There is an urgent global goal to eliminate new HIV infections among children by 2015 and to keep their mothers alive, with the targets of providing antiretroviral therapy to all HIV-infected pregnant women and children. Yet, barely a year away from the 2015 deadline, HIV programs are far from reaching these targets. A substantial proportion of HIV-positive pregnant women in sub-Saharan Africa are lost to follow-up after delivery. The exponential increase in the number of mobile phone connections in sub-Saharan Africa presents a unique opportunity to use mobile technology (mHealth) to promote retention in prevention of mother-to-child HIV transmission (PMTCT) programs. We previously found that text messages developed using a behavioral theoretical framework significantly improved maternal attendance at the six-week postpartum clinic appointments and rates of testing to facilitate timely infant diagnosis of HIV in a selected population and controlled setting. We are currently expanding this Texting to Improve Testing (TextIT) intervention using an implementation science framework in a cluster randomized, stepped wedge trial in 20 clinics operated by the Kenyan Ministry of Health in the high HIV prevalence Nyanza region. If our TextIT intervention is to have lasting effects beyond six weeks postpartum, it would need to be attuned to the nuances of navigating the complex PMTCT cascade over the long term, and still remain affordable for patients and programs. Therefore, we will extend the TextIT intervention beyond six weeks by adding a weekly two-way check-in message through 9 months postpartum, and conduct a cost-effectiveness evaluation of this extension. We hypothesize that a greater proportion of women at health facilities implementing the extended TextIT will be retained in care and have suppressed HIV RNA through 9 months postpartum, and that the extended TextIT intervention will be more cost-effective than current standard care.