I have learnt that our immune system distinguishes self from non-self by using MHC. So a mother during pregnancy should also develop antibodies against the fetus as it is also a genetically different individual. Answering by saying that antibodies won't cross the placental barrier seems wrong as there is so much research going on this thing that such a simple explanation wouldn't suffice. Why doesn't the mother reject the baby?
Awesome question! The immune system detects the presence of non-self using MHC and T cells and the lack of self using NK cells. The placenta first of all doesn't display MHC so T cells can't detect it, and expresses NK inhibitory receptors which stop NK cells killing it. Furthermore, the immune system isn't able to get to the foetus so it isn't able to activate against these antigens nor produce the antibodies which require T cells to be activated first, which then activate B cells. This is because the placenta is a syncitium, thus no spaces between cells for immune cells to slip through (a mechanism used by viruses). The placenta also secretes Neurokinin B, this makes it hard for the immune system to see the baby, and is a mechanism used by parasites. There are T reg cells which act to supress an immune response.