CUMULATIVE PREVALENCE RATES AND CORRECTED INCIDENCE RATES OF SURGICAL STERILIZATION AMONG WOMEN IN THE UNITED STATES,

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1 AMERICAN JOUBNAL or EPIDIMJOLOGY Vol. 116, No. 5 Copyright 1982 by The Johns Hopkins University School of Hygiene and Public Health Printed in UJ5A. All rights reserved CUMULATIVE PREVALENCE RATES AND CORRECTED INCIDENCE RATES OF SURGICAL STERILIZATION AMONG WOMEN IN THE UNITED STATES, - 1 TIMOTHY F. NOLAN, HOWARD W. ORY, PETER M. LAYDE, JOYCE M. HUGHES AND JOEL R. GREENSPAN Nolan, T. F., H. W. Ory, P. M. Layde (CDC, Atlanta, GA 30333), J. M. Hughes and J. R. Greenspan. Cumulative prevalence rates and corrected Incidence rates of surgical sterilization among women In the United States, -. Am J Epidemiol 1982;116: The authors used data from the 1970 National Fertility Survey and Centers for Disease Control surveillance of surgical sterilizations to estimate the cumulative prevalences of hysterectomy and tubal sterilization among women of reproductive age In the United States between and. In, the cumulative prevalence rate of tubal sterilization was more than twice as high for women aged years as it was in and at least three times as high for women under 30. Although the Increase In the cumulative prevalence rate of hysterectomy was not as marked, by,19% of women aged had undergone hysterectomy. The authors used the cumulative prevalence rate to estimate the population at risk for surgical sterilization, and calculated the corrected Incidence rates for these procedures. While corrected Incidence rates of tubal sterilization among women aged doubled between and, corrected hysterectomy rates remained stable. The largest agespecific Increase In Incidence rates of tubal sterilization was among women, with rates six times higher In than In. These findings can be used to recompute Incidence rates of endometrlal and cervical cancers, abortions, and ectopic pregnancies, allowing more precise analysis of related trends. hysterectomy; sterilization; sterilization, tubal Between 1970 and, 7.8 million determines the population at risk for women of reproductive age in the United other diseases or conditions, such as uter- States underwent either a hysterectomy ine cancer and ectopic pregnancy. Alor tubal sterilization (1, 2). The large though some researchers have corrected number of women involved makes these hysterectomy and endometrial cancer procedures an important public health rates by using an appropriate population concern. Moreover, because sterilized at risk, these studies have involved only women are at low risk for many repro- small areas (3) or a few years (4). We used duction-related events, the prevalence data from the 1970 National Fertility of tubal sterilization and hysterectomy Survey and from the Centers for Disease Control ongoing surveillance of tubal sterilizations and hysterectomies to calcu- Received for publication January 20,1982, and in late the annual cumulative prevalence final form May 25,1982. rates of surgical sterilization among US ' From the FamilyPlanning Evaluation Division, w o m e n for the years -. In addi- Center for Health Promotion and Education, Cen-...,. ters for Disease Control, Atlanta, GA (Retl0n. uam S the appropriate population at print requests to Dr. Layde.) risk, we calculated corrected age-specific 776

2 PREVALENCE AND INCIDENCE OF SURGICAL STERILIZATION 777 incidence rates of tubal sterilization and hysterectomy. METHODS To estimate the number of US women who had already been sterilized for each year from -, we needed the following information by single year of age: 1) the prevalence of these procedures for the starting year, 1970, by marital status (currently married, ever married, never married); 2) the annual number of tubal sterilizations and hysterectomies; and 3) the number of women living in the United States each year. The 1970 National Fertility Survey provided the data on the prevalence of tubal sterilizations and hysterectomies among currently and ever married women by five-year age group. We interpolated from these unpublished data to derive the prevalence of the procedures by single year of age. Because no data are available for 1970 on the prevalence of surgical sterilization among single women and since the incidence rate of these procedures among never married women is low (1, 2), we assumed that none of the women in this group in 1970 had undergone sterilization. The annual incidences of tubal sterilization and hysterectomy for - by single year of age were obtained from the Centers for Disease Control published surveillance of these procedures (1, 2). Population estimates of females by single year of age were obtained from the 1970 Census. We assumed that death and migration in women of reproductive-age were negligible compared with the size of the cohort, and therefore the size of each single-year cohort remained constant as the group aged. The actual calculation of the prevalence of sterilization by current five-year age groups was done as follows: the sterilizations subsequent to 1970 in each birth cohort (by single year of birth) were added to the prevalence of sterilization in 1970 for women of that cohort. For each calendar year during -, the cumulative prevalence of each sterilizing procedure (by current five-year age groups) was obtained by dividing the number of women who had undergone the procedure by the number of women in that five-year age group. To estimate the combined prevalence of surgical sterilization, we assumed that no women had both procedures and added the separate estimates for tubal sterilization and hysterectomy. As a check on the accuracy of our prevalence estimates, we repeated our analysis using only currently married women and compared our results for with those obtained by an independent survey, the National Survey of Family Growth (5). Since published data from the National Survey of Family Growth are based, in part, on contraceptive intent and do not report results separately for hysterectomy and tubal sterilization, we used their unpublished data. The cumulative prevalence rates of hysterectomy and tubal sterilization were then used to correct the incidence rates of these procedures. Women who had been previously sterilized and were no longer at risk of the procedure were subtracted from the population at risk. Women who had a tubal sterilization were considered still at risk of hysterectomy, while women with hysterectomies were removed from the population at risk for both operations. RESULTS The cumulative prevalence of tubal sterilizations among US women more than doubled between and for women aged years (table 1). Among women under 30 years of age, the prevalence more than tripled. Throughout the period, the prevalence of tubal sterilization rose steadily for women between ages 15 and 34, peaked in the age group, then declined slightly in the age group. By, 8 per cent of all women years of age and 17 per cent of all women aged had undergone a tubal sterilization. Although the cumulative prevalence of

3 778 NOLAN ETAL. TABLE 1 Estimated annual cumulative prevalence* oftubal sterilization, US women aged years, V-._ lear ( + ) * Per 100 women, by 5-year age group TABLE 2 Estimated annual cumulative prevalence* of hysterectomy, US women aged years, lear ( -i- ) * Per 100 women, by 5-year age group hysterectomy d for women of all ages during the period -, the s were less marked than those for tubal sterilization (table 2). Only among women with very low prevalence rates for hysterectomy (those less than 30 years old) did the rates at least double. The prevalence of hysterectomy for each year d steadily up to age 44. In, 19 per cent of all women aged had undergone hysterectomy. The total cumulative prevalence of surgical sterilization is shown in table 3. Except for women aged 15 to 19 years, the prevalence of surgical sterilization d steadily for each age group over the eight-year period. In relative terms, the greatest s in the prevalence of surgical sterilization were among younger women; for example, six times as many 20- to 24-year-old women were sterilized in as in. However, in absolute terms, the greatest was in older women; 14 per cent more women aged 35 years and over were sterilized in than in. In, 14 per cent of women of reproductive-age were surgically sterile. Our estimates of the prevalence of surgical sterilization among currently married women agree closely with estimates from the National Survey of Family Growth (5) (table 4). For both tubal sterilization and hysterectomy, and for

4 PREVALENCE AND INCIDENCE OF SURGICAL STERILIZATION 779 TABLE 3 Estimated annual cumulative prevalence* of surgical sterilization, US women aged years, I ear ( + ) * Per 100 women, by 5-year age group. TABLE 4 Comparison of hysterectomy and tubal sterilization prevalence rates* CentersforDisease Control (CDC) method vs. National Survey Family Growth (NSFG), Procedure Hysterectomy Tubal sterilization Total (hysterectomy and tubal) Age NSFG CDC * Per 100 currently married women, United States each age group for which survey data were available, our estimates and those from the National Survey of Family Growth agree within 1 per cent. The incidence rates for tubal sterilization and hysterectomy computed using the corrected population at risk are shown in tables 5 and 6, respectively. Corrected tubal sterilization incidence rates among US women were times as high in as in. Rates rose steadily in the age group and peaked in the age group before declining. Although dramatic s were seen in all age groups (figure 1), the largest relative in age-specific incidence rates was in the age group, in which rates were six times higher in than in. Corrected incidence rates for all age groups declined slightly in, and for the oldest age group in. Corrected incidence rates for hysterectomy remained relatively stable throughout the observation period. The year with the highest age-specific rate varied among individual age groups. Overall, hysterectomy rates d 2 per cent in absolute terms between and. To estimate the impact of the use of a corrected population at risk on the incidence rates of sterilization, we calculated the percentage in the corrected over the uncorrected incidence rate. As might be expected, this was greatest in older age groups during later years. The s in the corrected incidence rate of tubal sterilization over the uncorrected rate among the age group during the first four years of observation ranged from 0 to 8.4 per cent. Among older women (30-44 years) in -, the s ranged from 23.5 per cent to 72.5 per cent. The s were not as dramatic for the corrected incidence rates of hysterectomy: a per cent among women aged years during - and a per cent among those aged years for

5 780 NOLAN ETAL TABLE 5 Corrected incidence* of tubal sterilization, US women aged years, - i ear ( + ) * Per 1000 women with intact fallopian tubes and uteri. V... lear ( + ) 1.7 TABLE Corrected incidence* of hysterectomy, US women aged years, * Per 1000 women with intact uteri DISCUSSION Our estimates of the cumulative prevalence of surgical sterilization document the dramatic in the prevalence of tubal sterilization and hysterectomy among women in the United States from -. The National Survey of Family Growth (5) indicated that among currently married women surgical sterilization was the second most common method of contraception for women aged, and the most common method among women years. If the trend of increasing female surgical sterilization demonstrated in this report continues, it may soon become the single most common method of contraception in the United States for even younger groups of women The close agreement between our estimates of the prevalence of surgical sterilization and those obtained by an independent survey, the National Survey of Family Growth, indicates that our assumptions were reasonable. Considering never married women in 1970 as "nonsterilized" and not accounting for the number of sterilizing procedures done in outpatient facilities could falsely lower our cumulative prevalence estimates. On the other hand, not adjusting for mortality among sterilized women and not counting double procedures (i.e., a tubal sterilization followed by a hysterectomy) could slightly elevate the combined cumulative prevalence estimates in table 3. This latter bias would have a limited 1.1

6 PREVALENCE AND INCIDENCE OF SURGICAL STERILIZATION = = r AGE GROUP FIGUBE 1. Corrected tubal sterilization rates per 1000 women with intact fallopian tubes and uteri, US women aged years, -. effect on our estimate of surgical sterilization prevalence since the double procedures occur primarily in older age groups and are overshadowed by the larger number of single procedures performed on younger women. Even if women who have had a tubal sterilization are as likely to undergo a hysterectomy as other women of reproductive age, we calculate that the error in our estimates of the cumulative prevalence of surgical sterilization would be small never more than 2 per cent. In any event, because of the validation of our prevalence estimates by an independent method, these biases appear to have had little impact on the validity of our estimates. In a previous report (2), we noted a 9.1 per cent decrease in the incidence rates of tubal sterilization between and and suggested the use of uncorrected rates as a possible cause. Since our corrected incidence rates also decreased (7.0 per cent), the decline in tubal sterilization rates must be due to other factors, such as increasing frequency of outpatient sterilization. Our estimate for of 9.7 hysterectomies per 1000 woman-years among women aged years is lower than that of Walker and Jick (4) (13.0 per 1000 woman-years). This disparity is likely explained by the different age groups studied ( vs. >15 years). Our findings are important for several reasons. First, the correction for the number of women previously sterilized allowed more precise estimates of sterilization rates among US women and a more accurate assessment of secular trends in the incidence rates for these procedures. Second, the high prevalence of surgical sterilization, as derived from our calculation, reaffirms that these operations have been and continue to be of substantial public health importance. The large number of women affected underscores the need for continued surveillance of these procedures and any immediate or long-term complications. Third, our use of the population at risk to determine incidence rates for surgical sterilization can be applied to other areas of epidemiologic and public health research, such as the incidence of endometrial and cervical cancers, abortions, and ectopic pregnancies. Applications of our estimates of the population at risk will allow for more accurate epidemiologic analysis of trends in the incidence rates of these entities. REFERENCES Centers for Disease Control. Surgical sterilization surveillance, hysterectomy in women aged, -, March Centers for Disease Control. Surgical sterilization surveillance, tubal sterilization -, March Marrett LD. Estimates of the true population at risk of uterine disease and an application to incidence data for cancer of the uterine corpus in Connecticut. Am J Epidemiol 1980;lll: Walker AM, Jick H. Temporal and regional variation in hysterectomy rates in the United States, 1970-, Am J Epidemiol 1979; 110:41-6. National Cancer for Health Statistics. Contraceptive utilization, United States,. Rockville, MD. National Center for Health Statistics,. (Vital and Health Statistics, Series 23, no. 7.) r 781