| In the United States (U.S.), both unintended pregnancy and sexually transmitted infections (STIs) are common. When an unintended pregnancy and STI occur at the same time, it can result in serious health risks and complications, regardless of whether a pregnant person gives birth or has an abortion. Consequentially, unintended pregnancy and STIs are considered to have a syndemic relationship. There currently is no research on the frequency or correlates of unintended pregnancy and STI co-occurrence with a representative U.S. sample. In this study, data from the National Survey of Family Growth was used to calculate the frequency of STI occurrence within two years of an unintended pregnancy. The final sample included 1,692 pregnancies. Among unintended pregnancies, 11.5% had an STI within two years of the pregnancy occurring. Among intended pregnancies, 12.4% had an STI within two years of the pregnancy occurring. Multinomial logistic regression was used to identify correlates associated with unintended pregnancy and STI co-occurrence. The regression model displayed that using low effectiveness birth control (excluding condom use), having a higher number of male sexual partners, being single, being younger, being lower-income, and having less than a high school education was associated with higher odds of having an unintended pregnancy and an STI, compared to having an intended pregnancy and no STI. Keywords: sexually transmitted infections, unintended pregnancy, syndemics, National Survey of Family Growth |