My teeth are breaking?

ever since i had my 4th kid my teeth are breaking. not the front my back teeth. my front teeth are great. i dont know what is causing it. my hemoglobin is 14.6 and that was taken 3 days after i had a 2 in a half week long menstrual cycle. i weigh 127 and in am 5'3. my youngest son he had problems with his teeth something about calcium. i took my prenatal when i was only 5 weeks pregnant. my mom told me to take 2 intsead of one but i was scared that i would me and the baby. so i didnt listen to her. she says my teeth are breaking because i had so many kids. i brush my teeth at leat 3 times a day. and for my broken teeth should i get them pulled or get a crown. what is the difference between a fake tooth and a crown. and what are my other options. i seriously doubt they can put a filling on them because half of the tooth is gone.