Mouse with newborn litter (P0.5) Courtesy of Peter Bollen
Anyone who has kept mice as pets or in the lab will recognize the tiny pink pups, which are entirely dependent on parental care. They are poorly-developed (altricial) with closed eyes and no hair.
Newborn guinea pigs (one day old) Courtesy of Per Svendsen
Contrast this with the well-developed (precocial) litter of a guinea pig. They need to suckle but can move freely and they have open eyes and a full coat of hair. In a classic survey of 394 mammals, Martin and MacLarnon found that altriciality went with short gestations and large litters whereas precocial neonates came from small litters and long gestations. Some of the species they surveyed fell in between with for example eyes shut at birth and not opening for at least five days.
Where do human beings fit into the spectrum? Babies are clearly more helpless than newborn guinea pigs. Yet singletons are the norm and pregnancy lasts for nine months. As in precocial neonates much organ development occurs before birth in humans. The placenta has to function much longer than in a mouse and as a result acquires additional functions.