You really need to learn about the affect of another active organism working within the human body.
The fetal growth may actually help improve our knowledge of a whole range of health problems:
http://health.usnews.com/articles/health/healthday/2008/12/04/moms-cells-prime-fetal-immune-system.html
It has been proven, that the development of a fetus can strongly benefit a mother to be, as it can strengthen her against certain, bacterial, and fungal infections, (although the evidence about a virus is still up for debate)
http://www.news-medical.net/news/2007/09/20/30130.aspx
But you are correct in a sense: pregnancy usually suppresses a woman's immune system, otherwise the fetus might be rejected by her body as a 'foreign body'. This can make a pregnant woman more prone to infection which has implications for her own health as well as for her developing baby.
I do know this, as i am coming from a background of where people close to me, are involved in the field of medicine.
But the point simply remains, the field of medicine views pregnancy as a 'Symbiosis' not as a 'parasite'
The field of medicine does no such thing. Let's clear things up for a second here: I am married to an RN who has a B.Sc in Nursing and works exclusively with pregnant women, delivers babies, and does postpartum care. I myself have a B.Sc in Molecular Genetics with emphasis on Immunology and Infection, and significant course material in the area of early development and the immunology of pregnancy. I have a second B.A. in Sociology and Psychology.
Now then, while some forms of T-cell regulated immunity increase during pregnancy to increase resistance to microbial and viral infection by up-regulating B-cell production and increasing the overall response time of immune cells like macrophages, natural killer cell and T-cell response to organism-mediated infection drops off dramatically during pregnancy (as you've mentioned). The result is a marginal increase in efficacy of response to bacterial infection, and a sharp decline in the efficacy against parasitic infection. This is why pregnant women are much more susceptible to infection by parasitic worms and extracellular disease.
On top of that, pregnant women also faces problems of immune rejection of the fetus and pregnancy dramatically increases the risk of autoimmune diseases due to the up-regulated activity of B cells.
Beyond that, fetal development alters physiological structure and places tremendous demand on the digestive and cardiovascular systems of the body.
To be a symbiont, a fetus has to provide some benefit to the mother. There is no biological benefit to pregnancy (aside from the benefit to the new organism). It makes a woman weaker and more susceptible to illness and injury in virtually every way possible. The slight increase in the effectiveness of the immune response to bacterial infection is so negligible it can hardly be considered a benefit.