***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
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    Input: Time-related effects of a pregnancy on breast cancer risk were examined in a population -based prospect i ve study of 802,457 parous Norwegian women aged 20 - 56 years . The mean follow-up time was 16.4 years . A total of 4787 women were diagnosed with breast cancer . We observed a short-term increase in risk of breast cancer after a full-term pregnancy , with a maximum 3 - 4 years after delivery , followed by a long-lasting decrease in risk . The maximum risk was about twice the risk for women whose last delivery was 20 or more years previously ( incidence rate ratio = 1.99 , 95 % confidence interval = 1.70 - 2.33 ) . Compared with nulliparous women , those with one or two children were at higher risk in the first decade after the last pregnancy , whereas those with three or more children were at lower risk in most categories of time since the last birth . The positive association between breast cancer risk and age at last birth was markedly reduced after adjustment for time since last birth . We conclude that there is a non-linear relationship between breast cancer incidence and time since last birth . Part of the relation with age at last birth may be attributed to the association with time since last birth Results from case-control studies suggest that induced abortion may be associated with a small increase in risk of breast cancer . While risk estimates from cohort studies have generally not observed such an association , these studies have had limited information regarding abortion and possible confounding variables . Therefore , we conducted a study among a cohort of post-menopausal women from whom detailed information regarding pregnancy outcomes as well as risk factors for breast cancer had been collected . The study sample included 37,247 Iowa Women 's Health Study participants , 55 - 64 years of age at baseline in 1986 , who reported no history of breast , or other , cancer ( except non-melanoma skin cancer ) , and for whom information regarding pregnancy outcomes ( that is , live birth , stillbirth , spontaneous abortion , ectopic pregnancy or induced abortion ) was available . We used linkage with records of the State Health Registry of Iowa , part of the National Cancer Institute 's Surveillance , Epidemiology , and End Results ( SEER ) program , to estimate the incidence of breast cancer among cohort members through 1995 . We calculated age-adjusted relative risks and 95 % confidence intervals using Cox proportional hazards regression . Only 653 women ( 1.8 % ) reported an induced abortion . The age-adjusted relative risk of breast cancer among women with prior induced abortion compared with those without was 1.1 ( 95 % CI = 0.8 - 1.6 ) . Relative risks were higher among women whose age at first abortion was less than 20 or at least 30 years , for those whose abortion took place after their first birth or who never gave birth , and for those with early termination ( 0 - 2 months ) . These estimates varied from 1.3 - 1.7 , but the confidence intervals around each were wide . Since most women in this cohort were beyond their reproductive years when abortion became legal in 1973 , the low prevalence of induced abortion argues for a cautious interpretation We investigated whether induced abortion is associated with breast cancer when lifestyle confounders , including smoking and alcohol consumption , are adjusted for . Design . Prospect i ve cohort study Abstract Objective : Previous studies of induced abortion and breast cancer may have been limited by differential reporting of abortion history . We conducted a population -based case – control study to evaluate  abortion  ( both induced and spontaneous ) and breast cancer risk . Methods : All study subjects were aged 20–69 years and members of Group Health Cooperative of Puget Sound ( GHC ) . Incident invasive breast cancer cases ( n = 138 ) were identified from the linkage between the GHC enrollment file and the Seattle – Puget Sound SEER Cancer Registry . Controls ( n = 252 ) were r and omly selected from GHC enrollment files and matched to cases on age and enrollment period . All subjects had to have been enrolled at GHC for the 2 years preceding diagnosis ( cases ) or reference ( controls ) date . The unified medical record of each case was abstract ed for pregnancy history , including prior induced and spontaneous abortions , menopause status , height and weight , screening practice s , and other risk factors . Results : Compared to all women who had never had an induced abortion , the multivariate adjusted relative risk of breast cancer in women with an induced abortion was 0.9 ( 95 % confidence interval 0.5–1.6 ) . This risk was similar in parous women , and nulliparous women . There was no association between spontaneous abortion and breast cancer risk . Conclusions : These results do not support a relation between induced abortion and breast cancer incidence The possible influence of induced abortion on breast cancer risk was assessed in a cohort of 267 040 women enrolled in a r and omised trial of breast self-examination in Shanghai , China . Based on answers to a baseline question naire , subsequent breast cancer risk was not significantly associated with ever having an induced abortion . After adjustment for potential confounders , the relative risk estimate was 1.06 ( 95 % C.I. : 0.91 , 1.25 ) , and there was no trend in risk with number of abortions . Analysis of data from more detailed interviews of 652 cases and 694 controls from the cohort yielded similar results . There was also no overall increase in risk in women with  induced abortion  after first birth . Few women had undergone an abortion after 13 weeks gestation or before their first child . Although increases in risk were observed in such women , they were not statistically significant and could have been due to recall bias . Abortions as they have been performed in China are not an important cause of breast cancer Objective : To examine the relationship between breast cancer risk and the cumulative number of cycles before a first full-term pregnancy ( FTP ) and lifetime , taking age at menarche and at onset of regular cycling , periodicity and regularity of cycles , duration of periods of pregnancy , and lactation , oral contraceptive ( OC ) use , and age at menopause into account . Methods : The data were taken from the E3N prospect i ve cohort study of women aged 40–65 years in 1990 . A total of 1718 breast cancer cases were identified during the 579,525 person-years of follow-up . Results : There was a highly significant linear relationship between breast cancer risk and both the cumulative number of cycles before a first FTP ( p for trend < 0.0001 ) and lifetime ( p for trend < 0.001 ) , with multivariate relative risk ( RR ) of a similar magnitude for both variables . Compared to women with a lifetime number of cycles ≤ 402 ( ≤ 30 years ) , the RR for those with a lifetime total of 403–441 , 442–480 , 481–520 , and ≥ 521 cycles were 0.95 ( 0.75–1.21 ) , 1.21 ( 0.97–1.52 ) , 1.23 ( 0.96–1.58 ) , and 1.60 ( 1.25–2.04 ) , respectively . Results restricted to never OC users were similar . Conclusions : Further investigation is needed to clarify whether the underlying factor is repeated exposure to fluctuating hormones , the number of anovular/ovular cycles , or the relative importance of the follicular and luteal phases BACKGROUND Induced abortion has been inconsistently associated with breast cancer risk in case-control studies . Retrospective cohort studies using registry information in Sc and inavia have not suggested an increase in the incidence of breast cancer , although data on individual reproductive factors were not accounted for . METHODS We examined the association between induced and spontaneous abortion and the incidence of breast cancer in a prospect i ve cohort of young women , the Nurses ' Health Study II . The study included 105 716 women 29 to 46 years old at the start of follow-up in 1993 . Information on induced or spontaneous abortions was collected in 1993 and up date d biennially . During 973 437 person-years of follow-up between 1993 and 2003 , 1458 newly diagnosed cases of invasive breast cancer were ascertained . RESULTS A total of 16 118 participants ( 15 % ) reported a history of induced abortion  , and 21 753 ( 21 % ) reported a history of spontaneous abortions . The hazard ratio for breast cancer among women who had 1 or more induced abortions was 1.01 ( 95 % confidence interval , 0.88 - 1.17 ) after adjustment for established breast cancer risk factors ; among women with 1 or more spontaneous abortions , the covariate-adjusted hazard ratio was 0.89 ( 95 % confidence interval , 0.78 - 1.01 ) . The relation between induced abortion and the incidence of breast cancer did not differ material ly by number of abortions ( P for trend = .98 ) , age at abortion ( P for trend = .68 ) , parity ( P for interaction = .54 ) , or timing of abortion with respect to a full-term pregnancy ( P for interaction = .10 ) . CONCLUSION Among this predominantly premenopausal population , neither induced nor spontaneous abortion was associated with the incidence of breast cancer Controversy exists over the possible relationship between induced and spontaneous abortion and risk of breast cancer . Thus , the association of fatal breast cancer and spontaneous  abortion  was examined in a large prospect i ve study of United States adult women . After seven years of follow-up , 1,247 cases of fatal breast cancer were observed among 579,274 women who were cancer-free at interview in 1982 and who provided complete reproductive histories . Results from Cox proportional hazards models , adjusted for other risk factors , showed no association between a history of spontaneous abortion and risk of fatal breast cancer ( rate ratio [RR]=0.89 , 95 percent confidence interval [CI]=0.78–1.02 ) . The RR did not increase with increasing numbers of abortions . Parous women who had a spontaneous abortion before their first term birth were not at increased risk compared with parous women with no history of spontaneous abortion ( RR=0.76 , CI=0.54–1.05 ) . Women whose only pregnancy ended in a spontaneous abortion were not at increased risk compared with women who were never pregnant ( RR=0.61 , CI=0.27–1.38 ) or whose only pregnancy ended in a livebirth ( RR=0.72 , CI=0.32–1.65 ) . These findings do not support an association between spontaneous abortion and fatal breast cancer We recently provided data from a prospect i ve cohort study of postmenopausal women which suggested that a first livebirth at age 30 or older ( cf before age 20 ) was associated with a twofold increased risk of breast cancer  in women without a family history , but a 5.8-fold higher risk in women with a positive family history . To address the question of whether these observations reflect difficulty becoming pregnant or maintaining a pregnancy , we performed additional analyses in which the outcome of each pregnancy was considered . During five years of follow-up , 620 incident cases of breast cancer were identified in the 37,105 women at risk . There was little evidence for an increased risk associated with a history of spontaneous abortion ( relative risk [RR]=1.1 ; 95 percent confidence interval [CI]=0.9–1.4 ) , nor was the risk higher among women who reported two or more spontaneous abortions in consecutive pregnancies ( RR=1.0 , CI=0.7–1.4 ) . Although women who reported that they had tried unsuccessfully to become pregnant had only slightly and nonsignificantly elevated risks of breast cancer ( RR=1.1 , CI=0.9–1.3 ) , a more pronounced and statistically significant association was noted in women with a positive family history ( RR=2.0 , CI=1.4–3.2 ) . There was a strong inverse association between failure to become pregnant and parity ( P<0.0001 ) ; nearly 50 percent of the nulliparous married women reported having tried and failed to become pregnant , whereas the frequency was only 6.8 percent among married women with five or more livebirths . Thus , difficulties in becoming pregnant may characterize a subset of women at increased risk of breast cancer , especially in the presence of a family history Studies suggesting a positive association between pregnancy termination and  breast cancer risk have often been of retrospective case-control design , so subject to selection and recall biases . We undertook a registry-based analysis with minimal selection bias and prospect i ve record-based ascertainment of terminations . The source population comprised Massachusetts women with a record of giving birth between 1987 and 1999 in the Massachusetts Vital Statistics Registry . Primary breast cancer cases were 25 - 55 years old at diagnosis between 1988 and 2000 and had a record of the diagnosis in the Massachusetts Cancer Registry . We matched 3 controls to each case on maternal age , year of giving birth and birth facility . Information on terminations ( induced and spontaneous ) before the birth of record , the matched factors and potential confounders were collected from the birth certificate . After adjustment for the matched factors , age , parity and maternal and paternal education , the odds ratio associating any termination history with breast cancer risk equaled 0.91 ( 95 % CI = 0.79 - 1.05 ) . The marginally protective adjusted odds ratio largely derived from a protective effect among women with parity equaled to 1 ( OR for any termination = 0.68 ; 95 % CI = 0.45 - 1.03 ) , suggesting a protective effect of terminated pregnancy among women with one live birth Objective : There continues to be controversy about whether induced abortion influences the risk of breast cancer .  Because case – control studies of this relation are subject to recall bias , there is a 
    Output:
    
Conclusion The current prospect i ve evidence s are not sufficient to support the positive association between abortion ( including IA and SA ) and breast cancer risk