Nicole, don't worry about taking it at 7 weeks. That's actually still early. It does help prevent birth defects such as spina bifida, so make sure to take your vitamins everyday. The prenatal viatmin has folic acid, and you don't want to over do anything, but banana's have folic acid too. Think of it this way, most women (unless really tracking it) don't even know they're pregnant until about this time or maybe even a little later. You'll be fine! Goodluck!
Also, to sc17, some prenatal vitamins are big, but depending on which kind you buy and how many vitamins are in each, some aren't as big as others. You could always break the pill in half if you can't take big pills.