I have heard pregnancy described as being like a parasitic relationship: the embryo or fetus takes nutrients from the mother's body without giving anything in return. I am tired of hearing that; I don't think it's a fair description at all. I think the relationship is mutualistic, meaning each benefits the other.
First, some quick definitions. In biology, symbiosis describes any close relationship between two organisms. There are three basic types of symbiosis: mutualism, in which each organism benefits from the other; commensalism, in which one benefits and the other is neither helped nor harmed; and parasitism, in which one benefits and the other is harmed in some way.
When people refer to an embryo as a parasite, they usually mean that it benefits from its relationship with the mother, but the mother is harmed by having to sacrifice energy to maintaining it. For those of you who may be horrified to read that I have actually heard people say this, I want to quickly point out that there is always the implication that the mother is quite willing to make this sacrifice out of love. In fact, it is a rather crude and nerdy way of talking about a mother's love.
But I don't think parasitism is the best description of the relationship. Obviously the embryo benefits from the relationship, and I argue the mother does as well.
First, hormonal changes in pregnancy may protect against diseases of the reproductive system, including breast cancer. The science is still not completely clear on the subject.
Second, most mothers view their children positively. They would say they are benefitted by having children in many ways. They experience joy and learn patience.
Third, for those who prefer a clear and strictly scientific reason why a mother benefits, I will offer a Darwinian explanation. A mother benefits in the most profound way possible under Darwinian terms: by hosting her baby, she is able to pass on her genes. This is the Holy Grail of evolutionary success.
Some of you, especially mothers, may not understand how I can write about motherhood this way. Rest assured, I don't actually view motherhood as just an ecologically curious relationship between two organisms. I am writing here in a detached way as part of a thought experiment, nothing more.