National Survey of Family Growth.
"The women who are less likely to have access to healthcare are more likely to say 'I didn't use another method, and I turned to emergency contraception to protect myself,'" said Allina.
Some women may choose to use it occasionally if they cannot afford other methods, she added.
In a separate report on Thursday, CDC looked at overall contraceptive use and found that while the number of women using regular birth control pills has remained flat over time, the use of injections, patches and intrauterine devices has grown.
The number of women whose partners have used condoms also rose, the findings showed.
That trend may reflect increased wariness among Americans to have children amid the 2007-2009 economic recession, the effects of which are still being felt by many, according to researchers at the Guttmacher Institute, which also tracks birth control use.
"At the same time, it can make it harder for people to have access to birth control because of costs," especially for disadvantaged women who face higher rates of unintended pregnancies, said Lawrence Finer, head of domestic research for the reproductive research group.
That situation could change in the wake of the 2010 healthcare overhaul that required health insurers to begin covering birth control last year, although the law faces legal challenges.
Religious groups, particularly Catholics, charge that the provision violates their belief against artificial birth control and are fighting to block it.