It can be, but being as you are only 18 weeks it's almost impossible to tell what the placenta will do. As your uterus grows it could migrate away from the cervix and not present any problems at all. But if it doesn't and it covers the cervical opening either partially or completely it will become necessary for a c-setion. Your doctor should monitor the location of your placenta via ultrasuond throughout your pregnancy and you will get a better idea of how this will go as you get further along.
Most placentas move up as the uterus grows but if it doesn't and covers the cervix its usually necessary to have a c-section because labor/dilation can cause the placenta to separate and possibly come out before the baby.
Sounds like a low lying placenta or placenta previa. (I think I spelled that wrong) I was told I had a low lying placenta at 20 weeks and it's since moved up. Most will. Placenta previa is where the placenta covers part or all of the opening of the cervix and can make a vaginal delivery more complicated or make a c-section needed. It's normally nothing to worry about. You can google both of em for more info, and also have your doctor tell you which yours is. They'll be able to explain it in better detail.