Female contraception is an increasingly hot topic both in the world of politics and in public health circles. Convincing evidence that allowing women to control if and when to have children has a positive effect on society is steadily building up. For one, teen pregnancies negatively affect the ability of young women to graduate from high school and go to college, both of which are gateways to better jobs and increased financial security. For those women who do end up in competitive jobs, being able to space out pregnancies has been shown to decrease the gender pay gap. The accessibility of contraception to lower-income women as well as to those living in more conservative parts of the world is an issue that is being debated more and more both on national and international platforms. However, this story is about understanding the facts behind one of the most notorious claims about contraception – its connection to cancer.
First of all, it’s worth noting that hormonal contraception actually covers a relatively large variety of drugs and medical devices. About 16% of women between 15 and 44 in the United States use the pill, while about 7% use some form of long-term reversible contraception, such as an intrauterine device (also known as an IUD) or the contraceptive implant. There are two main types of pill: the “traditional” pill, which is composed of both estrogen and progesterone hormone, and the progesterone-only pill. While all of these methods (especially long-term reversible contraception) are relatively novel, there is a lot of data being published on the medium and long-term effects of hormonal contraception on cancer incidence. In particular, data on the pill is especially reliable since it has been prescribed to women throughout the wold for the longest and it is the most common . The data that is available on IUDs and on the contraceptive implant so far seems to be confirming the observations carried out on women who have been taking the pill. The idea of investigating the link between hormonal contraception and cancer goes back to the late Eighties and early Nineties and is a relatively logical step. Many cancers, especially breast cancer, are literally addicted to hormones and many types of breast cancer cells need a constant supply of estrogen in order to thrive. Therefore, it would make sense for extra doses of these hormones to promote cancer proliferation and progression. These hypotheses were tested as early as the Nineties and are still being explored by cancer scientists and epidemiologists throughout the world.
The link between increased breast cancer risk and use of the pill was first investigated in the Nineties, only a few decades after it had become accessible to women. These early reports found what would be confirmed by later and more sophisticated studies; women who are either on the pill or have recently stopped taking it have a slightly higher risk of breast cancer. However, the risk decreases with time and 10 years after having stopped taking the pill women are just as likely to get diagnosed with breast cancer as women who have never used hormonal contraception. What is interesting is that certain types of pill are more likely to increase the risk of breast cancer than others. For example, the triphasic pill (where the ratio of different hormones changes throughout the course of the natural hormonal cycle) affects breast cancer incidence much more dramatically than the progesterone-only pill. The interesting thing about looking at this data is that contraception use is often an indicator for life choices, class and standards of self-care. For example, while women who have stopped taking the pill for over 10 years are no more or less likely to develop breast cancer than women who have never been on the pill, the tumors they are diagnosed with tend to be at a much earlier stage than those detected in women who have never used hormonal birth-control. Since women on the pill tend to be wealthier, they have access to more screening programs and regular check ups. On the other hand, part of the reason that women on the pill might be more likely to develop breast cancer is that they are more likely to engage in other behaviors that are also predictors of breast cancer risk in their own regard, such as having children late or not at all, or to be in situations that already put them at higher risk for breast cancer, like starting their cycles early or experiencing late menopause. This is not to say that there isn’t a direct correlation between hormone administration and the invasive behavior or breast cancer cells, but rather to indicate that these studies have to contend with a huge variety of confounding factors.
Endometrial and ovarian cancer
The earliest studies on the effects of hormonal contraception on women identified a pattern of reduced risk of ovarian cancer in women who take the pill. After 5 years of taking hormonal contraception, the risk of ovarian cancer drops by half. However, these old studies did not pick up on how that happens. More recent work has shown that birth-control pills that have a higher proportion of the hormone progesterone are more effective at protecting women from ovarian cancer. One of the most interesting theories as to why progesterone would have this effect regards how progesterone-only pills affect the fertility of women. In fact, progesterone-based contraception is based on the simple principle that women with higher levels of progesterone will not ovulate. This is a way of “tricking” the body into thinking it is in the middle of its cycle (when progesterone is at its highest), when ovulation is not necessary. Since no ovulation occurs while on progesterone-based birth control, cells within the ovary are dividing at a much lower rate. Low rates of cell division tend to protect any given tissue from tumor formation, as cancer often results from cells becoming aberrant during replication. A similar mechanism could apply to endometrial cancer. Studies on women taking hormonal contraception have found that women on hormonal birth-control are at a much lower risk of endometrial cancer. Since women on oral contraception tend to have shorter periods, this could mean that the cells within their endometrium need to replicate less often, therefore lowering the overall risk of endometrial cancer.
The risk of cervical cancer is unfortunately increased in women who have been taking hormonal contraception for a long period of time (that is for 5 years of longer). The risk goes back down after patients stop taking hormonal contraception, going back to normal after 5 or more years free of hormonal contraception. The cause behind this dramatic change is another interesting example of how confounding factors can make interpreting this type of data particularly challenging. People who are on hormonal contraception are much more likely to contract an HPV infection. HPV is a virus that is transmitted sexually and has no or very mild symptoms in the majority of people. However, it also causes cervical cancer. HPV infections are easily prevented by using barrier contraception such as condoms, but women on hormonal contraception are much less likely to rely on condom usage as well. Therefore, women on the pill are more likely to get an HPV infection and therefore to be diagnosed with cervical cancer. After they stop the pill, they either adopt barrier contraception methods or settle down in a strictly monogamous relationship, therefore dropping their risk of HPV. As time goes on, the reduced risk of HPV translates into a reduced risk of cervical cancer.
The bottom line
All of these factors fail to answer the bottom-line question “is birth-control bad for you?”. It raises the risk of breast and cervical cancer (even though it might not d so if used with condoms to reduce the risk of HPV infection), but lowers the risk of ovarian cancer. The five-year survival rate for breast and cervical cancer is respectively around 90% and 70%, while ovarian cancer is much more life threatening with a five-year survival chance of 45%. While there is no statistical evidence suggesting that protection from high-mortality cancer is worth the increased risk of low-mortality cancer, this type of idea highlights the point that hormonal contraception is a choice to be made on balance. Increasing the risk of certain types of cancer, for example, can be offset by participating in screening programs that increase the chance of detecting malignancies early. Future studies extricating the direct effect of hormones on cancer formation from confounding factors are going to help us understand more about how to manage birth-control needs with our cancer-preventing behaviors.
Check out my follow-up post here.