In the long winded post below I referred to genetic conflicts in pregnancy. If you are curious about this, I highly recommend Mother Nature by Sarah Hrdy. Though the topic is placed in the context of female evolution (of our species) in general, it gets a thorough treatment. If you are interested in the technical literature, I recommend this review, Genetic Conflicts in Pregnancy by David Haig. Since it is a review, it is less analytical and formal than much of Haig's work, so it reads easily. You can get the pdf on Haig's website.