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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Obstet Gynecol. Author manuscript; available in PMC 2013 October 7.
Published in final edited form as:
PMCID: PMC3791619
NIHMSID: NIHMS181029

Ovarian Conservation at the Time of Hysterectomy and Long-Term Health Outcomes in the Nurses’ Health Study

William H. Parker, MD,1 Michael S. Broder, MD, MPH,2,3 Eunice Chang, PhD.,3 Diane Feskanich, ScD,4 Cindy Farquhar, MD,5 Zhimae Liu, PhD.,6 Donna Shoupe, MD,7 Jonathan S. Berek, MD, MMS,8 Susan Hankinson, ScD,4,9 and JoAnn E. Manson, MD, DrPH4,9,10

Abstract

Objective

To report long-term health outcomes and mortality after oophorectomy or ovarian conservation.

Methods

We conducted a prospective, observational study of 29,380 women participants of the Nurses’ Health Study who had a hysterectomy for benign disease; 16,345 (55.6%) had hysterectomy with bilateral oophorectomy and 13,035 (44.4%) had hysterectomy with ovarian conservation. We evaluated incident events or death due to coronary heart disease (CHD), stroke, breast cancer, ovarian cancer, lung cancer, colorectal cancer, total cancers, hip fracture, pulmonary embolus, and death from all causes.

Results

Over 24 years of follow-up, for women with hysterectomy and bilateral oophorectomy, compared with ovarian conservation, the multivariable hazard ratios (HR) were 1.12 (95% CI 1.03, 1.21) for total mortality, 1.17 (95% CI 1.02, 1.35) for fatal plus nonfatal CHD, and 1.14 (95% CI 0.98, 1.33) for stroke. Although the risks of breast (HR 0.75 95% CI 0.68, 0.84), ovarian (HR 0.04 95% CI 0.01, 0.09, NNT = 220), and total cancers (HR 0.92 95% CI 0.86, 0.98) decreased after oophorectomy, lung cancer incidence (HR =1.26, 95% CI 1.02, 1.56, NNH = 190) and total cancer mortality (HR=1.17, 95% CI 1.04, 1.32) increased. For never-users of estrogen therapy, bilateral oophorectomy before age 50 was associated with an increased risk of all-cause mortality, CHD, and stroke. With an approximate 35-year life span following surgery, one additional death would be expected for every 9 oophorectomies performed.

Conclusions

Compared with ovarian conservation, bilateral oophorectomy at the time of hysterectomy for benign disease is associated with a decreased risk of breast and ovarian cancer, but an increased risk of all-cause mortality, fatal and non-fatal coronary heart disease, and lung cancer. In no analysis or age-group was oophorectomy associated with increased survival.

Introduction

Bilateral oophorectomy at the time of hysterectomy for benign disease is commonly practiced in order to prevent subsequent development of ovarian cancer.(1) Data from the CDC show that for women having a hysterectomy between ages 40-44, 50% have concurrent oophorectomy and between ages 45-64, 78% have oophorectomy.(2) In all, approximately 300,000 U.S. women have a prophylactic oophorectomy every year.

Oophorectomy before menopause leads to an abrupt reduction in endogenous estrogen and androgen production.(3) Postmenopausal ovaries continue to produce significant amounts of testosterone and androstenedione which are converted to estrogen peripherally.(4,5) Later age of menopause has been associated with a reduced risk of death from coronary heart disease and stroke and studies show that preserving ovarian function is associated with a lower risk of coronary heart disease.(6,7,8,9) Among US women, ovarian cancer accounts for 14,700 deaths per year, whereas coronary heart disease accounts for 326,900 deaths and stroke for approximately 86,900 deaths each year.(10)

Ovarian conservation, therefore, might benefit overall survival in women not at high risk for ovarian cancer.(11) The objective of this study was to report long-term health outcomes and mortality after ovarian conservation or oophorectomy.

Materials and Methods

Study Population

We used the database from the Nurses’ Health Study (NHS) cohort, which included 122,700 married registered nurses who were ages 30 to 55 in 1976 when the initial questionnaires were mailed. Race was self-reported and the cohort was 94% white, 2% African-American, 1% Asian, 1% multi-racial and 2% other. The cohort was relatively homogeneous with regard to education, socioeconomic status and access to health-care. (12) Additional questionnaires, updating risk-factors and newly diagnosed health conditions have been sent every two years with response rates of approximately 90% for each cycle. In this cohort, a validation study found self-reported oophorectomy at the time of hysterectomy to be very accurate when compared with medical records.(13) NHS participants with a previous hysterectomy entered study follow-up in 1980. Others entered when they reported having a hysterectomy on the 1982 through 2002 questionnaires.

All eligible NHS participants were initially included prior to application of exclusion criteria. Through 2002, 50,432 NHS participants reported having a hysterectomy without a diagnosis of gynecologic cancer. Women were excluded from this study if they had unilateral or partial oophorectomy (n=4,817), unknown ovarian status at the time of hysterectomy (n=2,559), a prior history of an outcome of interest as described below (n=8,525) or an oophorectomy (n=465) prior to their hysterectomy, or an unknown age at hysterectomy (n=4,643). Women with missing information on past oral contraceptive use were excluded due to the small number in this category (n=43). The remaining 29,380 women were included in the analysis; 16,345 (55.6%) had a hysterectomy with bilateral oophorectomy and 13,035 (44.4%) had hysterectomy with ovarian conservation. Submission of a completed self-administered questionnaire was deemed to imply informed consent. The institutional review boards at John Wayne Cancer Institute at Saint John’s Health Center in Los Angeles and Brigham and Women’s Hospital in Boston approved this study.

Study Variables

Outcomes of Interest

We focused on incident events and death due to the following conditions: coronary heart disease (ICD-8:410—414), stroke (ICD-8:430—438), breast cancer (ICD-8:174), epithelial ovarian cancer (ICD-8:183), lung cancer (ICD-8:162), colorectal cancer, (ICD-8:153, 154), hip fracture (ICD-9:820.3 ), pulmonary embolus (ICD-8:450) and death due to all-causes. Hip fracture was confirmed by self-report alone; ovarian cancer was confirmed by medical record review, and all other events were confirmed either by medical record or by the participant in writing or by telephone interview.(13) If a diagnosis could not be confirmed or rejected, the event was excluded and the follow-up period was censored thereafter. Cause of death was determined using death certificates, autopsy reports and medical records. Mortality follow-up using the National Death Index and next of kin was more than 98% complete.(14)

Other Variables

Participant’s age in months and biennial questionnaire cycle were used as stratification variables in the Cox proportional hazards models. For each outcome analysis, we adjusted for related risk factors: age, age at hysterectomy, diabetes, high blood pressure, hypercholesterolemia, family history of MI before age 60, tubal ligation, family history of breast cancer, family history of ovarian cancer, body mass index (BMI), smoking status, use of estrogen therapy (ET), duration of oral contraceptive (OC) use, alcohol consumption, physical activity and ASA use (Table 1). Alcohol consumption, physical activity, and use of ASA were initially queried in 1980. All data were updated at biennial questionnaire cycles. Family history of ovarian cancer (mother or sister) was first asked in 1992 and, once reported, was not updated. For all variables, missing information was separately noted.

Table 1
Baseline* characteristics of the study population by oophorectomy status at hysterectomy

Statistical Analysis

Women contributed person-time from the return of the 1980 questionnaire or the questionnaire on which they first reported having a hysterectomy until report of an outcome of interest, oophorectomy subsequent to hysterectomy, death, or end of follow-up on June 1, 2004. In analyses of incident events, women were censored only upon report of the event under analysis, therefore the numbers of person-years varied for each outcome. We calculated incidence rates by dividing the number of incident cases by the total number of person-years for simple hysterectomy or hysterectomy with bilateral oophorectomy. For multivariable analyses, we used Cox proportional hazards models to estimate relative risk (RR) and corresponding 95% confidence intervals (95% CI). Age and questionnaire cycle were stratifying variables in the analyses and were controlled for multiple potential confounders, as described in Table 1 and listed in the footnotes of each table.

The study design stratified the cohort into three sub-cohorts based on age at hysterectomy: <45, 45-54 and ≥55 and we conducted modeling separately for each. In a secondary analysis of oophorectomy status among never-users of estrogen therapy, women were stratified into two age groups (<50, ≥50) to gain statistical power and all analyses were repeated. All data transformations and statistical analyses were performed using SAS© version 9.1. (SAS Institute, Cary, NC) All P values were based on two-sided tests with significance of 0.05.

RESULTS

Women with ovarian conservation and those with bilateral oophorectomy had similar baseline distributions of risk factors for cardiovascular disease and cancer, but the latter were slightly older and more likely to be current or past users of hormone therapy (Table 1). After adjustment for multiple relevant risk-factors, we compared the two groups in relation to the incidence of fatal and non-fatal events during 24 years of follow-up (Table 2 – cancer events, Table 3 – non-cancer events, Table 4 - deaths). Oophorectomy was associated with an increased risk of coronary heart disease; this increase was statistically significant for all women (HR 1.17 95% CI 1.02, 1.35), and for women having oophorectomy before age 45 (HR 1.26 95% CI 1.04, 1.54). Breast cancer was less frequent among all women having oophorectomy (HR 0.75 95% CI 0.68, 0.84), and the risk was lower among women having oophorectomy before the age of 45 (HR 0.62 95% CI 0.53, 0.74). Oophorectomy was associated with a markedly reduced risk of ovarian cancer (HR 0.04; 95% CI, 0.01-0.09), an increased risk of lung cancer (HR 1.26; 95% CI, 1.02-1.56), and a reduction in total cancers (HR 0.92 95% CI 0.86, 0.98). Risks of stroke, hip fracture, colorectal cancer and pulmonary embolism did not differ significantly between groups. We documented 3,197 deaths from any cause; 350 women (10.9%) died from coronary heart disease, 219 (6.9%) from stroke, 230 (7.2%) from breast cancer, 37 (1.2%) from ovarian cancer, 336 (10.5%) from lung cancer, 118 (3.7%) from colorectal cancer, none due to hip fracture, 12 (0.4%) from pulmonary embolism and 1895 (59.3%) from other causes.

Table 2
Risk of incident events (cancers) by oophorectomy status at time of hysterectomy
Table 3
Risk of incident events (non-cancers) by oophorectomy status at time of hysterectomy
Table 4
Risk of cause-specific and all-cause death by oophorectomy status at time of hysterectomy

Among women having a simple hysterectomy, 1242 died (527 per 100,000 person-years) and among women having a hysterectomy with bilateral oophorectomy 1955 died (648 per 100,000 person-years). In multivariable analysis, oophorectomy increased the risk of death from any cause (HR 1.12 95% CI 1.03, 1.21). For every 24 women having bilateral oophorectomy, at least one women will die prematurely from any cause as a result of the oophorectomy. Analysis of cause-specific mortality found an increased risk of death from CHD (HR 1.28 95% CI 1.00, 1.64), lung cancer ( HR 1.31 95% CI 1.02, 1.68), and all cancers ( HR 1.17, 95% CI 1.04, 1.32), a reduced risk of death from ovarian cancer ( HR 0.06; 95% CI, 0.02-0.21), and no overall difference in deaths from stroke, breast cancer, or colorectal cancer. For every 130 women having bilateral oophorectomy, one extra death from CHD will occur as a result of the oophorectomy. Analysis of death from pulmonary embolism was precluded by the small numbers of deaths.

Secondary Analyses

We performed an analysis of the 10,094 women who had either bilateral oophorectomy or ovarian conservation and had never used estrogen therapy (ET), stratified by age at hysterectomy <50 and ≥50 years. Never-users of ET who had oophorectomy before age 50 had a higher risk of incident coronary heart disease (HR 1.98 95% CI 1.18, 3.32). Oophorectomy was associated with a significantly increased risk of stroke for all women (HR 1.85 95% CI 1.09, 3.16) and for women younger than 50 at the time of surgery (HR 2.19 95% CI 1.16, 4.14). Oophorectomy was associated with an increased the risk of lung cancer (HR 2.09 95% 1.01, 4.33). The risk of all-cause death was significantly higher among women younger than 50 at the time of surgery (HR 1.40 95% CI 1.01, 1.96). The risks of breast cancer, colorectal cancer, total cancer, hip fracture and pulmonary embolus were no different among women who had never used ET.

DISCUSSION

This large prospective study of women having a hysterectomy for benign disease indicates that concurrent bilateral oophorectomy, after adjustment for multiple independent risk factors, is associated with a higher risk of all-cause mortality, primarily from coronary heart disease and lung cancer, when compared with ovarian conservation. Furthermore, prophylactic oophorectomy did not improve survival at any age. During 24 years of follow-up, among 13,305 women who had ovarian conservation, 34 (0.26%) died from ovarian cancer. We did not find increased risks for colorectal cancer, pulmonary embolus or hip fracture in any analysis. Whereas breast cancer, ovarian cancer and all cancers were less frequent, the overall risk of death from cancer was greater among women having oophorectomy. The basis for this paradox is unclear and warrants further study. In a secondary analysis of women who never used estrogen therapy, oophorectomy was associated with an increased risk for incident stroke and lung cancer, and oophorectomy before age 50 was associated with an increased risk of fatal plus nonfatal coronary heart disease, stroke, and deaths from all causes. Total cancer risk was neither increased nor decreased among women with oophorectomy who had never used ET.

Our study has several strengths. This is the largest prospective study, with the longest follow-up, to examine the effect of oophorectomy on health outcomes in women. Although our study is observational, the NHS cohort is particularly homogenous relative to a study in the general population, with regards to educational and socioeconomic factors that may possibly confound non-randomized studies. In order to reduce the possibility of confounding due to the indication for surgery, women with any prior diagnosis of cancer or prior unilateral oophorectomy were excluded from our analysis. In order to reduce the possibility of confounding due to the family history, our main analysis was adjusted for both family history of breast or ovarian cancer. We also performed a subset analysis that excluded women with a family history of ovarian cancer (approximately 4.5% of study subjects) and found results similar to those presented in our manuscript (data not shown).

Many previous studies were small or did not adjust for known risk factors for cardiovascular disease.(6,15,16) Our study included 29,380 women who had hysterectomies, nearly equally divided between bilateral oophorectomy and ovarian conservation. Although baseline characteristics differed somewhat between groups, we used multivariable analysis to correct for multiple known risk-factors associated with all the conditions of interest. Follow-up over the 24 years was high for reported incident diagnoses and updated information on risk factors, and identification of deaths is about 98% complete.

Several limitations of our study deserve comment. The study was observational and oophorectomy or ovarian conservation was self-selected. Despite the biologic plausibility of many of our results and despite accounting for multiple risk factors, it is possible that our findings could be related to the underlying indication for which participants chose oophorectomy or due to uncorrected differences between the groups. Most of the women in this study were white and the results may not be applicable to non-white women.

Our results for cardiovascular disease are biologically plausible and supported by experimental evidence. Reduction in endogenous estrogen increases serum lipids, reduces carotid artery blood flow, and increases sub-clinical atherosclerosis as measured by carotid artery intima-media thickness.(17,18,19)

Our results are consistent with other studies. A decision analysis found that ovarian conservation improved survival for women younger than 65 at the time of surgery.(20) A cohort study of 1,097 women who underwent hysterectomy and bilateral oophorectomy for benign disease who were matched by age to 2,390 woman choosing ovarian conservation found mortality to be higher in women who had prophylactic bilateral oophorectomy before the age of 45 years.(16)

Earlier age of surgical or natural menopause correlates with increased risk of cardiovascular events.(15, 21, 22) Previous reports from the Nurses’ Health Study found that women with oophorectomy between ages 40-44, compared with women with intact ovaries, had double the risk of myocardial infarction (RR 2.2 95% CI 1.2, 4.2).(7) Oophorectomy after age 50 increased the risk of developing a first MI compared to controls (RR 1.4, 95% CI 1.0-2.0).(8) When adjusted for age, death from stroke was reduced 6% per year of delayed menopause (RR 0.94 95%CI 0.89, 1.00).(6) A meta-analysis of observational studies found that oophorectomy doubled the risk of cardiovascular disease (RR 2.62 95% CI 2.05, 3.35).(9) In that cardiovascular disease is the main cause of death among US women, any increased risk would be expected to increase overall morbidity and mortality, as found in our study.

Ovarian cancer is a low-prevalence disease, and simple hysterectomy may reduce the risk of ovarian cancer. Suggested mechanisms include interruption of transport of potential carcinogens through the reproductive tract, alteration in hormone levels or induction of protective anti-MUC1 antibodies. (23, 24, 25) Our analysis found a decreased risk of breast cancer among women following oophorectomy. Women with oophorectomy before age 50 have been shown to have a 50% decreased risk of breast cancer which persisted for 10 years following surgery.(26)

We found the increased risk of dying of other cancers exceeded the risk of dying from ovarian cancer (low incidence) and breast cancer (high long-term survival rate) among women having an oophorectomy. The association of oophorectomy with lung cancer was unexpected and warrants further study.

While postmenopausal estrogen therapy may reduce some of the increased risks we found, following publication of the WHI results many women discontinued hormone therapy and 77% fewer women now start hormones at the time of menopause.(27) Likewise, continuation rates for medications that can reduce the risk of cardiovascular disease, such as statins, are as low as 18% for women after one year. (28)

Our findings provide evidence that, for women not at high risk for ovarian cancer, oophorectomy may adversely affect long-term health outcomes and mortality and at no age was oophorectomy associated with a survival benefit. Preventive surgery should not be performed if it does not clearly benefit the patient. Therefore, prophylactic oophorectomy, with the goal of improving survival by reducing ovarian cancer, appears not to be supported by our study. Given that approximately 300,000 U.S. women a year undergo elective oophorectomy, these findings have important public health implications.

Table 5
Risk of incident events and all-cause death by oophorectomy status at time of hysterectomy among never-users of estrogen therapy

Acknowledgements

The authors thank the women in The Nurses’ Health Study for their continuing contributions to the understanding of long-term health outcomes, and Dr. Shelley Tworoger, PhD and Dr. Bernard Rosner, PhD, for providing their advice on the study design and statistical analyses without compensation.

Funding: Funded by grants from Ethicon Women’s Health and Partnership for Health Analytic Research.

Footnotes

Financial Disclosure: Dr. Parker has been a consultant to Ethicon Women’s Health (Cincinatti, OH). Dr. Broder is president of Partnership for Health Analytic Research (Los Angeles, CA). Dr. Chang is an employee of Partnership for Health Analytic Research. Dr. Farquhar has received consulting fees from the World Health Organization (Geneva, Switzerland). Dr. Shoupe has received a research grant from the NIH National Center of Complementary and Alternative Medicine (Gaithersburg, MD). Drs. Liu, Manson, Feskanich, Berek, and Manson did not report any potential conflicts of interest.

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References

(1) American College of Obstetricians and Gynecologists Jan, 2008. Practice Bulletin number 89.
(2) Healthcare Cost and Utilization Project (HCUP), 1988-2001 A Federal-State-Industry Partnership in Health Data. Agency for Healthcare Research and Quality; Rockville, MD: Jul, 2003. http://www.cdc.gov/mmwr/preview/mmwrhtml/ss5105a1.htm.
(3) Judd H, Judd G, Lucas W, Yen S. Endocrine function of the postmentopausal ovary: concentration of androgens and estrogens in ovarian and peripheral vein blood. J Clin Endocrinl Metab. 1974;39:1020–1024. [PubMed]
(4) Fogle R, Stanczyk F, Zhang X, Paulson R. Ovarian androgen production in postmenopausal women. J Clin Endocrinol Metab. 2007;92:3040–3. [PubMed]
(5) Judd H, Lucas W, Yen S. Effect of oophorectomy on circulating testosterone and androsterone levels in patients with endometrial cancer. Am J Obstet Gynecol. 1974;118:793–798. [PubMed]
(6) Ossewaarde M, Bots M, Verbeek A, et al. Age at menopause, cause-specific mortality and total life expectancy. Epidemiology. 2005;16:556–562. [PubMed]
(7) Colditz G, Willett W, Stampfer M, Rosner B, Speizer F, Hennekens C. Menopause and the risk of coronary heart disease in women. N Engl J Med. 1987;316:1105–10. [PubMed]
(8) Falkeborn M, Schairer C, Naessen T, Persson I. Risk of myocardial infarction after oophorectomy and hysterectomy. J Clin Epidemiol. 2000;53:832–7. [PubMed]
(9) Atsma F, Bartelink M, Grobbee D, van der Schouw Y. Postmenopausal status and early menopause as independent risk factors for cardiovascular disease: a meta-analysis. Menopause. 2006;13:265–279. [PubMed]
(10) Kung H, Hoyert D, Xu J, Murphy S. [Retrieved May 10, 2008];Deaths: Final Data for 2005, April 24, 2008, National Vital Statistics Summary Report. Available at: http://www.cdc.gov/nchs/data/nvsr/nvsr56/nvsr56_10.pdf.
(11) Armstrong K, Schwartz J, Randall T, Rubin S, Weber B. Hormone replacement therapy and life expectancy after prophylactic oophorectomy in women with BRCA1/2 mutations: a decision analysis. J Clin Oncol. 2004;22:1045–54. [PubMed]
(12) Colditz G, Manson J, Hankinson S. The Nurses’ Health Study: 20 year contribution to the understanding of health among women. J Women’s Health. 1997;6:49–62. [PubMed]
(13) Colditz G, Stampfer M, Willett W, Stason W, Rosner B, Hennekens C, Speizer F. Reproducibility and validity of self-reported menopausal status in a prospective cohort study. Am J Epidemiol. 1987;126:319–25. [PubMed]
(14) Stampfer MJ, Willett WC, Speizer FE, et al. Test of the National Death Index. Am J Epidemiol. 1984;119:837–839. [PubMed]
(15) Van der Schouw Y, van der Graa Y, Steyerberg E, Eijkemans M, Banga J. Age at menopause as a risk factor for cardiovascular mortality. Lancet. 1996;347:714–18. [PubMed]
(16) Rocca W, Grossardt B, de Andrade M, Malkasian G, Melton J. Survival patterns after oophorectomy in premenopausal women: a population-based cohort study. Lancet Oncol. 2006 Oct;7(10):821–8. [PubMed]
(17) Cheung L, Pang M, Lam C, Tomlinson B, Chung T, Haines C. Acute effects of a surgical menopause on serum concentrations of lipoprotein(a) Climacteric. 1998;1:33–41. [PubMed]
(18) Mihmanli V, Mihmanli I, Kantarci F, Aydin T, Yilmaz M, Ogut G. Carotid pulsatility indices in surgical menopause. Arch Gynecol Obstet. 2002;266:96–100. [PubMed]
(19) Hodis H, Mack W. Atherosclerosis imaging methods: assessing cardiovascular disease and evaluating the role of estrogen in the prevention of atherosclerosis. Am J Cardiol. 2002;89:19E–27E. [PubMed]
(20) Parker WH, Broder MS, Liu Z, Shoupe D, Farquhar C, Berek JS. Ovarian conservation at the time of hysterectomy for benign disease. Obstet Gynecol. 2005;106:219–26. [PubMed]
(21) Løkkegaarda E, Jovanovicb Z, Heitmannc B, Keidingb N, Ottesend B, Pedersend A. The association between early menopause and risk of ischaemic heart disease: influence of hormone therapy. Maturitas. 2006;53:226–233. [PubMed]
(22) de Kleijn M, van der Schouw Y, Verbeek A, Peeters P, Banga J, van der Graaf Y. Endogenous Estrogen Exposure and Cardiovascular Mortality Risk in Postmenopausal Women. Am J of Epidemiology. 2002;155:339–345. [PubMed]
(23) Hankinson S, Hunter D, Colditz G, Willett W, Stampfer M, Rosner B, et al. Tubal ligation, hysterectomy, and risk of ovarian cancer. A prospective study. JAMA. 1993;270:2813–8. [PubMed]
(24) Cramer DW, Welch WR, Berkowitz RS, Godleski JJ. Presence of talc in pelvic lymph nodes of a woman with ovarian cancer and long-term genital exposure to cosmetic talc. Obstet Gynecol. 2007;110:498–501. [PubMed]
(25) Cramer D, Titus-Ernstoff L, McKolanis J, Welch W, Vitonis A, Berkowitz R, Finn O. Conditions Associated with Antibodies Against the Tumor-Associated Antigen MUC1 and Their Relationship to Risk for Ovarian Cancer. Cancer Epidemiol Biomarkers Prev. 2005;14:1125–1131. [PubMed]
(26) Schairer C, Persson I, Falkeborn M, Naessen T, Troisi R, Brinton L. Breast cancer risk associated with gynecologic surgery and indications for such surgery. Int J Cancer. 1997;70:150–4. [PubMed]
(27) Wegienka G, Havstad S, Kelsey J. Menopausal hormone therapy in a health maintenance organization before and after women’s health initiative hormone trials termination. J Womens Health. 2006;15:369–78. [PubMed]
(28) Huser MA, Evans TS, Berger V. Medication adherence trends with statins. Adv Ther. 2005;22:163–71. [PubMed]