Breast Development in Transwomen After One Year of Cross-sex Hormone Therapy: Results of A Prospective Multicenter Study

Context: Breast development is a key feature of feminization and therefore important to transwomen (male-to-female transgender persons). However, it is not exactly known when breast development starts after initiating cross-sex hormone therapy (CHT) and how much growth may be expected of CHT alone.

Objective: To investigate breast development in centimeters and bra cup-sizes in transwomen during their first year of CHT, and whether clinical or laboratory parameters predict the breast development.

Design: This study was performed as part of the European Network for the Investigation of Gender Incongruence (ENIGI), which is a prospective multicenter cohort study.

Setting: Specialized gender clinics in Amsterdam, Ghent, and Florence.

Participants: All transwomen who completed the first year of CHT (n=229) were eligible for analyses.

Intervention: Cross-sex hormone therapy.

Main outcome measures: Breast development in centimeter and cup-size.

Results: The median age of the included transwomen was 28 years (range 18;69 years). Mean breast - chest difference increased from 4.1±2.9cm at baseline to 7.9±3.1cm after one year of CHT, mainly resulting in less than an AAA cup-size (48.7%). Main breast development occurred in the first six months of therapy. Serum estradiol levels did not predict breast development after one year CHT (first quartile: +3.6cm(95% CI +2.7;+4.5cm), second quartile: +3.2cm(95% CI +2.3;+4.2cm), third quartile: +4.4(95% CI +3.5;+5.3cm), fourth quartile: +3.6cm(95% CI +2.7;+4.5cm)).

Conclusion: This study shows that after one year of CHT breast development is modest and occurs primarily in the first six months. No clinical or laboratory parameters were found that predict breast development.