Spock is a half-breed Vulcan/Human. Saavik is ahalf-breed Vulcan/Romulan. They are both mules. Mules are sterile. If they were not sterile, they would not be half-breeds. If Spock can father babies, or Saavik can become pregnant, then Humans and Vulcan/Romulans are not different species.
You can find the definition of species in the above link. Basically, if two animals produce viable offspring they are the same species. If they look different, but can produce viable offspring, then they are different races.
Since Trek lore maintains that Vulcans are a different species, Spock must by definition be sterile. Whether Saavik is sterile is another question altogether.
This however may be proven incorrect in Star Trek, as Worf has a child with a half-Klingon half-Human woman, Alexander. The child was a surprise, not planned, showing that there was probably no help in conception from a physician. This may be an odd case, but would nonetheless prove that it is indeed possible in the Star Trek universe.