Given a reference text about bull sharks, tell me the gestation period for bull sharks.

Bull sharks mate during late summer and early autumn,[9] often in bays and estuaries.[48] After gestating for 12 months, a bull shark may give birth to 1 to 13 live young.[9][49]

They are viviparous, born live and free-swimming. The young are about 70 cm (27.6 in) at birth. The bull shark does not rear its young; the young bull sharks are born into flat, protected areas.[49] Coastal lagoons, river mouths, and other low-salinity estuaries are common nursery habitats.[4]

The male bull shark is able to begin reproducing around the age of 15 years while the female cannot begin reproducing until the age of 18 years.[49] The size of a fully matured female bull shark to produce viable eggs for fertilization seems to be 175 cm to 235 cm. The courting routine between bull sharks has not been observed in detail as of yet. The male likely bites the female on the tail until she can turn upside down and the male can copulate at that point. Mature females commonly have scratches from the mating process.

The gestation period for bull sharks is 12 months.