Even if it seems impossible, this is usually only possible for a short period of time after a female has given birth(it can be induced in both males and females), or by a side effect of certain medications like: RisperidoneMetoclopramideRanitidineHaloperidol Or even by continuously using anabolic steroids with fast cancellation such as TestosteroneDHEA It is also common to see this in newborns due to the presence of maternal hormones
However, this is very often due to the artistic direction, particularly with Mpreg, Huge Pecs or Hyper kinks.