Hormonal contraception, when used correctly, is an highly effective way to prevent pregnancy. The pill is arguably the most popular of all hormonal methods, which may makes you wonder just how the pill works and why it is so successful?
To slightly back track, I would like to point out "the pill" means any type of hormonal contraceptive -- not just literally a birth control pill. For the most part, all hormonal contraceptives work the same way to prevent pregnancy. It does not make a difference if the hormonal birth control method contains both estrogen and progestin or has progestin-only. That being said, for some women, the progestin-only pill works a little bit differently, and this is also the case with the Mirena IUD. Both of these methods work in all the ways described below with the exception of ovulation.
The pill works to prevent pregnancy, primarily, because it suppresses a woman's body from ovulating during her monthly menstrual cycle. If the ovary does not release an egg, then there is nothing there for a sperm to fertilize. The pill (and hormonal contraception) may also make the fallopian tubes less likely to move an egg toward the uterus. As mentioned above, the progestin-only pill may prevent ovulation for some women, but it may also not be consistently prevented by using these pills. Likewise, Mirena may stop some women from ovulating, but this is not the main way that Mirena works to prevent pregnancy.
The next way that hormonal contraception and the pill work is that they help to thicken the cervical mucus (the substance at cervix/opening of the uterus). This makes the mucus sticky, so when sperm try to get through the cervix, this sticky mucus makes it much harder for them to swim through -- thus making it more difficult to reach and fertilize the egg.
The final way that the pill works to prevent pregnancy has to do with the uterine lining. Hormonal contraceptives may cause changes to the lining of the uterus. The hormones in these methods can thin out or prevent the growth of uterine tissue. This may reduce the likelihood of implantation.
Hormonal contraception and the pill can work in any or all of these ways to prevent pregnancy. To be most effective, these methods must be used consistently and correctly. Hormonal contraceptives (on the whole) are between 92% - 99.9% effective. This means that of every 100 women who use hormonal contraception, 8 will become pregnant (with typical use) within the first year and less than 1 will become pregnant with perfect use (again, the exact percentages depend of the specific hormonal method as their effectiveness rates vary slightly - see: Effectiveness of Prescription Birth Control for specific rates).
One final thing to point out has to do with the placebo week of combination birth control pills, and the fourth week for the NuvaRing and the Ortho Evra Patch. These birth control methods can still prevent pregnancy during this week, so even though you may be experiencing a withdrawal bleed and not actively using any hormones, you are still protected against getting pregnant if you choose to engage in intercourse during this week.