molar pregnancy or retained placental tissue ?

At my 10 week check up , no fetal heart beat could be detected. After a trans-vaginal u/s only a gestational sac could be seen very clearly on the screen. It measured approx. 7w4days.As there was no heartbeat or fetus the doctor suggested a D&C, but I had a blood test just to confirm. The hcg levels were 3500. The doctor said a heartbeat should have been heard.I had the d&c on May 23 and at the 2 week check-up the hcg was 490 then at 4 weeks was 157. I have since had a menstrual cycle ( at 4.5 weeks) it was quite heavy and I was even convinced that there was some suspicious tissue expelled. I now feel as if everything is back to normal and want to start ttc again. Is there any risk of Molar pregnancy, I had no symptoms and my hcg levels are going down well. Can any fetal tissue that is still inside the placenta cause an unusual hcg level ? can this also be confused with Molar Pregnancy ?