Teratology is the study of the effects that drugs, medications, chemicals and other exposures may have on the unborn child during pregnancy.  Particulary when a mother is taking a medication that is helping her stay well, it can be a difficult decision whether or not to stop taking the medication because of a possible risk to the fetus ... or because of a possible risk that could be transmitted through breastfeeding.  This decision is made harder because we know that if a pregnant woman becomes unwell, for example with depression, this too risks damaging the fetus, so it's not necessarily the case that stopping medication is going to be in the unborn baby's best interest.