The mammalian sexual cycle
All of this makes for great biochemical complications. But there are further ones. The mammalian sexual cycle involves the regular production of a new egg. This causes no problem to the marsupial, for in every species, the neonate emerges before the next egg is due to be produced. In the placental animal the foetus, however, stays in the uterus for a very much longer period. So the placentalfoetus secretes a hormone which suspends the mother's sexual cycle for as long as the placenta is in place so that no more eggs are produced to compete with the foetus in the uterus.