Breast Development in Transwomen After 1 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). It is not exactly known when breast development starts after initiating cross-sex hormone therapy (CHT) and how much growth may be expected.

OBJECTIVE

To investigate breast development in transwomen during their first year of CHT and whether clinical or laboratory parameters predict breast development.

DESIGN

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

SETTING

Gender clinics in Amsterdam, Ghent, and Florence.

PARTICIPANTS

Transwomen who completed the first year of CHT (n = 229).

INTERVENTION: CHT.

MAIN OUTCOME MEASURES: Breast development in centimeter and cup size.

RESULTS

The median age of the included transwomen was 28 years (range, 18 to 69). Mean breast-chest difference increased to 7.9 ± 3.1 cm after 1 year of CHT, mainly resulting in less than an AAA cup size (48.7%). Main breast development occurred in the first 6 months of therapy. Serum estradiol levels did not predict breast development after 1 year of CHT (first quartile, 3.6 cm [95% confidence interval (CI), 2.7 to 4.5], second quartile, 3.2 cm [95% CI, 2.3 to 4.2], third quartile, 4.4 cm [95% CI, 3.5 to 5.3], and fourth quartile, 3.6 cm [95% CI, 2.7 to 4.5]).

CONCLUSION

This study shows that, after 1 year of CHT, breast development is modest and occurs primarily in the first 6 months. No clinical or laboratory parameters were found that predict breast development.