For a moment, I want to share with you something that is a bit unrelated, but you post triggered something for me. On last night's news (nightly news with Brian Williams), they talked about immunotherapy, which is a wonderful new treatment for cancer. They use your own immune system/cells to treat cancer, and it has worked! Just wanted to share.
Okay, on to your question. I have no idea if there is a correlation between too much folate and cancer, however, I know it doesn't matter how many vitamins you take if your vitamins are water soluble vitamins, because the excess (whatever you body does not need) gets eliminated in your urine. Are your pre-natal vitamins water soluable?
Better option - EAT your vitamins! I eat two eggs a day (either organic, or vegeterian fed, cage free with no hormones), and make myself a yummy veggie powder smoothie every day. My smoothie: 1.5 cups of milk, one cup of blueberries (Costco sells a big bag of frozen organic blueberries), a scoop of my green veggie powder (that I buy at my health food store), 1 TBSP cocoa powder (it has a lot of antioxidants), 2 TBSP ground flax seed, and some raw honey. With my 2 eggs and my smoothie every day, I do not need to take vitamins. Eggs are considered to be "natures vitamin," and I also get all the vitamins I need in my veggie powder smoothie. If you eat eggs and a smoothie (like mine) every day, you may not need to take a prenatal vitamin. And I didn't even get to dinner - what about a salad with organic kale (kale is the healthiest type of lettuce for you - even better than spinach, I hear) and a lean meat/protein? It's always better to eat your vitamins, so you may want to look into this alternative. Best wishes!