BACKGROUND: Female sex workers in low-income and middle-income countries face high risks of unintended pregnancy. We developed a 12-month, multifaceted short messaging service intervention (WHISPER) for female sex workers in Kenya who had the potential to become pregnant, to improve their contraceptive knowledge and behaviours. The aim of this study was to assess the effectiveness of the intervention to reduce the incidence of unintended pregnancy among sex workers in Kenya compared with an equal-attention control group receiving nutrition-focused messages (SHOUT). METHODS: Our two-arm, cluster-randomised controlled trial was done in sex-work venues in two subcounties of Mombasa, Kenya (Kisauni and Changamwe). Participants, aged 16-34 years, not pregnant or planning pregnancy, able to read text messages in English, residing in the study area, and who had a personal mobile phone with one of two phone networks, were recruited from 93 randomly selected sex-work venues (clusters). Random cluster allocation (1:1) to the intervention or control group was concealed from participants and researchers until the intervention started. Both groups received text messages in English delivered two to three times per week for 12 months (137 messages in total), as well as additional on-demand messages. Message content in the intervention group focused on promotion of contraception, particularly long-acting reversible contraception and dual method contraceptive use; message content in the control group focused on promotion of nutritional knowledge and practices, including food safety, preparation, and purchasing. The primary endpoint, analysed in all participants who were randomly assigned and attended at least one follow-up visit, compared unintended pregnancy incidence between groups using discrete-time survival analysis at 6 and 12 months. This trial is registered with Australian New Zealand Clinical Trials Registry, ACTRN12616000852459, and is closed to new participants. FINDINGS: Between Sept 14, 2016, and May 16, 2017, 1728 individuals were approached to take part in the study. Of these, 1155 were eligible for full screening, 1035 were screened, and 882 were eligible, enrolled, and randomly assigned (451 participants from 47 venues in the intervention group; 431 participants from 46 venues in the control group). 401 participants from the intervention group and 385 participants from the control group were included in the primary analysis. Incidence of unintended pregnancy was 15.5 per 100 person-years in the intervention group and 14.7 per 100 person-years in the control group (hazard ratio 0.98, 95% CI 0.69-1.39). INTERPRETATION: The intervention had no measurable effect on unintended pregnancy incidence. Mobile health interventions, even when acceptable and rigorously designed, are unlikely to have a sufficient effect on behaviour among female sex workers to change pregnancy incidence when used in isolation. FUNDING: National Health and Medical Research Council of Australia.
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