Design and Methods of the Apple Women's Health Study: A Digital Longitudinal Cohort Study.

BACKGROUND Prospective longitudinal cohorts assessing women's health and gynecologic conditions have been historically limited. OBJECTIVES The Apple Women's Health Study was designed to gain a deeper understanding of the relationship between menstrual cycles, health, and behaviors. This paper describes the design and methods of the ongoing Apple Women's Health Study and provides demographic characteristics of the first 10,000 participants. STUDY DESIGN Mobile-application based longitudinal cohort study with survey and sensor-based data. We collected data from 10,000 participants who responded to the demographics survey upon enrollment between November 14, 2019 and May 20, 2020. Participants were asked to complete a monthly follow-up through November 2020. Eligibility criteria include having ever menstruated, installing the Apple Research app on iPhone with iOS version 13.2 or later, living in USA, age of 18 years or older in most states (19 years old in Alabama and Nebraska, 21 years old in Puerto Rico), proficiency in written and spoken English, sole user of an iCloud account or iPhone, and providing informed consent to participate in the study. RESULTS Mean age at enrollment was 33.6 years old (+/- standard deviation 10.3). Race/ethnicity was representative of the US population [69% White/Non-Hispanic (6,910/10,000)], while 51% (5,089/10,000) had a college education or above. Participant geographic distribution included all U.S. states and Puerto Rico. Seventy-two percent (7,223/10,000) reported use of an Apple Watch, and 24.4% (2,438/10,000) consented to sensor-based data collection. For this cohort, 38% (3,490/9,238) did not respond to the Monthly Survey: Menstrual Update after enrollment. At the six-month follow-up there was a 35% (3,099/8,972) response rate to the Monthly Survey: Menstrual Update. 82.7% (8,266/10,000) of the initial cohort and 95.1% (2,948/3,099) of participants who responded to month 6 of the Monthly Survey: Menstrual Update, tracked at least one menstrual cycle via HealthKit. Participants tracked menstrual bleeding days an average of 4.44 (25-75% range: 3 - 6) calendar months during the study period. Non-White participants were slightly more likely to drop out than White participants; those remaining at 6 months were otherwise similar in demographic characteristics to the original enrollment group. CONCLUSIONS The first 10,000 participants of the Apple Women's Health Study were recruited via the Research app and were diverse in race/ethnicity, educational attainment, and economic status, despite all using an Apple iPhone. Future studies within this cohort incorporating this high-dimensional data may facilitate discovery in women's health in exposure outcome relationships and population-level trends among iPhone users. Retention efforts centered around education, communication, and engagement will be utilized to improve survey response rates, such as the study update feature.