Researchers recommend monitoring for women who receive fertility treatment.
The breast consists of two types of tissue: dense, fibroglandular and non-dense, fatty tissue.
Previous research has shown that the chance of women with very dense breasts developing breast cancer is four to six times higher than that of women whose breasts are not dense.
Somewhere between 12-28% of couples globally are thought to experience infertility. In the US, 6.7 million, or 10.9% of women aged 15-44 years have difficulty either becoming pregnant or carrying a baby to term, according to the Centers for Disease Control and Prevention (CDC).
While no clear association has emerged between ovarian stimulation and breast cancer, researchers believe that raising estrogen and progesterone levels may increase the risk of breast cancer.
History of infertility associated with denser breasts
Researchers at Karolinska Institutet in Sweden, led by Frida Lundberg, surveyed 43,313 women aged from 40-69 years, who had mammograms from 2010-2013.
- Apart from some skin cancers, breast cancer is the most common cancer among women in the US
- In 2012, 224,147 American women and 2,125 men received a diagnosis of breast cancer
- 41,150 women and 405 men died in 2012 of the disease.
The screening was part of the KARolinska MAmmography (KARMA) project for risk prediction of breast cancer.
Participants then gave details about age, weight, height, alcohol consumption, smoking status, history of infertility and family history of breast cancer, by completing a questionnaire.
The researchers compared mammographic density levels between fertile and infertile women. They then compared the breast density of infertile women who had never had hormonal fertility treatment with those who had.
Of all the participants, 8,963 women reported fertility problems. Of these, 1,576 had undergone COS, 1,429 had received hormonal stimulation without COS, and 5,948 had not undergone fertility treatment.
Results showed that women who had a history of infertility had denser breasts than those who did not. Their breasts contained more dense, fibroglandular tissue than women who were fertile. Infertile women who had undergone COS had higher absolute dense volume than those who had not had hormone treatment.
Researchers do not know whether COS or the underlying fertility is responsible for the increase in breast density.
This is the first population study to look into the effect of infertility and hormone stimulation on breast density.
The authors believe breast density may be useful in evaluating the effect of hormonal fertility treatment on breast cancer risk, especially among women who develop breast cancer before the age of 50 years, the age below which a breast cancer diagnosis is less common.
Lundberg believes the findings may indicate a higher risk of breast cancer among women who receive COS.
She cautions, however:
"While we believe it is important to continue monitoring these women, the observed difference in breast tissue volume is relatively small and has only been linked to a modest increase in breast cancer risk in previous studies."
Limitations include the fact that the study relied on self-reported information, and the KARMA data did not provide specific diagnoses. Hence, it may be unclear exactly who has infertility and who does not. Women reporting infertility, for example, may be fertile women with an infertile partner.
Questions that remain unanswered
A lack of information about the timing and number of treatment cycles received by each woman means that the authors cannot know whether high breast density results specifically from more recent treatment.
However, a stronger association emerged between COS and breast density in older women, suggesting that COS may have a long-term effect on breast tissue.
Being cross-sectional rather than longitudinal, the current study does not prove a causative link between hormonal fertility treatment, infertility and breast density.
However, since there appears to be a moderate association between infertility, hormonal treatments and breast density, the researchers recommend ongoing monitoring for women undergoing COS.
Medical News Today reported earlier this year on research suggesting that measures of breast density are not always consistent.