"We had broken up, so I'd gone off the pill," a woman might say.
"Just this once, we skipped the condom."
"I was still breastfeeding, so I thought I couldn't get pregnant."
Though birth control is widely available, about half of the 6 million pregnancies in the United States each year are accidents.
Contraception experts have long known that even couples who want to avoid pregnancy can be strikingly lax about actively preventing it. This month, a survey from the Guttmacher Institute, a top think tank on reproductive issues, casts new light on the whys and why nots.
Greater awareness of the contraceptive pitfalls, specialists say, may help women use birth control better. It estimated that of 50 million sexually active adult American women of childbearing age, 28 million are at risk of unintended pregnancy.
"The main problem is that taking a pill every day or using a method every time you have sex is really hard to do for 20 or 30 years," said the report's principal investigator, Jennifer Frost.
It is especially hard, the survey found, when women dislike their method, are in the midst of break-ups or other life changes, have problems getting pills or other contraceptive supplies, or are not totally sure they want to avoid pregnancy.
Conducted in 2004, the survey included 1,978 American women ages 18 to 44 who were sexually active, not sterile, and not pregnant or trying to become pregnant. They were asked about their contraceptive use in the previous year. The survey did not determine how many of the unplanned pregnancies led to births, though previous Guttmacher research has suggested that 42 percent end in abortion, 14 percent in miscarriage, and the rest in births.
The report found that nearly two-fifths of women on the pill miss their daily dose at least once in the course of three months, raising their risk of pregnancy; about three-fifths of women who rely on their partners to use condoms reported that a condom was skipped or put on late at least once during the previous three months.
Nearly four in 10 women using birth control do not much like their method, and such dissatisfaction also significantly raises the risk of pregnancy, it revealed. About 5 percent of the women said they used their birth control method reliably, but it had failed - possibly because of a break in a condom or, rarely, pills that don't work - allowing them to get pregnant.
Birth control providers familiar with the report say that it helps show women that if they fall down on birth control, it is not because they are stupid or lazy; there are complex and powerful factors at work.
For pill-takers, for example, studies have shown that it is a huge challenge for people without immediate symptoms to take a pill every day, whether it is birth control or blood pressure medication.
"Everyone always wants to indict young, sexually active women, and this is a behavior that is a human nature problem" not specific to them, said Dr. Katharine O'Connell of Columbia University Medical Center. "Which is why we're lucky to have other methods that don't require daily remembering."
Researchers are looking into various methods to prompt regular pill-taking, she said, such as text-messages on a woman's cellphone every morning.
Another persistent challenge, providers say, is that societal attitudes about sex often make women who are not in steady relationships reluctant to prepare in advance for possible sexual encounters, for fear that will make them seem promiscuous.
Dr. Moira Cunningham routinely advises her female patients to buy their own condoms and to keep a supply of Plan B, the emergency "morning after" pill, at home. When physicians discuss birth control, she said, they must be nonjudgmental.
"The way I explain it to my patients is, Mother Nature wants the species to survive, so all of us are hard-wired for sex. And that's what you're up against. It's not about 'good person' or 'bad person,' " said Cunningham, who is chief of the internal medicine practice at the Copley office of Harvard Vanguard Medical Associates.
Yet another barrier is trouble gaining access to birth control, whether because of cost, insurance changes, or logistical problems getting to the doctor for a new prescription or method.
"A fairly common scenario is that patients take their pills, and then their prescription runs out," said Dr. Alisa Goldberg of Planned Parenthood. "They just can't get in on time, and so they have a gap, and eventually they come back, but maybe they come back pregnant."
According to the Guttmacher report, more than half of women who have a risky gap of at least one month in using birth control are in the midst of an "important life event," whether a job change, a move, a break-up, a new love, or some kind of crisis.
Cunningham said that she encourages patients in that situation to simply call her office and ask for a "bridge prescription" to tide them over until they can come in. But many practices do not offer that option.
Karen Caponi, a nurse-practitioner at the Worcester Health Center of Planned Parenthood for 22 years, said that faulty birth control practices also often stem from myth and misinformation.
A classic myth she has fought for years, she said, "is that you can only stay on the pill for two years, and then you have to come off it, your body needs a rest. It doesn't need a rest!"
She tells her patients, "Call!" she said. "If you have any questions, please call us!"
Providers mentioned a variety of other myths that they encounter: That you cannot get pregnant over 40, during your period, or in a first sexual encounter. More abound.
Yet a trickier problem is the ambivalence many women feel about possible pregnancy. The Guttmacher study found that women who said they were trying to avoid pregnancy but would not mind if they had a baby were far less likely to use birth control reliably.
That has no easy fix, doctors said; in the course of a 15-minute visit, it is hard enough to do basic counseling about methods without getting into a patient's hopes and dreams. But they did caution that planned pregnancies tend to do best, because mothers who are "trying" can take better care of themselves, such as avoiding alcohol and taking prenatal vitamins to prevent certain birth defects.
Given all the "human error" problems with pills and condoms, some providers say they encourage patients to give added consideration to longer-term methods: the Intra-Uterine Device, or IUD, which can stay in for up to five years and has recently been approved even for women who have not yet borne children; the Nuva ring, which stays in the vagina for three weeks; and hormonal implants such as Implanon that last three years.
"They take the requirement to do something out of the patient's hands, which for most people is a good thing," Goldberg said.
Carey Goldberg can be reached at Goldberg@globe.com.
How to better protect yourself
1) Consider longer-term methods, such as IUDs and hormonal implants, which offer protection for years.
2) If on the pill, set up a daily reminder system.
3) Women who are not sexually active should keep condoms on hand, just in case.
4) Consider keeping a stash of "morning-after" pills, known as Plan B, which is available without a prescription for those over 18.
5) Keep track of when pill prescriptions are set to expire and get a new prescription or an extension before they run out.
6) Be aware that you may need to change your method as your life circumstances change.
7) Watch out for times of heightened stress and change, when birth control use is likeliest to fall down.
8) Talk to your provider right away if you have any problems or are not satisfied with your method; the doctor may be able to help you figure out other method.
9) Dispel myths. Get information from established sources such as 4women.gov/faq/birthcont.htm.
10) Try to think through your feelings about having a baby before you take birth control risks; doctors say planned pregnancies are optimal.