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I understand your concern.
Since there was a sudden fall in HbA1c level, your doctor might have been surprised by the change. He might want to follow up for a couple of weeks or a month to ascertain that everything is going well. That is probably why your wife is now on Melmet 500 (Metformin) which is the first-line drug for diabetes. If her blood glucose levels are stable with diet and exercise, then she will definitely be able to go off medication. But that needs to be confirmed by the doctor after subsequent testing. Her HbA1C came down to 5.6 while she was on earlier medication. Now the doctor has reduced the dosage to a minimum and your wife's reports will be followed up for further progress. Depending on the next report, your doctor will take further action. That is how it works usually. So do not be in a hurry to stop medication and do not make any hasty decisions.
Regarding the efficiency of Ayurvedic medicine, there have been reports of effective medicines in Ayurveda, but that needs to be discussed with an Ayurveda expert. Be sure that you are consulting a doctor who practices pure Ayurveda and prescribes genuine medicines. You may be aware - all that is labeled herbal may not be ayurvedic.
Regarding pregnancy, I think what your doctor meant is, when your wife gets pregnant, if her blood sugar is not under control, she will have to take Insulin. It is true because oral medications are not recommended in pregnancy. Insulin is the safest medicine for use in pregnancy. In case her blood sugar is normal during pregnancy (without any medication), she may not need Insulin or any other medication. But she will still need to be tested regularly to be sure that she is in the normal range. You do not have to be apprehensive about Insulin use, there are a lot of myths associated with Insulin use. It is a safe drug, easy to manage, and after pregnancy and lactation, she can slowly move to oral medication again.
Since you have another year to plan the pregnancy, she can now focus on lifestyle modification - balanced diet and exercise - and take a decision based on how things unfold. Stay positive, stay healthy.
For more information consult a diabetologist online --> https://www.icliniq.com/ask-a-doctor-online/diabetologist