“No, not the first [Vulcan-Human hybrid]. But the first to survive. As you must know, an Earth-Vulcan conception will abort during the end of the first month. The fetus is unable to continue life once it begins to develop its primary organs. The fetus Spock was removed from Amanda’s body at this time—first such experiment ever attempted. His tiny form resided in a test tube for the following two Earth months, while our physicians performed delicate chemical engineering, introducing over a hundred subtle changes that we hoped would sustain life. At the end of this time, the fetus was returned to Amanda’s womb. At the ninth Earth month, the tiny form was again removed from Amanda, prematurely by Vulcan standards, and spent the following four months Vulcan term pregnancy in a specially designed incubator. The infant Spock proved surprisingly resilient—there seems to be something about the Earth-Vulcan mixture, which created in that.. tiny body.. a fierce determination to survive.”
Sarek (as played by Mark Lenard) as interviewed by Gene Roddenberry about his son for the 1976 album, Inside Star Trek.