Gestation was the period of time it took for a fetus to develop to maturity. The length of the gestation period depended on the species. Gestation periods were sometimes divided into groups of three months, known as trimesters. Babies carried for the full gestation period were said to be carried to term. (DS9: "Body Parts", "Ties of Blood and Water")
The normal gestation rate for Betazoids was ten months, while the Bajoran gestation period of just five months was considered short. (TNG: "The Child"; DS9: "Body Parts")
An hybrid, half Human and half Ktarian, had a double gestation period compared to a full Ktarian. (VOY: "Fury")
Doctor Phlox denied the request to extract the Xyrillian embryo from Commander Tucker as the embryo had integrated with Tucker's pericardium and Phlox was unaware what would happen to it following the extraction. (ENT: "Unexpected")
Klingons sometimes opted to have gestation take place ex utero, although this might result in a smaller infant. (DIS: "Point of Light")
The Borg drone One needed an hour to enter the fourth gestation phase, the way from an embryo into a child of six years. (VOY: "One")
One who was born before their species' normal gestation period ended was said to be premature. (TNG: "Galaxy's Child")
Given that James T. Kirk and his alternate reality counterpart's birthdays differ by over two months, the latter being born first, it is likely that his mother gave birth to him prematurely.
In 2367, Dr. Beverly Crusher speculated that "Junior" was still premature after being aided in its birth by the USS Enterprise-D, not having possessed the strength to break through its mother's body. (TNG: "Galaxy's Child")