Hi there, dietitian here. In Canada (not sure about specific recommendations for other countries), it is recommended that all women of childbearing age take a prenatal multivitamin that contains folic acid because many pregnancies are unplanned and it reduces the risk of neural tube defects. Even though you are taking birth control, you still have a small chance of becoming pregnant which is likely why your doctor recommended this.

These vitamins will not increase or decrease your risk of becoming pregnant, as other commenters have mentioned.

As with any sort of recommendation a health care professional makes, you have the right to choose to follow or not follow it.

As far as brands, I usually generally recommend store brands or whatever is on sale. It's generally recommended that prenatal vitamins contain 0.4mg or 400mcg of folic acid (amount recommended may very depending on your medical history) so look for one that contains that, if you choose to take this.