I was concerned about this with both pregnancies because I went overdue. First off, don't worry - the doctor will not let you go longer than your body will allow. Amniotic fluid levels are very important so they keep a close watch and will of course err on the side of caution.
Secondly, both of my midwives told me to drink a lot of water and up my vitamin intake. You need a super healthy placenta & uterus to maintain your fluid levels.
It's normal for your fluid level to get lower as you near your due date.