The goal of this study is to learn about the role of estrogen and other hormones in bone development in adolescent and young adult women with cystic fibrosis (CF). The study has two main components, an observational study and a feasibility sub-study. The main questions it aims to answer are: * How do different forms of hormonal contraception (e.g. birth control pill or other progesterone only methods) impact bone health? * Is a study of transdermal estradiol (estrogen skin patches) feasible in this group? * How does transdermal estradiol impact bone health and quality of life? Participants in the observational study will have three study visits (baseline, 6 months, and 12 months). At all visits, participants will have blood drawn and will complete questionnaires. At the baseline and 12 month visits, participants will additionally have x-rays and DXAs (bone density assessment). Between visits, participants will complete brief questionnaires. Researchers will compare various measurements over time to assess the impact of different forms of hormonal contraception. Participants in the feasibility sub-study will apply transdermal estradiol patches once weekly for 12 months, fill out brief quality-of-life surveys in between study visits, and complete blood draws and questionnaires at four study visits (baseline, 3 months, 6 months, and 12 months). The participants will also have x-rays and DXAs performed at the baseline and 12 month visits. Researchers will examine changes in various measurements to determine the impact of transdermal estradiol.