I am 31weeks pregnant, went to the doctor for cramps and backpain, babies head was down, not dilated but i am 70% effaced. what does that mean?

Ask your doctor: You should ask your doctor these questions. That's what you are paying for. Also, your insurance company might have a nurse question line. For a baby to come out, you need to be dilated (the cervix where the baby comes out, needs to open from zero to 10 cm or so) and 100% effaced Effaced = cervix was thick, it needs to thin and open so the baby can come out.