Introduction
Dramatic population growth in the US-Mexico border region suggests more effective family planning services are needed, yet binational data are scarce. The Brownsville-Matamoros Sister City Project for Women’s Health collected binational, standardized data from 947 postpartum women in Cameron County (Texas) and Matamoros (Tamaulipas, Mexico) hospitals from August through November 2005.

Methods
We analyzed these data to estimate the proportion of women with unintended pregnancy and the proportion of these women who reported contraceptive use, and to identify associated factors.

Results
The current pregnancy was unintended for 48% of women overall. Almost half of these women reportedly used birth control at conception, but many used low-efficacy methods. Among women with unintended pregnancy who did not use contraception, 34.1% of Mexico residents believed they could not become pregnant and 28.4% of US residents reported no reason for nonuse. Overall, contraceptive use to prevent pregnancy was less common among younger than older women and among women who had not graduated high school compared with those who had. Among Mexico residents, those who had a source of routine health care were more likely than those who did not to have used contraception.

Conclusion
More effective contraceptive practices are needed in this population, especially among younger and less-educated women. A cooperative binational approach that integrates reproductive and family planning services may be most effective.