It ' s also just one of the main mammalian ways to ensure infant survivability . Mammals have what are called altricial young , or babies that are unable to take care of themselves after birth . This is because our reproductive system has branched away from the labor intensive process of egg forming and laying and having lots of babies those are called precocial young and are usually fully formed , smaller versions of the adult and are independent from their first breath . Instead , mammals doubled down that energy on ensuring max infant survival rate . Instead of putting all of your energy into laying a bunch of eggs , having to guard them while the incubate , risk getting food and abandoning them or starve a wait , etc , mammals just go about their business and develop smaller fetuses , then expend energy on protection and milk after birth . All in all , it leads to a drastically different infant mortality rate between the two natality types and yet both are highly feasible and functional in their own regards . Evolution is awesome .