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The mother needs to stop taking this medication and request Doctor for alternative medication while trying to get pregnant. For what reason are you both taking this, you haven't specified. You probably have developed drug reaction. Do get your blood sugar tested to rue out Diabetes
as sometimes it can present with dark thick patches in the armpits. Kindly ask the Doctor to change the medication as tetracyclines can arrest bone growth in children. The mother can take Folic acid
tablets under medical supervision once a day from one month before conception for safety from Neural tube defects in the baby.