B.C.
Taking tetracycline (an antibiotic) while pregnant can affect your child's teeth.
Some people have a certain plaque which can form up really fast (like over night) that will discolor teeth.
For this, use a slightly firmer tooth brush and just a little adult tooth paste (baking soda might work, too).
Don't forget to floss her teeth.
http://www.livestrong.com/article/512787-what-are-the-cau...