The placenta could be the surprising reason why so many women get autoimmune diseases – even in women who have never been pregnant. Women are diagnosed with 80% of all autoimmune diseases in the U.S.
Humans in hunter-gatherer communities have an average of 8 to 12 children – a stark contrast to today’s U.S. average, which is less than 2 children. During these years of childbearing, the placenta sends signals to the immune system to weaken in order to prevent rejection of the fetus. This constant weakening of immunity could be dangerous for the mother, so it’s hypothesized that other aspects of the female immune system are programmed to ramp up during adulthood to compensate (women actually contain more immunity genes than men, possibly for this very reason).
It’s possible that because our bodies have evolved to bear such large numbers of children over hundreds of thousands of years, the immune system anticipates the presence of a placenta and this immune ramping occurs whether or not a placenta is actually present, leading to an overactive immune response, especially in those who bear none or only a few children. This hypothesis is called the pregnancy-compensation hypothesis. Cell
Takeaway: While this theory as to why women experience so many more autoimmune diseases than men has yet to be proven, most experts agree it’s a very viable one. A future study that will shed light on the theory’s accuracy will compare autoimmune diagnoses with number of pregnancies. If the theory is true, those women who bear more children will be less likely to have an autoimmune diagnosis.