Well, i would like to suggest you that , your daughter first needs to become Symptom free , as in , she needs to be free from any complaints , and has been stable for atleast 6-12 months , and then she can think of pregnancy. As far as her diagnosis is concerned, i believe you are talking about Schizophrenia
, and for this condition anti psychotic drugs and mood stabilizers are the mainstay of treatment ,and both these group of drugs are teratogenic ,as explained by me earlier.
You have talked about relapse, yes there is a possibility of relapse of her illness , if drugs are stopped , but , if they are not stopped , we will be putting the future baby at risk for contacting fetal abnormality , and believe me that would not be appreciable neither by you nor by your daughter. At the most, what we can do is , we can stop her drugs for atleast 1st trimester ,and then start her on maintenance dose of the drugs , but still , no one would take gaurantee that the baby born will be 100 percent free from any abnormality, but yes there will be a good chance that the baby will be healthy. But, please , take my advice , and let your daughter be on the treatment for atleast 6 months and let her to be symptom free before planning pregnancy.
I have laid out to you the various options available to you , now , i can only wish you take the right decision,putting all the possibilities into proper perspective.
I hope my answer serves your query according to your satisfaction.