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Use-effectiveness of the condom in a selected family planning clinic population in the United Kingdom
USE-EFFECTIVENESSOF THE CONDOM IN A SELECTED FAMILY PLANNING CLINIC POPULATION IN THE UNITED KINGDOM
Roger Glass,
M.D.*
Martin Vessey,
M.D.
Peter Wiggins,
B, SC.
Department of Social and Community Medicine, University of Oxford, England
ABSTRACT A prospective study of the beneficial and adverse effects of different methods of contraception is in progress at 17 clinics run by the British Family Planning Association. The present report is concerned with the findings so far on the efficacy of the condom. Overall, 62 accidental pregnancies have been observed among 2057 couples using this method of birth control during 1543 womanyears of exposure to risk, representing a use-effectiveness pregnancy rate of 4.0 per 100 woman-years, It is concluded that there is little reason to encourage couples who are highly motivated towards family planning and who find the condom aesthetically acceptable to change to a more modern method of birth control with its attendant risks unless a very high degree of security is essential.
* Present address: Accepted
for
DECEMBER
Mount Sinai Hospital,
publication
1974
October
VOL. 10
15,
New York City,
U.S.A.
1974
NO. 6
591
CONTRACEPTION
INTRODUCTION In 1968, a prospective study of the beneficial and adverse effects of different methods of contraception was started at two clinics run by the British Family Planning Association. The study was subsequently extended with the result that 17 clinics are now participating and almost 17,000 women are under observation, We have already described our findings about the efficacy of the diaphragm as a method of birth control (I)* A substantial amount of information about the use-effectiveness of the condom has also accumulated, which is summarised in the present report, MATERIALAND METHODS For a clinic patient to be eligible for recruitment to the study, she has to be: (a) aged 25-39 years: (b) married; (c) a white British subject: and (e) either a current user of oral contra(d) willing to participate; ceptives of at least five months standings a current user of the diaphragm or an intrauterine device of at least five months standing, without prior exposure to oral contraceptives. At the time of recruitment, an admission form is completed for each subject. Among other items, this form includes questions about age, social class, smoking habits, intended family size, and obstetric history. After remuitment, each subject is questioned at return visits to the clinic by a doctor or a nurse and certain items of information are recorded on a special form, including the dates of all pregnancies, both planned and unplanned, and details of all changes in contraceptive practices. Women who default are sent a postal version of the follow-up form and, if this is not returned, are contacted by telephone or visited and interviewed in their homes, Losses to follow-up in the study so far, for all reasons combined, have been less than one per cent of women per annum. From the above account, it will be clear that all the information that has been obtained about the use-effectiveness of the condom relates to couples who changed to this method of birth control after Some couples have, of cours=ed recruitment to the investigation, the condom for more than one period of time since the date of entry, In these instances, only the first such period has been included in the It should also be noted that in some clinics, it is a common analysis. practice to encourage couples to use the condom for a month or so after discontinuation of oral contraceptives with a view to planning a pregnancy: the very short periods of use of the condom resulting from implementation of this advice have been discounted in all that follows.
DECEMBER
1974
VOL. 10 NO. 6
CONTRACEPTION With this exclusion, 2057 couples have used the condom so far as their method of birth control at some time during the follow-up period. Of these, 66 per cent had previously been relying on oral contraception, 9 per cent on the diaphragm and 9 per cent on an intrauterine device. Almost all of the remaining 16 per cent started to use the condom on resumption of contraception after a delivery. Side effects of oral contraception were responsible for 49 per cent of couples adopting the condom. The remaining 51 per cent adopted the method for one or other of a wide variety of reasons. No information is available as to which couples used a spermicide in conjunction with the condom and which did not. In the analysis which follows, each subject has been considered to be “at risk” from the time at which the condom started to be used until the date of discontinuation of use (for accidental pregnancy or for any other reason) or until the latest date for which follow-up data are available. RESULTS Overall, 62 accidental pregnancies have been observed among the 2057 subjects during 1543 woman-years of exposure to risk, representing a use-effectiveness pregnancy rate of 4.0 per 100 woman-years. Table I shows the net cumulative discontinuation rates per 100 subjects by reason for discontinuation, at selected ordinal months of use, calculated by the methods recommended by Tietze and Lewit (2). While the cumulative accidental pregnancy rates increase fairly steadily with duration of use, the cumulative rates of discontinuation for all of the other reasons level off markedly with the passage of time. The extremely high rates of discontinuation during the early months of use because of doubts about the efficacy of the condom reflect the fact that many couples adopted the method on a temporary basis before returning to oral contraceptives, the intrauterine device or the diaphragm, Because the continuation rates were so low, gross cumulative accidental pregnancy rates per 100 subjects were also calculated. The figures obtained at different ordinal months of use were as follows: 3rd month, 0.7; 6th month, 2.2: 9th month, 3.4; 12th month, 4.2; 15th month, 5.0; 18th month, 5.9; 21st month, 7.0; 24th month, 8.4. It will be noted that these rates show an almost exactly linear increase with the passage of time. Table II provides summary information about discontinuation of use of the condom in relation to some characteristics of the female members of the couples who were using the method. The data given
DECEMBER
1974
VOL. 10 NO. 6
593
5.5
75.8 5368
Woman+nonthsof use
4.5
Continuationrate
Other personal reasons
8789
51.8
7.4
28.8
2.8
Desire for more effectivemethod 14.0
2.3
Aestheticobjections
4.8
0.6
Planning pregnancy
1.7
6
Accidentalpregnancy
3
11145
42.8
8.3
33.7
6.6
6.3
2.3
9
12908
36.7
9.0
36.5
7.3
7.9
2.6
12
14204
32,8
9.8
37.8
8.0
8.7
2.9
15
15216
29.0
10.8
39.3
8.2
9.5
3.2
18
Ordinal months of use
24.8
11.8
41.0
8.7
9.8
3.9
24
16001 16628
26.5
11.4
40.4
8.3
9.8
3.6
21
Net cumulativediscontinuationrates per 100 subjects by reason for discontinuation,at selected ordinal months of usd of the condom.
Reason for discontinuation:
Table I
10.7 8.3 11.5 8.2
810 206 331 196 (22) ( 8) 20 12 I1
1.6 29.4
1063 qso 2.7 3.9 6.1 6.1
14.1 8.0 6.6
8.1 2.2
30.1
20.4 4.8 0.5
Planning prw=w
10.7 9.2 6,4 10.7
3::
10.5 8.6
9.6
8.0 9.1 11.6
9.8
X2
Aesthetio objections
.
z:6'
14.0 9.7 12.7 10.2
13.5 10.8
42.6 50.2 46.5 43.7
12.8 12.0 15.2
8.8 14.1 12t.l
11.1 12.7 15.1
42.3 46.3 49.0
46.5 45.0
39.7
;:i
Other personal reason8
risk for:
Deeire for more effective method
rate8 per 100 roman-yeareat
603 789 151
.z
2.62
648 498 397
Aoeidental pregn=w
Discontinuatim
(i) I?=bers of aacidentalpregnanciesshorn in parentheses (ii) Regisfrar-Generalta Claseificationof Social Class based on occupationof head of family: I- Professional,II - Intermediate,III - Skilled, IV - Semi-Skilled,V - lJnt3killed (iii) Bo enquirywas made about the age and smokinghabits of the male members of the couples wing the condom
:;:oY4ydeJ 15 or more/day
Social Claea (RegietrarGeneral's Classification) I-II III IV-v Ccewletionof Famile Completed Hot completedor uncertain CiaaretteSmokiq Bone ever
3 or more
Parity (no. of previous birtha) 0 l-2
3G34 35 or more
&z5b;~s
Characterietic
Bumrberof wxaan~eare at risk
Table II Condom discontinuationrates in relation to some characteristicsof the female members of the couples ueimz the method
CONTRACEPTION
are simple rates per 100 woman-years of use, but these figures are adequate for our present purpose because the distributions of duration of exposure during the follow-up period in the various subgroups which are compared are reasonably similar. The lowest accidental pregnancy rate of I,1 per 100 woman-years was recorded among nulliparous women although this figure is based on only 262 woman-years of obsenration. Accidental pregnancy rates were slightly lower among women aged 35 years or more than among younger women and among women who stated that they had completed their families than among women desiring more children or uncertain about their intentions in this respect. There was no clear relationship between the rate of accidental pregnancy and social class, but there was a strong positive correlation between the rate and cigarette smoking (P
Clearly, some of the characteristics examined in Table II are highly correlated; for example, older women tend to be less frequently nulliparous and to have completed their families more often than we undertook an analysis in which the younger women. Accordingly, accidental pregnancy rates for each characteristic in turn were standardised (by the indirect method) for the other four characteristics. This analysis had little effect save to increase slightly the difference in rates between the women who stated they had completed their families and the women who stated they had not (or might have not) done so. DISCUSSION The condom is the most widely used mechanical form of contraception in the United Kingdom and, indeed, in the world. It is cheap, easy to use, available without prescription or medical advice, free from medical complications, and offers at least some protection against venereal disease. Failure rates reported in the medical literature range almost lOO-fold from 31.9 per 100 woman-years (3) to 0.4 per 100 womanyears among consistent users (4). The present investigation, which is one of the largest to have been published, is consistent with the studies of John (4), Fisher (5), Westoff et al (6) and Peel (7) in showing the condom to be a contraceptive method of high effectiveness. Of these studies, the one most similar to ours in design and analysis is that reported by Fisher (5). In contrast to her findings, however, we observed little relationship between the accidental pregnancy rates and the duration of use of the
DECEMBER
1974
VOL. 10 NO. 6
CONTRACEPTION
method. This result presumably reflects the high degree of motivation towards family planning among the participants in the present investigation, both those who used the condom merely as an interim measure and those who adopted the method as a long-term practice,, The analysis of accidental pregnancy rates in relation to characteristics of the female members of the couples who were using the condom yielded two unexpected results. First, there was a strong positive correlation between the risk of accidental pregnancy and cigarette smoking., This is likely to be an indirect association attributable to some other variable, perhaps a personality characteristic. It should be noted, however, that we have been unable to find any association between smoking habits and the risk of accidental pregnancy among diaphragm users 0 Secondly, there was no obvious association between social class and the risk of accidental pregnancy. The most likely explanation is that the relatively few individuals from the lower social classes who make use of family planning clinics are as highly motivated towards contraception as the relatively large numbers of individuals from the upper social classes who avail themselves of the same service. It might reasonably be asked whether all the unplanned pregnancies occurring among users of the condom were identified by the survey methods. We believe this to be so for two reasons. First, it has been possible to check the completeness of reporting of pregnancies by women attending two of the clinics (8). The results of this enquiry indicated that no pregnancies had been missed by our follow-up system. Secondly, the use-effectiveness of combined oral contraceptives, progestogen-only oral contraceptives and intrauterine devices observed so far in our investigation has been very similar to the experience in other large-scale studies. It might also be asked whether, perhaps, the couples using the sheath in our study are relatively infertile. The data accumulated so far strongly suggest that this is not so; indeed our current findings indicate that couples discontinuing the sheath to plan a pregnancy conceive at least as readily as those discontinuing oral contraception for the same reason. The condom is a method of contraception that is widely used by the public but is thought by many to be relatively ineffective. This is illustrated by the large number of couples in the present study who discontinued using the method because they desired greater security. Our findings show that for couples who are highly motivated towards family planning, the degree of security can be greater than is commonly
DECEMBER
1974
VOL. 10 NO. 6
597
CONTRACEPT1ON
supposed. There would, therefore, seem to be little reason to encourage such couples to change to a more modern method of birth control with its attendant hazards, provided that there are no aesthetic objections to the use of the condom and that a small risk of pregnancy is acceptable. ACKNOWLEDGEMENTS We express our thanks to the Medical Research Council for financial support: to Mrs. B, Johnson, Mrs. D. Collinge and Mrs. S. Jeffreys for their careful work in the day-to-day running of the study; and to the research assistants, doctors and nurses working in the family planning clinics who have made great efforts to collect reliable data. REFERENCES 1.
of the diaphragm Vessey, M. and Wiggins, P, Use-effectiveness in a selected family planning clinic population in the United Kingdom. Contraception 9, 15 (1974).
2.
Tietze, C, and Lewit, S. Statistical evaluation of contraceptive methods, Clin.Obstet.Gynecol, 17, 121 (1974).
3.
Tietze, C., Pai, D.N., Taylor, C-E, planning service in rural Puerto Rico. 81, 174 (1961).
4.
John, A. P. K, Contraception in a practice Gen, PractiL 23, 665 (1973),
5.
Fisher, M. A local authority contraceptive clinic. Med. Offr. 110, 175 (1963). its effectiveness.
6.
The third child: Westoff, C.F., Potter, R.G, and Sagi, P,C. Princeton University Press, A study in the prediction of fertility. Princeton, N. J. 1963.
7.
Peel, J. A male-orientated fertility Practitioner 202, 677 (1969),
8.
Vessey, M.P. I Johnson, B, and Donnelly, J. Reliability of reporting by women taking part in a prospective contraceptive study. Brit. J. Prev, Sot. Med. 28, 104 (1974).
598
and Gamble, C, J. A family Am. J. Obstet. Gynecol.
community.
A survey of
control experiment.
DECEMBER
1974
J. Roy. Coll.
The
VOL. 10 NO. 6
Roger Glass,
M.D.*
Martin Vessey,
M.D.
Peter Wiggins,
B, SC.
Department of Social and Community Medicine, University of Oxford, England
ABSTRACT A prospective study of the beneficial and adverse effects of different methods of contraception is in progress at 17 clinics run by the British Family Planning Association. The present report is concerned with the findings so far on the efficacy of the condom. Overall, 62 accidental pregnancies have been observed among 2057 couples using this method of birth control during 1543 womanyears of exposure to risk, representing a use-effectiveness pregnancy rate of 4.0 per 100 woman-years, It is concluded that there is little reason to encourage couples who are highly motivated towards family planning and who find the condom aesthetically acceptable to change to a more modern method of birth control with its attendant risks unless a very high degree of security is essential.
* Present address: Accepted
for
DECEMBER
Mount Sinai Hospital,
publication
1974
October
VOL. 10
15,
New York City,
U.S.A.
1974
NO. 6
591
CONTRACEPTION
INTRODUCTION In 1968, a prospective study of the beneficial and adverse effects of different methods of contraception was started at two clinics run by the British Family Planning Association. The study was subsequently extended with the result that 17 clinics are now participating and almost 17,000 women are under observation, We have already described our findings about the efficacy of the diaphragm as a method of birth control (I)* A substantial amount of information about the use-effectiveness of the condom has also accumulated, which is summarised in the present report, MATERIALAND METHODS For a clinic patient to be eligible for recruitment to the study, she has to be: (a) aged 25-39 years: (b) married; (c) a white British subject: and (e) either a current user of oral contra(d) willing to participate; ceptives of at least five months standings a current user of the diaphragm or an intrauterine device of at least five months standing, without prior exposure to oral contraceptives. At the time of recruitment, an admission form is completed for each subject. Among other items, this form includes questions about age, social class, smoking habits, intended family size, and obstetric history. After remuitment, each subject is questioned at return visits to the clinic by a doctor or a nurse and certain items of information are recorded on a special form, including the dates of all pregnancies, both planned and unplanned, and details of all changes in contraceptive practices. Women who default are sent a postal version of the follow-up form and, if this is not returned, are contacted by telephone or visited and interviewed in their homes, Losses to follow-up in the study so far, for all reasons combined, have been less than one per cent of women per annum. From the above account, it will be clear that all the information that has been obtained about the use-effectiveness of the condom relates to couples who changed to this method of birth control after Some couples have, of cours=ed recruitment to the investigation, the condom for more than one period of time since the date of entry, In these instances, only the first such period has been included in the It should also be noted that in some clinics, it is a common analysis. practice to encourage couples to use the condom for a month or so after discontinuation of oral contraceptives with a view to planning a pregnancy: the very short periods of use of the condom resulting from implementation of this advice have been discounted in all that follows.
DECEMBER
1974
VOL. 10 NO. 6
CONTRACEPTION With this exclusion, 2057 couples have used the condom so far as their method of birth control at some time during the follow-up period. Of these, 66 per cent had previously been relying on oral contraception, 9 per cent on the diaphragm and 9 per cent on an intrauterine device. Almost all of the remaining 16 per cent started to use the condom on resumption of contraception after a delivery. Side effects of oral contraception were responsible for 49 per cent of couples adopting the condom. The remaining 51 per cent adopted the method for one or other of a wide variety of reasons. No information is available as to which couples used a spermicide in conjunction with the condom and which did not. In the analysis which follows, each subject has been considered to be “at risk” from the time at which the condom started to be used until the date of discontinuation of use (for accidental pregnancy or for any other reason) or until the latest date for which follow-up data are available. RESULTS Overall, 62 accidental pregnancies have been observed among the 2057 subjects during 1543 woman-years of exposure to risk, representing a use-effectiveness pregnancy rate of 4.0 per 100 woman-years. Table I shows the net cumulative discontinuation rates per 100 subjects by reason for discontinuation, at selected ordinal months of use, calculated by the methods recommended by Tietze and Lewit (2). While the cumulative accidental pregnancy rates increase fairly steadily with duration of use, the cumulative rates of discontinuation for all of the other reasons level off markedly with the passage of time. The extremely high rates of discontinuation during the early months of use because of doubts about the efficacy of the condom reflect the fact that many couples adopted the method on a temporary basis before returning to oral contraceptives, the intrauterine device or the diaphragm, Because the continuation rates were so low, gross cumulative accidental pregnancy rates per 100 subjects were also calculated. The figures obtained at different ordinal months of use were as follows: 3rd month, 0.7; 6th month, 2.2: 9th month, 3.4; 12th month, 4.2; 15th month, 5.0; 18th month, 5.9; 21st month, 7.0; 24th month, 8.4. It will be noted that these rates show an almost exactly linear increase with the passage of time. Table II provides summary information about discontinuation of use of the condom in relation to some characteristics of the female members of the couples who were using the method. The data given
DECEMBER
1974
VOL. 10 NO. 6
593
5.5
75.8 5368
Woman+nonthsof use
4.5
Continuationrate
Other personal reasons
8789
51.8
7.4
28.8
2.8
Desire for more effectivemethod 14.0
2.3
Aestheticobjections
4.8
0.6
Planning pregnancy
1.7
6
Accidentalpregnancy
3
11145
42.8
8.3
33.7
6.6
6.3
2.3
9
12908
36.7
9.0
36.5
7.3
7.9
2.6
12
14204
32,8
9.8
37.8
8.0
8.7
2.9
15
15216
29.0
10.8
39.3
8.2
9.5
3.2
18
Ordinal months of use
24.8
11.8
41.0
8.7
9.8
3.9
24
16001 16628
26.5
11.4
40.4
8.3
9.8
3.6
21
Net cumulativediscontinuationrates per 100 subjects by reason for discontinuation,at selected ordinal months of usd of the condom.
Reason for discontinuation:
Table I
10.7 8.3 11.5 8.2
810 206 331 196 (22) ( 8) 20 12 I1
1.6 29.4
1063 qso 2.7 3.9 6.1 6.1
14.1 8.0 6.6
8.1 2.2
30.1
20.4 4.8 0.5
Planning prw=w
10.7 9.2 6,4 10.7
3::
10.5 8.6
9.6
8.0 9.1 11.6
9.8
X2
Aesthetio objections
.
z:6'
14.0 9.7 12.7 10.2
13.5 10.8
42.6 50.2 46.5 43.7
12.8 12.0 15.2
8.8 14.1 12t.l
11.1 12.7 15.1
42.3 46.3 49.0
46.5 45.0
39.7
;:i
Other personal reason8
risk for:
Deeire for more effective method
rate8 per 100 roman-yeareat
603 789 151
.z
2.62
648 498 397
Aoeidental pregn=w
Discontinuatim
(i) I?=bers of aacidentalpregnanciesshorn in parentheses (ii) Regisfrar-Generalta Claseificationof Social Class based on occupationof head of family: I- Professional,II - Intermediate,III - Skilled, IV - Semi-Skilled,V - lJnt3killed (iii) Bo enquirywas made about the age and smokinghabits of the male members of the couples wing the condom
:;:oY4ydeJ 15 or more/day
Social Claea (RegietrarGeneral's Classification) I-II III IV-v Ccewletionof Famile Completed Hot completedor uncertain CiaaretteSmokiq Bone ever
3 or more
Parity (no. of previous birtha) 0 l-2
3G34 35 or more
&z5b;~s
Characterietic
Bumrberof wxaan~eare at risk
Table II Condom discontinuationrates in relation to some characteristicsof the female members of the couples ueimz the method
CONTRACEPTION
are simple rates per 100 woman-years of use, but these figures are adequate for our present purpose because the distributions of duration of exposure during the follow-up period in the various subgroups which are compared are reasonably similar. The lowest accidental pregnancy rate of I,1 per 100 woman-years was recorded among nulliparous women although this figure is based on only 262 woman-years of obsenration. Accidental pregnancy rates were slightly lower among women aged 35 years or more than among younger women and among women who stated that they had completed their families than among women desiring more children or uncertain about their intentions in this respect. There was no clear relationship between the rate of accidental pregnancy and social class, but there was a strong positive correlation between the rate and cigarette smoking (P
Clearly, some of the characteristics examined in Table II are highly correlated; for example, older women tend to be less frequently nulliparous and to have completed their families more often than we undertook an analysis in which the younger women. Accordingly, accidental pregnancy rates for each characteristic in turn were standardised (by the indirect method) for the other four characteristics. This analysis had little effect save to increase slightly the difference in rates between the women who stated they had completed their families and the women who stated they had not (or might have not) done so. DISCUSSION The condom is the most widely used mechanical form of contraception in the United Kingdom and, indeed, in the world. It is cheap, easy to use, available without prescription or medical advice, free from medical complications, and offers at least some protection against venereal disease. Failure rates reported in the medical literature range almost lOO-fold from 31.9 per 100 woman-years (3) to 0.4 per 100 womanyears among consistent users (4). The present investigation, which is one of the largest to have been published, is consistent with the studies of John (4), Fisher (5), Westoff et al (6) and Peel (7) in showing the condom to be a contraceptive method of high effectiveness. Of these studies, the one most similar to ours in design and analysis is that reported by Fisher (5). In contrast to her findings, however, we observed little relationship between the accidental pregnancy rates and the duration of use of the
DECEMBER
1974
VOL. 10 NO. 6
CONTRACEPTION
method. This result presumably reflects the high degree of motivation towards family planning among the participants in the present investigation, both those who used the condom merely as an interim measure and those who adopted the method as a long-term practice,, The analysis of accidental pregnancy rates in relation to characteristics of the female members of the couples who were using the condom yielded two unexpected results. First, there was a strong positive correlation between the risk of accidental pregnancy and cigarette smoking., This is likely to be an indirect association attributable to some other variable, perhaps a personality characteristic. It should be noted, however, that we have been unable to find any association between smoking habits and the risk of accidental pregnancy among diaphragm users 0 Secondly, there was no obvious association between social class and the risk of accidental pregnancy. The most likely explanation is that the relatively few individuals from the lower social classes who make use of family planning clinics are as highly motivated towards contraception as the relatively large numbers of individuals from the upper social classes who avail themselves of the same service. It might reasonably be asked whether all the unplanned pregnancies occurring among users of the condom were identified by the survey methods. We believe this to be so for two reasons. First, it has been possible to check the completeness of reporting of pregnancies by women attending two of the clinics (8). The results of this enquiry indicated that no pregnancies had been missed by our follow-up system. Secondly, the use-effectiveness of combined oral contraceptives, progestogen-only oral contraceptives and intrauterine devices observed so far in our investigation has been very similar to the experience in other large-scale studies. It might also be asked whether, perhaps, the couples using the sheath in our study are relatively infertile. The data accumulated so far strongly suggest that this is not so; indeed our current findings indicate that couples discontinuing the sheath to plan a pregnancy conceive at least as readily as those discontinuing oral contraception for the same reason. The condom is a method of contraception that is widely used by the public but is thought by many to be relatively ineffective. This is illustrated by the large number of couples in the present study who discontinued using the method because they desired greater security. Our findings show that for couples who are highly motivated towards family planning, the degree of security can be greater than is commonly
DECEMBER
1974
VOL. 10 NO. 6
597
CONTRACEPT1ON
supposed. There would, therefore, seem to be little reason to encourage such couples to change to a more modern method of birth control with its attendant hazards, provided that there are no aesthetic objections to the use of the condom and that a small risk of pregnancy is acceptable. ACKNOWLEDGEMENTS We express our thanks to the Medical Research Council for financial support: to Mrs. B, Johnson, Mrs. D. Collinge and Mrs. S. Jeffreys for their careful work in the day-to-day running of the study; and to the research assistants, doctors and nurses working in the family planning clinics who have made great efforts to collect reliable data. REFERENCES 1.
of the diaphragm Vessey, M. and Wiggins, P, Use-effectiveness in a selected family planning clinic population in the United Kingdom. Contraception 9, 15 (1974).
2.
Tietze, C, and Lewit, S. Statistical evaluation of contraceptive methods, Clin.Obstet.Gynecol, 17, 121 (1974).
3.
Tietze, C., Pai, D.N., Taylor, C-E, planning service in rural Puerto Rico. 81, 174 (1961).
4.
John, A. P. K, Contraception in a practice Gen, PractiL 23, 665 (1973),
5.
Fisher, M. A local authority contraceptive clinic. Med. Offr. 110, 175 (1963). its effectiveness.
6.
The third child: Westoff, C.F., Potter, R.G, and Sagi, P,C. Princeton University Press, A study in the prediction of fertility. Princeton, N. J. 1963.
7.
Peel, J. A male-orientated fertility Practitioner 202, 677 (1969),
8.
Vessey, M.P. I Johnson, B, and Donnelly, J. Reliability of reporting by women taking part in a prospective contraceptive study. Brit. J. Prev, Sot. Med. 28, 104 (1974).
598
and Gamble, C, J. A family Am. J. Obstet. Gynecol.
community.
A survey of
control experiment.
DECEMBER
1974
J. Roy. Coll.
The
VOL. 10 NO. 6