I went for an ultrasound 3 weeks ago and my doctor said retained placenta. however my last child was born 5 years ago. what are the risks of this ?

Not possible: I am not an OB but as a radiologist, I have had to diagnose this. As far as I know, you could not go for five years with retained placenta. It is more likely scarring or polyp/tumor of the uterus. I recommend you go to an independent laboratory for a repeat ultrasound including an endovagional US. If there is still uncertainty, a MRI is a great way to diagnose uterine abnormalities.

Slim unless pregnant: If you were not pregnant recently or if your blood betaHCG level is negative, the probability of a "retained placenta" is minimal to practically zero. The sonographic appearance of blood clots or menstrual debris inside the uterus may be mistaken for products of conception or retained placenta. A simple blood test and a careful Gyn evaluation (history+ exam) may provide you reliable answers.