Birth defects can occur if the pregnant mother is not doing her obligation being a good mother. I mean she should avoid all the bad habits and lifestyles during pregnancy if she wants her baby to come out well and good. So, she needs to avoid smoking, drinking alcohol and taking over the counter medicines.
You can look after yourself very well and still have a baby with birth defects. This can happen very early on in the pregnancy often before the mom knows she is pregnant.