How do I know if I missed a period since I dont have one while on Microgestin?

How long have you been taking MIcrogestin?

Were you ever advised to take a 7 day break at the end of the 21 day pill cycle? I hope you were advised to take it as explained below.

When initiating a Sunday start regimen, the first tablet may be taken on the first Sunday after menstruation begins. If a period begins on a Sunday, the first tablet may be taken on that day. When initiating a Sunday start regimen, another contraceptive method should be used until after the first 7 consecutive days of administration. For a 28 day package, one tablet is taken daily for 28 days and a new package begun on the following day. For a 21 day package, one tablet is taken daily for 21 days followed by 7 days with no medication. A new package of contraceptives is begun on the following day.

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When initiating a Day 1 start regimen, the first tablet is taken on the first day on menstruation. Such initiation may increase the risk of spotting and breakthrough bleeding but decrease the risk of early ovulation and pregnancy. For a 28 day package, one tablet is taken daily for 28 days and a new package begun on the following day. For a 21 day package, one tablet is taken daily for 21 days followed by 7 days with no medication. A new package of contraceptives is begun on the following day.

Many clinicians recommend that additional contraceptive methods be used during the first cycle of hormonal therapy in order to reduce the risk of unintended pregnancy.

http://www.drugs.com/mtm/ethinyl-estradiol-and-norethindrone.html