Hi R.,
The title of your message caught my eye because I've been dealing with something similar. My daughter was delivered at 39 weeks in December 2006. The birth was uneventful except for the fact that there was difficulty delivering the placenta. Unlike you, mine was actually delivered so no D&C was required. The ob examined it and deemed it in tact so we thought that was that.
Fast forward to December, 2007... My husband and I began trying for baby #2. With our daughter, we conceived on the first try. After several months of trying this time around with no success, we saw a reprodcutive endocrinologist (infertility doc) who performed uterine surgery and found a little (1 cm) piece of retained placenta in my uterus, 19 months after I had my daughter - yikes! Apparently, it was acting like an IUD and preventing pregnancy. After two months of healing post-surgery, we have just begun to get back to trying.
It seems to me that retained placenta is a bit uncommon common (I can't find too much about it on Google!). I hope I'm not over sharing but thought it might be helpful to share my experience. Most likely, you won't have any trouble with a second pregnancy, but if you don't have luck conceiving in the timeframe you'd expect, you might want to consult an RE to make sure that your uterus is in good shape for pregnancy. I understand that infrequently, D&Cs can result in scarring of the uterus that could make it tough to get pregnant. As for the risks of pregnancy iteself, I only wish I could speak to that, but we haven't gotten that far yet!
Best of luck to you.