The prevalence of HIV-positive pregnant women in relationships with HIV-negative men in the United States is unclear. The purpose of this study was to calculate the prevalence of HIV-positive pregnant women with a serodiscordant (HIV-negative) partner within a single clinic population, assess disclosure of their HIV status, and examine factors associated with disclosure. All HIV-positive pregnant women who received prenatal care at the Harris County Hospital District Women's Program at Northwest Health Center in Houston TX between 1/1/2006 and 4/1/2011 were identified. Data were obtained from electronic medical records. Prevalence of serodiscordance and disclosure was calculated, and predictors of disclosure were evaluated. We identified 212 HIV-positive pregnant women. About 40% had a serodiscordant partner, and 34% had a partner with an unknown HIV status. Disclosure occurred in over 90% of women with a serodiscordant partner and in 68% of women with partners whose HIV status was unknown. Among pregnant women who knew their HIV status prior to the current pregnancy and had a serodiscordant partner, 92% reported disclosing their status prior to conception. Our data indicated that serodiscordant relationships are common in our clinic population. Suboptimal disclosure rates were observed, especially among women who have a partner with an unknown HIV status. Further research is needed to evaluate the prevalence of serodiscordance and disclosure in other United States populations.