Your friends are experiencing something that is VERY rare. Many women do experience bleeding during a pregnancy, but it's often very obvious that it is not a period. It does not come once a month, last for a 5 days, yadda yadda yadda. On to the fact your friends had bleeding up to 6 months, that is VERY dangerous. They were at serious risk for a miscarriage, if they did not go see a doctor, I'm very surprised they have children. Bleeding that late during a pregnancy is very rare, which makes your statement almost statistically impossible. Anyone bleeding past 3 months into their pregnancy should see a doctor immediately. Only 20-30% of women bleed during the first trimester, not even close to half, and about half of those women lose their children to a miscarriage. I know you're just trying to be helpful, but what you have experienced is so very rare, it scares the crap out of young girls who are still getting their periods and now think "Great, I could be six months pregnant."
Edit: Here's a good link
http://www.americanpregnancy.org/pregnancy
complications/bleedingduringpreg.html