More than half of women who got an abortion last year were using at least one form of contraception, according to data released by the British Pregnancy Advisory Service, a United Kingdom charity that provides reproductive health services, including abortions.

Furthermore, nearly a quarter of women had been using either hormonal contraception like birth control pills or a long-acting contraceptive method like an IUD. The two are among the more effective types of contraception, with long-acting contraceptives considered the most effective.

The statistics speak to a dirty little secret that isn’t much of a secret at all: Contraception—which runs the gamut from condoms to birth control pills and patches to IUDs, injections and implants—usually works, but it doesn’t always work.

When used perfectly, contraception can be 98% or 99% effective. Though those figures are commonly bandied around, it is less effective in real life use, with issues like human error and difficulty renewing one’s supply getting in the way, reproductive health experts told MarketWatch.

For women in their childbearing years—of which there were about 61 million in 2012—the consequences of contraceptive failure are swift and have immediate implications for their lives.

Moreover, many forms of contraception can mask the signs of pregnancy, which poses a problem for women when it comes to detecting the condition in time to make a decision like abortion, they said.

“Our data shows women cannot control their fertility through contraception alone, even when they are using some of the most effective methods,” said BPAS Chief Executive Ann Furedi. “Abortion is birth control that women need when their regular method lets them down.”

Though contraceptives do have a risk of failure, they are effective, experts emphasized. Better contraceptive use has helped contribute to a declining U.S. abortion rate, which reached a record low in 2014, according to the non-partisan Guttmacher Institute.

Contraceptives have also become far more affordable for U.S. women since the Affordable Care Act was passed, with the requirement that most health plans cover them.

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But access to contraception and abortions are both threatened amid attempts to repeal the Affordable Care Act and defund Planned Parenthood, which provides reproductive health services to women along with abortions.

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Even without the reassurance of infallibility, it’s still important that women have access to contraception, are able to choose the type that best works for them and use it consistently, experts said.

“No method of contraception is foolproof, and that’s what we have to tell patients,” said American Congress of Obstetricians and Gynecologists President Dr. Haywood Brown. But “we also don’t want to scare people. We want them to know they’re doing the right thing.”

The BPAS data didn’t break down into categories beyond use of contraceptives, but experts said it’s likely that all of the woman who reported using contraception aren’t necessarily using it faithfully.

Of those with unintended pregnancies, 5% used contraception consistently and correctly, while 41% used contraception inconsistently or incorrectly, according to 2008 Guttmacher Institute statistics.

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Long-acting contraceptives are considered the most effective, including implants, IUDs and permanent sterilization, which have failure rates below 1%.

But they can still fail in various ways. IUDs can be in the wrong position or even get pushed out, which is more likely to happen immediately after a woman gets one but can happen later, too. And because IUDs can last up to 10 years, a woman may forget to get the device replaced.

Even permanent sterilization fails in rare cases, experts noted.

Any contraceptive that’s user-controlled, or require a woman to do something every day, has the highest chance of failure, said Dr. Maria Rodriguez, an assistant professor in Oregon Health & Science University’s Department of Obstetrics and Gynecology.

Guttmacher

Male condoms, which have an 18% failure rate, may well be the best-known example of contraceptive failure.

But there are also many ways that human error can affect a birth control pill, which has a 9% failure rate, especially if the pill is one with a lower dose of hormones.

One common problem happens when a woman doesn’t take the pill at the same time every day, experts said.

Missing doses is another problem. Even if the missed doses occurred earlier in the week, for example, the woman will need emergency contraception, ACOG’s Brown said.

Birth control pills “really just keep ovaries from presenting a follicle, but follicles are still in a stage of development,” Brown said. “If you miss one pill, two pills in a week, you run the risk of ovulating.”

Skipped doses can also happen when a woman has run out of pills and needs a new supply.

Many health insurance plans only allow for a month’s worth at a time, which means a woman needs to resupply fairly frequently, said Dr. Philip Darney, professor and director of the Bixby Center for Global Reproductive Health at the University of California, San Francisco. “It only takes missing a couple of birth control pills for a person to become pregnant.”

Similarly, injectable contraceptives like Pfizer Inc.’s PFE, -0.51% Depo-Provera requires resupply every three months, the patch each week and the vaginal ring each month, he noted. The contraceptives have failure rates of 6%, 9% and 9% respectively.

Other medications can also interfere with the birth control pill’s effectiveness. They include an antibiotic for tuberculosis and seizure medications, which may also be used for bipolar disorder and other conditions, Rodriguez said.

“Contraception, and women’s health in general, is siloed from the mainstream idea of health care,” she said. “Their internist, if they have seizures, or neurologist, isn’t always thinking about contraception. It should be part of the overall health care we give to women, whether they have complicated conditions or not.”

For women who are concerned about their contraception failing, symptoms may include missing a period, irregular bleeding, nausea, unexplained fatigue, bloating or weight gain and breast tenderness. They should go to their doctors or take a pregnancy test, experts said.

But it’s also important that women understand that contraception is fallible. Patients who were using contraception when they got pregnant tend not to believe that it’s possible, and often delay coming to the doctor as a result, experts said.

Those patients often feel betrayed by their contraception, Darney said, making it especially important to have a variety of contraceptive options available and continue to make progress on developing new kinds.

The BPAS data, released July 7, surveyed the 60,592 women who had an abortion at BPAS clinics last year.

Contraceptive manufacturers include Pfizer, Merck & Co. Inc. MRK, +0.19% , Bayer AG BAYRY, +0.59% , Allergan US:AGN, Johnson & Johnson JNJ, +1.36% , Teva Pharmaceutical Industries Ltd. TEVA, +2.04% and Church & Dwight Co. Inc. CHD, -1.89% .

This story was first published on August 2.