I always thought that B12 occurred naturally in Marmite but a bit of t'internets shows that it's added. Either way 7g of Marmite contains all of the B12 and adult needs. It's also added to stuff like breakfast cereal, soya milk etc.

I don't usually take vitamins as I think I get enough nutrition from my food. If I were planning to have a baby I'd probably take them to be extra sure I wa getting everything I needed.