Each year, half of American women who would rather not get pregnant will have an unplanned pregnancy, often because they failed to use their contraceptive properly or forgot to use it at all, US researchers said.
As a result, 28 million women in the United States are at risk for an unintended pregnancy, according to the study conducted by the Guttmacher Institute in New York.
They found one in four women is very likely to become pregnant because of inconsistent contraception use.
Some of this gap is due to lack of access to healthcare, with many women saying they cannot afford some of the more effective methods of contraception such as birth control pills that require a doctor's visit and prescription.
"It is critical to have a better understanding of what is preventing women from using contraception consistently and correctly, or even at all," Dr. Jennifer Frost, a senior research associate at Guttmacher, said in statement.
The research involved surveys of women and family planning providers nationwide. Among the results, they found that more than half of women who have a gap in contraceptive use of at least one month have experienced some type of major life event - such as the end of a relationship, a move, job change or personal crisis.
They also found that many women are not satisfied with their current method of contraception, a problem that can lead to missing birth control pills or failure to keep a condom handy, for example.
Last month the National Centre for Health Statistics reported there were almost 6,4 million pregnancies in 2004, down 6 percent from 1990. Forty-five percent were to women who were not married, and there were 1,22 million abortions and 1,06 million stillbirths and miscarriages.
The researchers also found that many women who are lax about birth control are simply ambivalent about preventing a pregnancy and confessed that they would be very pleased if they found out they were pregnant.
The researchers said women who are the least motivated to avoid pregnancy are far less likely to use birth control pills, or any contraceptive method at all on a consistent basis.
They said a woman's attitude toward pregnancy, her satisfaction with her method and her experiences with gynaecologists and other providers of contraception play a far bigger role in a woman's risk of pregnancy than other major risks, such as poor education and poverty.
Frost said doctors and other health providers should start by helping women to find the best contraceptive method for them.
"The more we can remove the remaining barriers to consistent use, the better we will be at ensuring that all women can avoid unwanted pregnancies and plan the children they want, when they want them," Frost said.