birth from taking place naturally. If this complication occurs, a fetal transport may be needed. (VOY: "Deadlock")
Kira Nerys carried Keiko and Miles O'Brien's second child, Kirayoshi, to term in her womb due to a medical emergency following a shuttle accident. (DS9: "Body Parts")
Harry Kim slept wearing an eye mask and claimed that it reminded him of being in his mother's womb, something he insisted to Tom Paris that he could remember. (VOY: "The Cloud")
B'Elanna Torres described her simulations in EV suit at Starfleet Academy as feeling "peaceful, like floating in the womb," but described an experience of a real space walk as making her a little sick to her stomach. (VOY: "Day of Honor")