This might be TMI, but a baby is in some ways a parasite. It uses Mums body for nutirent, baby gets enough, mum goes without. Apart from the reward at the end, physically speaking a baby does nothing to the mothers benefit during PG.
But I would never go without my DS and won't stop me from having more babies.

During the course of my work I came across a woman who had picked up an African parasite, she said she could feel it moving around in her.