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.
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")
One who was born before their species' normal gestation period ended was said to be premature.
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")