The Tyrannosaurus rex, like other dinosaurs and their surviving modern-day bird relatives, reproduced sexually. The female then laid fertilized eggs from which newborn tyrannosaurs would hatch.
Because the fossil record does not provide many clues about the reproduction of tyrannosaurs or dinosaurs in general, much of what scientists know about dinosaur mating habits is from observing modern animals that might reproduce similarly. The awkward problem of getting into a position for the male to fertilize the female in such a large animal as a Tyrannosaurus rex can only be hinted at by the mating procedure for modern-day large animals such as giraffes and elephants. Once in place, the most likely method of sexual reproduction was through the cloaca, an orifice also present in birds, which would have been used for urination, defecation and also fertilization. The transfer of sperm from the male to the female might only take a few seconds, which is the case with birds.
When laying eggs, the females in both birds and Tyrannosaurs grow a medullary bone. This bone has very a high calcium content and is used to make the shells of eggs in both species. This discovery has helped scientists distinguish between female and male dinosaur skeletons.
Less is known about the amount of eggs laid and any adult presence before and after hatching in a Tyrannosaurus rex's life. There is reliable evidence that some sauropods (Seismosaurus) would lay 20 to 30 eggs and leave them unattended. This might yield a survival rate of two or three hatchlings. Others, such as the duck-billed dinosaur Maiasaura, would hatch underdeveloped young and care for them until they could walk. Tyrannosaur young could probably walk and run right out of the shell, and like other sauropods, they were most likely born from large clutches of eggs. However, scientists are uncertain.