Try not to stress, Didi. I had a low lying placenta, close to the cervix, and it caused me no problems throughout my pg. I had a friend with complete placenta praevia whose placenta was completely covering her cervix (therefore high risk) and she did have two bleeding episodes, at 28 weeks and 32 weeks, but neither time was she put on complete bed rest. The last one she stayed in hospital for a few weeks for monitoring and extra risk, but more so that she was right there if there was a need to perform a c/s quickly. She had a healthy baby boy at 37 weeks.

If your placenta is not over the cervix in all likelihood it will move up as the uterus and placenta grow. I was warned there was a chance I would experience bleeding, especially after sex (I never did), but that unless it was more than spotting and accompanied by other things like pain, cramping, etc, there was nothing to worry about. They only told me so that I wouldn't worry!

I would think you can relax and just keep an eye on it. The 18 week u/s should tell you if it's moving further away and you can have a later one at 32 weeks if there is any concern remaining. Almost all are resolved by this point.