I took prenatal vitamins occasionally during my first and second pregnancies. They always made me sick. I wasn't about to make myself sick, especially when I had kids to take care of, on top of already being sick, so for the other pregnancies I stopped taking them. I was ok for a long time and all my babies were perfect. I did find that if I took a whole foods vitamin (the Alive capsules or tablets) they didn't make me sick in the slightest.
I only took the Alive tablets in my last pregnancy because I didn't know about them before that, but I was also really lazy about taking them too.
So guess what happened? My whole life I've had perfect teeth and gums. From one pregnancy to the next my teeth became more and more sensitive even though I was going to the dentist. I gave up on dental and my teeth felt a little better for it for a while. But after I had this baby I had to see a dentist finally because my teeth started to hurt so badly........ and I had 12 cavities. My dentist told me it was because I've been giving my body out to babies in pregnancies and breastfeeding for 8 years straight. My diet wasn't perfect, but it wasn't entirely junk food either.
I know I've had 6 kids very close together (in less than 8 years) and breastfed all of them, but if I could do something differently, I'd take that Alive the whole time for all the pregnancies. I know that I'm rather an extreme case, because no one here is probably going to have 6 kids like I did. You still may not see the effects of not taking a vitamin now, and your babies will come out fine...because if you don't eat right, the baby just taps into your stores...you will see a long term effect somewhere down the line.
As with anything, you should get things okay-ed with your doctor...and if you are having any sort of problems or concerns, it's never a bad thing to talk about it