The goal of this clinical trial is to test the acceptability, feasibility, accuracy and precision of the 'GrowthMonitor' smartphone app in a pilot population.

The GrowthMonitor app will calculate height data using smartphone images of children taken by parents or carers in the home at two or more set time points. The app will convert height data from the image to a numerical height measurement in centimeters and this will be transferred to a central database hosted on a secure UK based server with additional relevant demographic data (gender, ethnicity, weight, date of birth, and parents' height).

The height and demographic data are transferred with an anonymous identifier to a GDPR-approved secure database on a UK based server. Critically, patient images and children's names will not be transferred, and only authorized users will have access to the data stored within the secure database.

The app will display a virtual growth chart providing the patient with a digital history of the child's growth over time. BMI will also be calculated and plotted on an age- and gender-appropriate chart. Ultimately, users of the app will need to create an account using a username (email address) and password. We will use their email address so that password resets can be sent along with reminders or alerts about their account.

Child's height in relation to his/her parental heights as well as longitudinal growth data will be analyzed within the secure UK based web server using existing growth monitoring algorithms.

1. Height compared to population-based height references (HSDS)
2. Distance from target height (calculated from parental heights, THSDSDEV)
3. HSDS change over time (ΔHSDS) -when more than one height measurement is available (taken >6 months apart)

Recorded heights will be compared with pre-established cut-off standards that define normal growth in an appropriate population. We envisage being able to incorporate ethnicity-matched reference population data as the app evolves. An automated outcome will be returned to the parent/carer, via in app notification or email, and will offer advice, for example, confirming normal growth or recommending appropriate medical contact.

Data will be stored in such a way that users can sync their old growth measurements on to any new / replacement smartphone handset and also 'sync' any measurements taken offline.

An accurate standing height measurement will be obtained using a calibrated stadiometer during a patient clinic visit. This 'gold standard' height measurement will be performed by a trained member of the research team and documented. This will be used to compare with the height obtained by the growth monitoring app in the clinic and subsequently at home.

Inclusion Criteria:

• Children and young people (male and female) aged 1-18 yr receiving height measurements as part of their routine / prearranged attendance to a paediatric endocrinology or other paediatric out-patient appointment at Barts Health Trust.
• Participants require a smartphone to take part in the study.

Exclusion Criteria:

• Children under the age of 1 years
• Young people aged 16-18yr who lack capacity to consent
• Other individuals unable to reliably stand for an accurate height measurement e.g.

individuals with severe disability and/or confined to a wheelchair

Patients attending prearranged new and follow-up outpatient appointments at the Children's Hospital at Barts Health Trust, London. Participants will be recruited through convenience sampling in order to maximize recruitment efforts during the time of the study. Data on gender, age, and height will be collected.