Context: For transgender women undergoing gender-affirming hormone therapy (GAHT), breast development is an important treatment outcome. Since earlier studies showed small breast volumes in transwomen treated with GAHT, we hypothesized that testosterone exposure during puberty might negatively influence breast development and that early initiation of puberty suppression (PS) might have a positive effect on breast development.

Aim: To investigate the influence of timing of PS and subsequent GAHT on breast volume and satisfaction with breast size.

Methods: Twenty-three transwomen who started PS in early puberty (Tanner stage G2-3), and 19 who started in late puberty (Tanner stage G4-5) were included. Twelve individuals who had breast augmentation were excluded (early pubertal n=5, late pubertal n=7). 3D-scans of the breasts were used to calculate breast volume. Participants filled out a questionnaire on breast satisfaction.

Results: Mean age at time of study was 19.4±1.9 years in the early group and 21.0±1.8 years in the late pubertal group. Both groups had used GAHT for 4.2±1.6 years. Fat percentage in the late group was was 8.9% (95%CI 4.5 to 13.4) higher compared to the early group. Mean breast volume was 114 cc (IQR 58 to 203), i.e. bra cup-size <A. Breast volume was 47 cc (95% CI -9 to 104) larger in the late group but this difference was only 4 cc (95% CI -67 to 75) after correction for fat percentage. In total, 64% of subjects were satisfied with their breast size (57% vs 74% in the early and late group, respectively). 

Conclusion: In this study, early start of PS was not found to result in larger breast volume. There even was a trend towards larger breast volume in the late pubertal group, but this seems related to a higher fat percentage. These findings suggest that pubertal testosterone exposure does not affect breast development during GAHT.  However, exclusion of those who had undergone breast augmentation may have introduced bias. The impact of timing of PS on other breast characteristics, such as breast shape and positioning, deserves further study.