(Reuters Health) - Transgender women who take sex hormones to feminize their bodies may not experience as much breast development as they expect, a new European study suggests.

Researchers followed 229 people transitioning from male to female for one year after they started taking what’s known as cross-sex hormones. For this type of transition, hormone therapy consists of both anti-androgens to block the activity of male sex hormones and curb male characteristics as well as estrogens to encourage breast development and feminization.

Only 21 of the trans women attained a bra size of an A cup or larger after one year of hormone therapy, the study found.

“In our experience, trans women have high expectations of the breast size they gain with hormone therapy,” said lead study author Dr. Christel de Blok of the Center of Expertise on Gender Dysphoria at the VU University Medical Center in Amsterdam.

“This study provides doctors with some evidence that it is less than often thought or hoped,” de Blok said by email.

Participants were patients at specialized clinics in Amsterdam; Ghent, Belgium; and Florence, Italy.

To assess breast growth, researchers measured the circumference of the chest underneath the breasts and at the largest part of the breasts. The difference between these two measurements can be used to determine breast growth and bra sizes.

At the start of the study, participants’ average difference between chest and breast circumference was 4.1 centimeters (1.6 inches). After one year on hormones, the average difference increased to 7.9 centimeters (3.1 inches).

Most of the breast growth happened during the first six months of treatment.

During the first three months on hormones, participants had average breast growth of 1.8 centimeters, followed by growth of about 1.3 centimeters over the next three months. After that, average growth tapered off, with 0.5 centimeters between six and nine months and 0.2 centimeters during the final three months.

After translating the growth into bra sizes, the results were modest.

Almost half of the trans women had a bra cup size of less than AAA, or less than 8 centimeters, after one year of hormone therapy, researchers report in the Journal of Clinical Endocrinology & Metabolism.

Another 26 percent had an AAA cup, which runs from 8 to 10 centimeters, and 14 percent had an AA cup, from 10 to 12 centimeters.

Average breast growth didn’t change even after researchers accounted for how large participants’ breasts were at the start of the study.

Breast development also didn’t vary based on whether the trans women gained or lost weight. Although obese trans women had the biggest breast size after six months, their development was similar to growth for other participants by the end of the study.

It also didn’t vary based on the type of estrogen women used or hormone levels in the blood.

One limitation of the study is that breathing can influence breast size measurements, and it’s possible not all participants were breathing in the same way when they were assessed, the authors note.

Measuring breast volume, which wasn’t done in the study, may also give a more accurate cup size than the method based on breast and chest circumference that was used in the study, the authors also point out.

It’s also possible that development might look different over a longer period of time or with different doses of estrogen, said Dr. Stuart Chipkin, a researcher at the School of Public Health and Health Sciences at the University of Massachusetts Amherst who wasn’t involved in the study.

“We know very little about how long this process takes in trans women, and that’s one of the reasons this study is important,” Chipkin said by email.

“We also know very little about what are reasonable breast size expectations for trans women,” Chipkin added. “Society has tended to place probably more emphasis on larger breasts than is needed - that’s true for all women.”

SOURCE: bit.ly/2o0T3E6 Journal of Clinical Endocrinology & Metabolism, online November 20, 2017.