By Aly W. | First published February 14, 2020 | Last modified October 23, 2020
The role of progesterone in breast development and its possible usefulness for helping with breast development in transfeminine hormone therapy can be informed by the normal biological circumstances of puberty in cisgender females. Progesterone essentially doesn’t appear during puberty until ovulatory menstrual cycles begin. Menarche, the onset of menstruation and hence menstrual cycling, occurs on average at Tanner breast stage 4, although it occurs at Tanner breast stage 3 or Tanner breast stage 5 (complete breast development) in significant subsets of girls (Marshall & Tanner, 1969; Hillard, 2007). Tanner breast stage 4 is on average about 2.5 years into breast development, while breast development as a whole takes on average about 3.5 years. Hence, the appearance of progesterone in normal female puberty is a relatively late event (Marshall, 1978; Begley, Firth, & Hoult, 1980; Drife, 1986).
The reproductive axis in pubertal and adolescent cisgender girls is immature (Rosenfield, 2013; Gunn et al., 2018; Sun et al., 2019; Carlson & Shaw, 2019). In the first 1 to 2 years postmenarche, most menstrual cycles are anovulatory (i.e., ovulation does not occur) (Döring, 1963 (Table); Apter, 1980; Lemarchand-Béraud et al., 1982; Talbert et al., 1985; Venturoli et al., 1987; Rosenfield, 2013; Gunn et al., 2018; Carlson & Shaw, 2019). Without ovulation, the corpus luteum doesn’t form from a ruptured ovarian follicle and progesterone production doesn’t commence. Only about half of menstrual cycles are ovulatory by Tanner breast stage 5 (Talbert et al., 1985). In addition, menstrual cycles are unusually long for some time after menarche (e.g., 50 days vs. 28 days for adult cycles) and thus there are fewer menstrual cycles per reproductive year (Rosenfield, 2013; Gunn et al., 2018; Carlson & Shaw, 2019). Luteal-phase progesterone levels are also lower in postmenarche adolescents than in adulthood even when ovulation does occur (McArthur, 1966 (Figure); Lemarchand-Béraud et al., 1982; Apter et al., 1987; Venturoli et al., 1987; Venturoli et al., 1989; Sun et al., 2019). Consequently, progesterone exposure is sporadic and limited even during late female puberty. Moreover, this is the case not only by the time of Tanner stage 5 but for many years after it as well. It takes more than 6 years after menarche for menstrual cycling to become fully mature and consistently ovulatory (Lemarchand-Béraud et al., 1982; Venturoli et al., 1987; Carlson & Shaw, 2019). Over this period of time the rate of ovulatory cycles increases progressively until it reaches approximately 100% (Lemarchand-Béraud et al., 1982; Venturoli et al., 1987; Carlson & Shaw, 2019). Only then is full adult-level exposure to progesterone finally achieved (Lemarchand-Béraud et al., 1982; Venturoli et al., 1987).
There are findings suggesting that progesterone is dispensable for pubertal breast development. One is that women with complete androgen insensitivity syndrome, who have no progesterone, have excellent and full breast development (Aly W., 2020). Another is that, as mentioned above, a significant portion of girls reach Tanner breast stage 5 (complete breast development) before experiencing menarche. There is some more discussion on this topic, including other relevant findings, here on Wikipedia. We don’t know for certain whether progesterone is involved in pubertal breast development or not due to a lack of studies, but what we do know isn’t promising in terms of a potential role of progesterone. There is also theoretical concern that premature introduction of progestogens might have an adverse effect on final breast development, although more research is needed to confirm this possibility (Kay C. & Aly W., 2019). Moreover, it may only apply to high doses of progestogens (Kay C. & Aly W., 2019).