Birth Control Pills Force the Body to Have a 28-Day Menstrual Cycle
Standard, traditional birth control pills come in packs of 21 or 28. The first 21 pills in a 28-pack of combination pills contain artificial reproductive hormones (usually an estrogen and progestin combination) that prevent the release of an egg (ovulation) during a woman's monthly cycle. The last seven pills in a 28-day pack have no hormones. When your body stops receiving the artificial hormones, it responds by bleeding or having a period. Women who opt for a 21-pack of pills must remember to start their new pack of pills seven days after taking the last hormone-filled pill.
Birth Control Pills Can Eliminate Monthly Menstrual Periods
There is another type of birth control pill called the mini-pill. This pill is a low-dose progesterone-only pill (it does not have estrogen). Women who take this low-dose birth control pill at the same time every day will not get a period until they stop taking the pills. Their cervical mucus will also thicken when on this pill and the uterus lining becomes unripe for egg implantation. If you are getting your period while taking the mini-pill, there is a chance you could become pregnant.
Although it's usually not advised, women who skip the non-hormone-containing pills in the traditional 28-day pack of combination birth control pills can achieve the same effect. The dose of hormones in the traditional combination pill is a bit higher than in the specialized pills, thus skipping the placebos is not advised.
Birth Control Pills Can Decrease the Frequency of Menstrual Periods
Another formulation of the combination birth control pill can be taken for 12 weeks straight. This pill decreases the frequency of periods from one per month to one every three months. The mini-pill (or low-dose progesterone pill) can also decrease the frequency of menstruation, as you can stop it and induce a period when it's convenient.