Comes with Large/XL (12.5 in- 18.5 in) arm band and extender. After ovulation, your body temperature rises and remains in a higher range until your next cycle starts. Tracking your sleeping temperature allows you to identify ovulation at home. Tempdrop contains multiple sensors that measure your core temperature continuously throughout the night. Tempdrop’s patented algorithm learns your unique nightly and monthly temperature patterns, filtering out disturbances for accurate results. Combining temperature readings with other fertility symptoms helps you more accurately identify your fertility window. The battery can be changed using a coin or flathead screwdriver. Temp drop uses a super easy app to track your cycle. This is a great tool to get pregnant or naturally prevent pregnancy. If you have questions, let me know!