If you have had a previous pregnancy where the membranes ruptured earlier than you'd like you may want to make sure you're getting enough in your diet. According to the American Pregnancy Association, a pregnant woman should have 85 mg of vitamin C daily. There is 70 mg in ONE medium-sized orange! So couldn't you add an orange or the equivalent each day and get the same benefit, plus more because of the extra fiber and all the other nutrients?
Vitamin C is water-soluble, meaning it's not easy to overdose because excess will be flushed from your body in your urine. However, there IS an upper limit of 2,000 mg a day and side effects from taking large amounts. This may happen if you're taking a prenatal vitamin, plus other supplements containing vitamin C "just to be safe", and then add vitamin C because you heard it helped with PROM. Just keep in mind that a healthy diet gives you plenty of vitamin C.