TITLE: Induced abortion and the risk of breast cancer [see comments]

AUTHORS: Melbye M; Wohlfahrt J; Olsen JH; Frisch M; Westergaard T; Helweg-Larsen K; Andersen PK

AUTHOR AFFILIATION: Department of Epidemiology Research, Danish Epidemiology Science Center, Statens Serum Institut, Copenhagen, Denmark. SOURCE: N Engl J Med 1997 Jan 9;336(2):81-5

COMMENT: Comment in: N Engl J Med 1997 Jan 9;336(2):127-8
Comment in: N Engl J Med 1997 Jun 19;336(25):1834; discussion 1835
Comment in: ACP J Club 1997 Jul-Aug;127(1):18 

NOTE FROM THE COALITION: The 1997 Melbye study, which is also known as the Danish study, received a considerable amount of press coverage because the study concluded that there was no significant link between abortion and breast cancer. Journalists were impressed with the study's size: 1.5 million women. However, this study has been severely criticized for its misclassification and data adjustment errors. For example, the researchers followed non-abortive women more than twice as long as the women who did have elective abortions. They compared a smaller group of younger women (280,691) who had had abortions to a far larger number of older women (1,248,541) who hadn't procured abortions. The difficulty with this practice is obvious. Women who live longer have a greater chance of developing breast cancer. (See Dr. Joel Brind's web site, "Rotten in Denmark," www.abortioncancer.com.)

Moreover, even this study reported that "[w]ith each one-week increase in the gestational age of the fetus . . . there was a 3 percent increase in the risk of breast cancer." The relative risk was found to increase from 0.81 due to induced abortion at under seven weeks of gestation to a statistically significant 1.89 after eighteen weeks of gestation. The Melbye researchers noted that "The increased risk among women who had had second-trimester abortions finds biologic support in experiments in rats and is in line with the hypothesis of Russo and Russo."

Dr. Brind and his colleagues believe that this study actually masked a statistically significant risk increase. It is interesting that the Melbye researchers corrected some of these errors in a 1999 study, but declined to admit that they were doing so and did not provide a reanalysis of their induced abortion data from the earlier study. [Joel Brind and Vernon Chinchilli, Letter, "Induced Abortion and Risk of Breast Cancer," II Epidemiology 234-235 (2000)]. The Brind team intends to do this job for the Melbye researchers.

The Melbye team reported in their 1999 study that premature birth before 32 weeks gestation more than doubles the breast cancer risk which the woman would have had had the pregnancy continued to full term.

Interestingly, the New England Journal of Medicine which published the 1997 study acknowledged abortion as a risk factor less consistently associated with breast cancer in the text of an article discussing risk factors for breast cancer written by Katrina Armstrong in February, 2000. However, the NEJM's acknowledgment understated the increased risk. [Armstrong (2000) NEJM 342:564-71].

ABSTRACT:

BACKGROUND: It has been hypothesized that an interrupted pregnancy might increase a woman's risk of breast cancer because breast cells could proliferate without the later protective effect of differentiation. METHODS: We established a population-based cohort with information on parity and vital status consisting of all Danish women born from April 1, 1935, through March 31, 1978. Through linkage with the National Registry of Induced Abortions, information on the number and dates of induced abortions among those women was combined with information on the gestational age of each aborted fetus. All new cases of breast cancer were identified through linkage with the Danish Cancer Registry.

RESULTS: In the cohort of 1.5 million women (28.5 million person-years), we identified 370,715 induced abortions among 280,965 women (2.7 million person-years) and 10,246 women with breast cancer. After adjustment for known risk factors, induced abortion was not associated with an increased risk of breast cancer (relative risk, 1.00; 95 percent confidence interval, 0.94 to 1.06). No increases in risk were found in subgroups defined according to age at abortion, parity, time since abortion, or age at diagnosis of breast cancer. The relative risk of breast cancer increased with increasing gestational age of the fetus at the time of the most recent induced abortion: <7 weeks, 0.81 (95 percent confidence interval, 0.58 to 1.13); 7 to 8 weeks, 1.01 (0.89 to 1.14); 9 to 10 weeks, 1.00 >12 weeks, 1.38 (1.00 to 1.90) (reference category, 9 to 10 weeks).

CONCLUSIONS: Induced abortions have no overall effect on the risk of breast cancer.