Four-legged animals don't have the same degree of birthing challenges as two-legged ones because of the way the birth canal is shaped. Consider for example how horses are able to be up and walking (both baby and mother) of some number of minutes of birth. That's because the baby comes out more developed, which is possible because of the larger birth canal (in relative proportions). Human have to come out underdeveloped because otherwise we wouldn't be able to give birth at all, and then that makes human infants especially helpless out of the womb.