system
Given the following post, respond as if you're commenting on Reddit:
human
Why does birth control fail?

If a woman takes it exactly as prescribed, or has an IUD, then how can they get pregnant? Why is it only 99% effective?
gpt
A full answer to this would be extremely long and involve a lot of "we don't know."

There are a lot of mechanisms geared towards reproduction, and biological mechanisms aren't precisely engineered.  They have some tolerances built-in, and these vary from person to person (and from cycle to cycle!).

The combined oral contraceptive contains progestin and estrogen.  These work to inhibit ovulation.  When the pill was first released, the dose was four times higher than it is now, which wasn't safe, long-term.  The dose was decreased.  Estrogen is still a risk factor for clots -- but so is pregnancy.  So we have it at a tolerably safe dose with a significant reduction in risk of pregnancy.

Let's get more specific: the most effective form of birth control we have is Nexplanon.  When Merck was replacing their previous contraceptive implant, Implanon, with Nexplanon, they did a study on all the causes of failure in Implanon.

Again, this is the most effective contraceptive we have.  It is 99.9%+ effective.  It is more effective than tubal ligation.

Of the 127 causes that they found:

* 84 were a failure to insert implant -- one of the biggest changes between Implanon and Nexplanon was a package redesign to make it much harder to neglect to insert the device, plus changes to protocol to require the provider to check that the device is present in the needle prior to insertion and absent after insertion.

* 19: incorrect timing -- that means that the patient was either already or imminently pregnant at the time of insertion, or became pregnant in the first week after insertion.

* 8: interaction with hepatic-enzyme-inducing meds -- progestin is digested by a set of liver enzymes that some other medications up-regulate.

* 3: expulsion -- the device came out because it was poorly-inserted.

* 13: product/method failure: as in, unexplained.

There were some theories that those 13 unexplained cases may have been related to obesity, because fat tissue is hormonally active and increases the volume of distribution of the medication.

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For IUDs, it's also the case that most failures are due to the IUD not actually being there, or placement being poorly-timed.