My mate had it on her first and basically there is a possibility it moves as the baby grows. The plecenta is not at the top basically. If it is sort of half way down is does not seem to be a problem. If it is at the bottom then it does not cause a problem till birth when basically the placenta would be born first causing major trouble for mother and baby. They moniter you carefully to keep a check on the position and do a section a week before due date to ensure natural labour does not start if the placenta is still at the bottom. Her midwife will explain it all, be carefull as some web sites will paint a very bleak and scary picture. My mate is fine and had a perfect second pregnancy and natural birth after.