My tutor in Maternal and Reproductive Health said that it is still one of the fairly unknown areas of medical science. For example, it is believed that only 15% of babies conceived with Down's Syndrome develop beyond 1st Trimester - the other 85% being spontaneously aborted. Even then these are only estimates. It is possible that an unusually heavy menstruation is actually a spontaneous abortion - again still only a theory though. 
There is still a lot that's not fully understood about what happens, especially in the first Trimester of pregnancy.