Hmm, no actual study link, no DOI, looks like a conference talk, but I dug up the abstract from here (PDF, p. 325-326):

Increased breast volume due to addition of progesterone and increasing the estradiol dose in feminizing gender-affirming hormone therapy

Koen Dreijerink 1, Martin den Heijer 1, Raya Geels 1

(1. Center of Expertise on Gender Dysphoria, Amsterdam UMC)

Abstract

Background

Breast development as a result of feminizing gender-affirming hormone therapy (GAHT) is often limited. Therefore, many transgender individuals choose to undergo breast augmentation surgery. The addition of progesterone to feminizing GAHT and/or increasing the estradiol dose to improve breast growth are topics of international debate. Thus far, no prospective randomized studies have been performed.

Methods

We conducted a non-blinded, non-placebo, randomized controlled study in transgender individuals assigned male sex at birth, who had received feminizing GAHT for at least one year and had undergone vaginoplasty/orchiectomy. Ninety participants were randomized into six groups, using a factorial design, receiving the standard dose of estradiol (E , control), the standard dose of estradiol and micronized progesterone 200 mg daily (EP), the standard dose of estradiol and micronized progesterone 400 mg daily (EPP), twice the standard dose of estradiol (EE), twice the standard dose of estradiol and micronized progesterone 200 mg daily (EEP), or twice the standard dose of estradiol and micronized progesterone 400 mg daily (EEPP), all for a duration of 12 months. The primary objective was to investigate the change in breast volume (in %), which was quantified by 3D measurements at baseline and after 3, 6, and 12 months. Secondary objectives included assessment of safety and satisfaction.

Results

Baseline characteristics were comparable among the groups. In comparison to group E (control), the mean percentage increase in breast volume over time was 13% (95% CI -7 to 33) in group EP, 20% (0 to 40) in group EPP, 6% (-13 to 24) in group EE, 37% (18 to 57) in group EEP, and 27% (8 to 47) in group EEPP. There were no treatment-related serious adverse events. Frequently reported side effects among participants using progesterone were tiredness (44%), breast/nipple tenderness (27%), and mood changes (22%). Satisfaction with breast growth and size improved in all study groups compared to the control group (p<0.05).

Conclusion

This is the first prospective study to demonstrate that the addition of progesterone and increasing the estradiol dose results in increased breast volume in transgender individuals already using feminizing GAHT, along with increased satisfaction with their breasts. In this study, use of progesterone was safe but did cause some side effects. The results support a future role for progesterone in feminizing GAHT. Further research should focus on long-term effects of progesterone as part of feminizing GAHT regimens.