What is the X in X-mas?
1530s, "riding on horseback, etc., as a form of exercise," from Latin gestationem (nominative gestatio) "a carrying," noun of action from gestare "bear, carry, gestate," frequentative of gerere (past participle gestus) "to bear, carry, bring forth" (see gest). Meaning "action or process of carrying young in the womb" is from 1610s.
gestation ges·ta·tion (jě-stā'shən)
The period of fetal development from conception until birth; pregnancy.
in mammals, the time between conception and birth, during which the embryo or fetus is developing in the uterus. This definition raises occasional difficulties because in some species (e.g., monkeys and man) the exact time of conception may not be known. In these cases the beginning of gestation is usually dated from some well-defined point in the reproductive cycle (e.g., the beginning of the previous menstrual period)