Breast Cancer and Abortion

Chris Kahlenborn, MD
Reproduced with permission
(chapter six)
Breast Cancer:Its link to Abortion and
the Birth Control Pill

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Does an induced abortion prior to a woman's first full–term pregnancy (FFTP) increase her risk of developing breast cancer? This central question will be addressed here, but first it would be helpful to review the history of this issue.

One of the main questions that people have when they hear or read that having an abortion at a young age increases a woman's risk of breast cancer is: Why? That is, what link could there be between abortion and cancer of the breast?

Two renowned researchers, Jose and Irma Russo, were among the first prominent scientists to offer a possible explanation of this phenomenon in their work with rats in the 1980s and l990s. The Russos took a number of rats and divided them into different groups. One of the groups were rats that had one pregnancy in the past, the second group were virgin rats, and the third group were pregnant rats who underwent an abortion at a young age. Each of these three groups was then subjected to a cancer producing agent called DMBA (7,12 dimethylbenz(a)anthracene). They found that none of the 9 rats that had a full-term pregnancy deueloped breast cancer, 15 out of 22 of the virgin rats (de, 68%) developed breast cancer, and 7 out of 9 (77.7%) of the rats that had an abortion developed breast cancer. It was further noted that when one looked microscopically at the breast tissue of each of the aforementioned groups, the full–term pregnancy group had many more mature (differentiated) breast cells than did the rats that had an abortion. "Therefore, while pregnancy and lactation protected the mammary gland from developing carcinomas and benign lesions by induction of full differentiation, pregnancy interruption did not elicit sufficient differentiation in the gland to be protective…" [1, p.497]. The correlation between the rat model and the human breast could not be ignored. The Russos continued: "In women, protection against breast cancer is provided when pregnancy occurs before age 24. In contrast, abortion is associated with increased risk of breast cancer. The explanation for these epidemiologic findings is not known, but the parallelism between the DMBA–induced rat mammary carcinoma model and the human situation is striking." [2, p.27] These words, written in 1980, led a number of researchers to study the link between induced abortion and breast cancer in women.

The next question was: What is it about an abortion at a young age that leaves the human breast especially vulnerable to carcinogens (ie, an agent that causes cancer)? That is, why would a woman's breast cells be left in a more vulnerable state if she were to have an induced abortion early in her life, instead of completing her pregnancy?

Figure 6A offers a visual explanation. Here we see that the levels of both of the sexual hormones, estradiol and progesterone, as well as hCG (human Chorionic Gonadotropin), all rise rapidly in early pregnancy. These hormones trigger the breast cells to begin dividing and maturing. The combination of these hormones, as well as others, causes the cells of the breast to divide rapidly and start a maturing process that will continue over the next 9 months, after which the breast cells will be matured or differentiated. What happens to the woman who has an abortion early in her life? Russo and Russo again provide an insight into this puzzle in their work on the rat. They found that the group of virgin rats who received the carcinogen DMBA and the pheromone hCG, developed breast cancer far less often than those that received the DMBA alone [3]. The implication is that in the human, a woman needs to complete her pregnancy in order for the breast cells to receive the full protective effect of the pheromone hCG.

Figure 6A: Hormone Levels During Pregnancy and After an Induced Abortion

The hormone levels during pregnancy and after induced abortion are depicted with curved lines in Figure 6A.

In Figure 6A we see that if a woman has an induced abortion — at about the 10th week of pregnancy (point A) — her estrogen, progesterone and HCG levels plummet to baseline levels (ie, the levels of a woman in the non–pregnant state). These hormones' among others, are critical for the full development and maturation of a woman's breast cells, especially in her first pregnancy. Once the levels drop, the process of breast cell differentiation is in a sense "frozen." This state, in which a breast cell has started to divide but has never completed the differentiation process, leaves the cell far more susceptible to becoming cancerous than breast cells which have completed their differentiation. Now one can see why a woman's first pregnancy is so important. If the pregnancy goes to term, her breast cells will have undergone the natural maturation process and be less likely to become cancerous. If the natural process in a woman's first pregnancy is interrupted via an induced abortion, those same cells are left in a vulnerable state.

What about miscarriages, also called spontaneous abortions. Do they increase the risk of breast cancer if a woman experiences one in her first pregnancy? Most authors have found that miscarriages increase the risk of breast cancer less than an induced abortion does (see Chapter 7 for details). Why is this so?

Table 6A: Comparison of Hormone Levels in Women with Healthy Pregnancy vs. Miscarriage
Hormone Level--> Progesterone (ng⁄ml) Estradiol (pg⁄ml) hCG (mLU⁄ml)
Healthy Pregnancy 25.3 766 65,717
Miscarriage 5.2 117 10,643

If we look at Table 6A [data from Witt et al, 4], we note that women who experience a healthy pregnancy (eg, women considering induced abortion) have far higher hormone levels than women who are about to miscarry. Because hormone levels drop rapidly after an abortion or miscarriage, women who choose abortion will experience a greater change in hormone levels than women who miscarry. Their breast cells essentially experience a greater "hormonal blow" (ie, a rapid drop in hormone levels) which leaves their breast cells more vulnerable to carcinogens. Thus, we would expect that women who miscarry would have less risk than women who have induced abortions, although as Chapter 7 will show, women who miscarry before their first full–term pregnancy (FFTP) still appear to be at increased risk.

Q–6A. What has the history of various research studies shown?

As early as 1957, the first research study to formally show a link between abortion and breast cancer was published by a Japanese researcher named Segi. During the years of 1953 to 1955 he and his group pooled most of the cancer patients from nearly all of Japan's major hospitals; 644 of the women had breast cancer. He noted that the women who had "an artificial interruption of pregnancy" experienced at least a 2–fold statistically significant increased risk in breast cancer rates compared to "controls" (ie, those who did not have an abortion [5, p.43]). Segi et al also noted a number of other important findings: 1) women who had an early pregnancy, 2) those who had a later age of the menarche (ie, the onset of a woman's menstrual cycles), and 3) those who had more children, all had lower rates of breast cancer than the norm. Unfortunately, this remarkable study went largely unheeded.

The next major breakthrough came with a stunning study [6] published in 1981 by M.C. Pike and B.E. Henderson et al from the University of California, entitled: "Oral contraceptive use and early abortion as risk factors for breast cancer in young women." Pike studied 163 women, all of whom were under the age of 32 when they discovered that they had breast cancer. He made two important discoveries. First, women who had a first trimester abortion before their first full–term pregnancy (FFTP) had a 2.4–fold significantly increased risk of breast cancer compared to "controls." Second, the risk from early oral contraceptive use — specifically, the use of OCPs for at least 4 years before a FFTP — resulted in a 2.25–fold statistically significant risk (125% increased risk). This result sparked the medical world's continued study of both of these findings until the current time.

Since 1981 a number of studies from around the world, have continued to show that having an abortion at a young age is a significant risk factor for breast cancer [7, 8, 9, 10, 11, 12]. A major development occurred in late 1994, after Janet Daling et al [9] revealed the results of a study which commanded the attention of both the medical and the lay realms. In her study of 845 women who had breast cancer and were under the age of 45, she noted a number of significant findings: 1) In general, women who had an abortion experienced a 50% increased risk of breast cancer; 2) Women who had an abortion before the age of 18 had a 150% increased risk of developing breast cancer; 3) Women who were over the age of 30 at the time of their abortion had a 110% increased risk; and 4) Women who were less than the age of 18 at the time of their abortion and had a later abortion (ie, after 8 weeks of pregnancy) experienced an 800% increased risk of breast cancer! In addition, Daling noted that women who had an abortion at a young age (before the age of 18) and who also had a positive family history for breast cancer, were at infinitely increased risk compared to young women who had no abortion and a positive family history for breast cancer (confidence intervals 1.8 to infinity)! Daling was paraphrased by another writer in the Journal of the National Cancer Institute as saying: "Daring indicated that abortion should be included as a risk factor if the cohort — women who had abortions prior to the age of 18 — is specified" [13].

These stunning findings led to an abrupt criticism of Daling's study. In fact, the very same issue of the journal which published the study — The Journal of the National Cancer Institute — printed an editorial by Lynn Rosenberg which severely criticized the study. Rosenberg argued that Daling's study might be limited by recall bias, as was discussed in Chapter 5. This was a remarkable claim, because Daling et al had taken great pains to address this issue, even going so far as performing a side study comparing abortion and cervical cancer, which would have specifically identified reporting bias [9, p.1590]. Rosenberg also made the bizarre claim that "While the findings of Daling et al add to the limited evidence that induced abortion increases the risk of breast cancer, neither a coherent body of knowledge nor a convincing biologic mechanism has been established." She wrote this comment despite the work by Russo and Russo, done more than a decade earlier, which implied a clear biological mechanism.

Finally, in the Fall of 1996, a group of researchers led by Dr. Joel Brind published the most comprehensive and meticulous meta–analysis of all the abortion⁄breast cancer related studies performed until that time [14]. In their analysis, they reviewed 61 different studies ranging between the years 1957 to 1996. From those 61, they selected out the studies which did not adequately distinguish between induced and spontaneous abortion, leaving 28 studies, which were further reduced to 23 studies after combining the results of the studies that overlapped. He and his group also reviewed studies from around the world, including Japan, Russia, Yugoslavia, France, Denmark, Sweden, Norway, Greece, and many others including the U.S.

The paper by Brind and colleagues provided an extensive commentary on many of the previous studies and included a thorough historical review of the entire field. One conclusion stood out clearly:

Women who had an induced abortion had a 30% increased risk of developing breast cancer, whereas parous women who had an induced abortion prior to their FFTP had a 50% increased risk of developing breast cancer, compared to women who did not have an induced abortion. The meta–analysis led to a flurry of discussion but left few able to argue with its methods or results. Janet Daling, an epidemiologist at the Fred Hutchinson Cancer Research Center in Seattle, defended Dr. grind's paper as "very objective and statistically beyond reproach. (The authors did) a fair job of compiling the data, having taken pains to include studies of every point of view." [15]

What did Dr. grind's paper state regarding the major studies which examined the effect of an early induced abortion and the consequent risk of developing breast cancer? The studies are noted in Table 6B:

Table 6B:
Findings of Studies Which Examined Breast Cancer Risk From Induced Abortion Prior to a Woman's First Full-Term Pregnancy (FFTP)
Researchers Percent Change Year Country Relative Risk
Brinton [8] 34% increase* 1983 USA 1.34 (0.3–5.6)
Rosenberg [12] 10% decrease* 1988 USA 0.9 (0.5–1.4)
Harris [10] 18% decrease* 1989–90 Sweden⁄Norway 0.82 (0.44–1.51)
Daling [9] 40% increase* 1944 USA 1.4 (1.0–2.0)
Lipworth [11] 106% increase* 1995 Netherlands 2.06 (1.45–2.90)
Rookus [16] 160% increase* 1996 USA 2.6 (1.0-6.8)
Totals 50% increase     1.5 (1.2–1.8)

* This result reflects a trend toward an increased or decreased risk but does not attain statistical significance.

Table 6B, whose data comes from Table 2 in the Brind meta–analysis, clearly shows that when data from all the major studies from around the world were pooled and conservatively analyzed via a meta–analysis, it indicated that women had a 50% increased risk of developing breast cancer from an abortion performed before a woman's first fullterm pregnancy. Why does this author refer to it as a "conservative" estimate? Because, a closer look at each of the individual studies would have yielded an even higher risk, had they adjusted for the many factors which could affect the results. A comprehensive review of those factors is presented at the end of this chapter but two of them stand out.

Lynn Rosenberg's 1988 study suffers profusely from the "age factor" problem. Ms. Rosenberg compares "cases" with a mean age of 52 years old to "controls" with a mean age of 40 years old. As noted in Chapter 5, this is a serious error and one that results in a lower reported risk from abortion at a young age than would have been, had she designed the study properly.

The second example comes from the Daling study in 1994. Although it was one of the best–designed studies, it still had a huge "stack effect." The researchers ended up "stacking" the younger spectrum of the data. They had 8% of the "cases" in the 21 to 30 year–old age group, whereas 17% of the "controls" were in this age group. If the study had distributed the "case" and "control" populations proportionally among the various age groups (ie, have the same percent of each population in each of the various age brackets), the "stack effect" would have been averted, most likely yielding a higher relative risk from abortion at a young age, because young women who are disproportionately represented in the "control group" will most likely report having had more abortions prior to their FFTP, given that the percentage of young women who have had abortions in their early reproductive years has increased dramatically since 1973.

In short, almost every study noted in Table 6B has a factor or factors (these factors are all reviewed in Chapter 5) which would have served to increase the relative risk markedly, had they been accounted for. These include: 1) the "death factor" [8, 9;], 2) the "stack effect" and⁄or ⁄age mismatching⁄ [9, 12], 3) too short of a latent period [8, 12], and⁄or 4) a financial bias from a sponsor of one of the studies [10, 12]. What does all this mean? The short answer, is that the 50% increased risk which Dr. Brind et al reported from induced abortion prior to a FFTP is a very conservative estimate. Had the above–mentioned factors been properly adjusted for, the real risk from an abortion before a FFTP would almost certainly have been significantly higher. It should again be emphasized that many of the studies that evaluated the risk of an abortion in young women took their data from the late 1970s or early 1980s. We noted earlier that the latent period for risk factors in breast cancer (eg, DES or radiation) could be 20 to 30 years or longer. This implies that the full impact of having an abortion at a young age upon a woman's risk of developing breast cancer may only be fully appreciated in studies from the late l990s and the first decade of the next century.


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