There is often a correlation between a large body size and a long gestation period.  Therefore, because the Paraceratherium were such large mammals, they most likely had a long gestation period, similar to elephants (Prothero, 2013).  Elephants have a gestation period of 22-24 months and produce a calf every three to four years. This pattern is suspected to happen in the Paraceratherium as well.