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If your periods were regular while being on Metformin treatment, there are chances that you were having regular ovulation. The absence of periods could suggest possibility of pregnancy. Look out for symptoms indicating early pregnancy like sore and tender breasts and nipples, increased hunger, increased frequency of urination, bloating, mild cramps, back pain, mood changes, headaches, fatigue.
Sometimes urine pregnancy tests can keep showing you false negative results due to less concentration of pregnancy hormone (HCG) in urine during early pregnancy. Visit your gynecologist for an examination and a blood beta HCG level test to confirm pregnancy (it can detect pregnancy accurately than the urine pregnancy tests as early as 11 days post conception).
Take care.