I find this part of the paper interesting:

Interestingly, the most common situations involve touch, sound, or movement--three things that are abundant for the infant while in the womb. Recall that human babies are born at least nine months early compared to other animals because of head size (if they got any bigger they could not get through the birth canal; see Trevathan, 2011), so for at least nine months their bodies expect an “external womb.”
i've read about the 4th trimester, with the reasoning being that human babies are born probably 3 months early. Not 9 months early. However, I suppose if we look at human development versus other mammal development, this makes some sense. A horse's gestation is 11 months, but the foal can walk at birth. A human baby is much more helpless for much longer.

I'm not sure about the reasoning, but it helps manage expectations better, thinking that baby won't suddenly stop needing an external womb at 3 months, but that it may extend to 9 months.