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Birth control pills is one of the more common methods of protection from pregnancy. You can find many posts indicating that the probability of getting pregnant while using birth control pill is very very unlikely (for example - This post indicates 0.1).

Other claim that this statistics is misleading, and that perfect use is very unlikely, or that there are many other reasons not to trust birth control pills.

Has the perfect use statistics really proven in practice? How error prone is it (missing a dose, etc.)?

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    ... By the same reasoning, the more often you use a seatbelt the more chances it has to fail. This is a really bloody stupid argument. – Shadur Sep 3 '15 at 18:32
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    "By the same reasoning, the more often you use a seatbelt the more chances it has to fail." Well, but that is true isn't it? The amount of times you drive affects the chances that you will die in an accident. A lot of stats I see for birth control essentially say "within a year X amount of couples will become pregnant", which is kind of meaningless if we don't know how much or little sex they were having. If I have sex once a year the odds of pregnancy within a year while on birth control are obviously much lower than someone who does it every day. – Andrew Whatever Sep 4 '15 at 18:46
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Per UK NHS,

1. Combined contraceptive pill is more than 99% effective if taken correctly. Less than 1 woman in 100 will get pregnant in a year while taking the combined pill.

2. Progestogen-only pill is 99% effective if taken correctly. One woman in 100 will get pregnant in a year while taking the progestogen-only pill.

The AHRP site clearly mentions "to be fully effective, birth control pills need to be taken correctly. Missing even a few doses greatly increases your chances of pregnancy: while only 0.1 percent of women get pregnant when using the pill properly, pregnancy rates soar by 30-80 times when pills are missed. Because of the missed pills, you have a much greater chance of getting pregnant. So even though you’re back on schedule, you also need to use a back-up method of contraception — like a condom, or spermicide — for the rest of your cycle." Also if emergency contraceptive Pills treatment is initiated within 72 hours after unprotected intercourse, this reduces the risk of pregnancy by at least 75%. Quoting Oddthinking, "differences among probabilities of pregnancy for various methods will increase over time."

  1. Per US CDC, the effectiveness of the Pill each year is between 88% to 94% which means 6 to 12 pregnancies per 100 women can occur each year.

  2. Per US HHS, the effectiveness of the combined pills is 91% each year and the effectiveness of the mini pills is 95% each year which means of 100 women who use the pill each year, about nine women may get pregnant on the combined pills and five will get pregnant on the mini-pills. The risk of pregnancy is much less for women who take the pill correctly every day at about the same time.

Per James Trussell in 2011, the progestin-only pill is less effective than the combined pill during typical use, since the progestin-only pill is probably less forgiving of nonadherence to the dosing schedule. Whether the progestin-only pill is also less effective during perfect use is unknown.

Although the lowest reported pregnancy rate for the combined pill during typical use is 0%, recent studies indicate that pregnancies do occur, albeit rarely, during perfect use. Hence, we set the perfect-use estimate for the pill at the very low level of 0.3%. The lowest reported pregnancy rate for the progestin-only pill exceeds 1%.

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