1 Division of Obstetrics and Gynaecology and 2 Division of Paediatric and Perinatal Epidemiology, University of Bristol, Bristol, UK
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Abstract |
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Key words: Caesarean section/population-based study/subfertility
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Introduction |
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Materials and methods |
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Specific fertility information included previous obstetrical and gynaecological history, sexual experience, oral contraception including duration of use, duration of cohabitation, paternity, whether the pregnancy had been planned and how long the parents had been trying to conceive. Other factors of interest included age of mother and her partner at the time of conception, their ethnic origins, highest educational level, the woman's body mass index (BMI = wt/ht2), cigarette exposure, alcohol exposure and parity. Subfertility was defined at two levels, trying to conceive for >1 year and for >3 years. Subfertility has traditionally been defined as failure to conceive after 12 months, therefore 1 and 3 year time periods were selected for the original questionnaires to reflect short- and long-term subfertility. The mode of delivery was self-reported and cross-checked with the STORK maternity database.
Parous women who had a previous Caesarean section were compared with parous women who had no previous Caesarean section at the two subfertility levels. The associations between previous Caesarean section and subfertility were further evaluated for the influence of parity. Caesarean section rates were compared between nulliparous women who had a previous history of subfertility and nulliparous women with no history of subfertility. Results are presented as unadjusted and adjusted odds ratios (OR) and 95% confidence intervals (CI) calculated by logistic regression using SPSS (version 10.0). Factors were included in the multivariable model based on a statistically significant difference between the two groups in the univariable analyses (P < 0.05) or if there was a biologically plausible potential for confounding. For our sample of 3994, the confounders used suffered from between 0 and 12% missing data. The method of creating an additional category denoting missing information was used when appropriate. There did not appear to be any relationship between the amount of missing data and our subfertility outcome (as one might expect) and restricting the sample to the completely observed cases (n = 3240) made little difference to the size of effect or width of CI.
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Results |
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Discussion |
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The direction of causality for an association between subfertility and delivery by Caesarean section is complex. The previously reported association between maternal history of subfertility and subsequent delivery by Caesarea~ section is confirmed by this study (Li et al., 1991; Venn and Lumley, 1993
; Reubinoff et al., 1997
; Dulitzki et al., 1998
; Pandian et al., 2001
). It appears to be independent of maternal age, BMI, and educational level. The women in this sample represented all types of subfertility and both elective and emergency Caesarean sections. The current analysis is based on nulliparous women and is not confounded, therefore, by any previous obstetric experience. Previous studies have reported specific associations between subfertility, obstetric complications and Caesarean section (Venn and Lumley, 1993
; Pandian et al., 2001
) but others have reported a higher rate of Caesarean section without any increased risk of maternal or fetal complications (Li et al., 1991
; Reubinoff et al., 1997
). Maternal choice and obstetrical anxiety may be contributing to increased Caesarean section rates for this group of women and this area warrants further research.
The observed association between previous Caesarean section and an increased risk of subsequent subfertility is more difficult to explain. It is possible that this association reflects a pre-existing history of subfertility which played some part in the indication for a previous Caesarean section, however, the association persists even after controlling for important contributors to baseline fertility. It is interesting to note that the association between Caesarean section and subsequent subfertility is stronger for women of higher parity. This would appear to be contradictory as women of higher parity (parity 2) are generally less likely to be affected by subfertility and an association with only one previous pregnancy (parity = 1) precludes a dilutional effect of an intervening normal pregnancy between the Caesarean section and the index pregnancy. However, the higher parity group includes women with two or more previous Caesarean sections and it may be that there is a cumulative effect of repeat Caesarean sections on the risk of subfertility. Alternatively, it may be that women of greater parity with one or more previous Caesarean sections have a higher background rate of subfertility which persists through successive pregnancies. Prospective studies evaluating successive pregnancies are required to shed further light on this complex relationship.
The underlying mechanisms for an association between Caesarean section and subsequent subfertility are unclear and may relate to infection, adhesion formation or placental bed disruption, which in turn may be influenced by the indication for Caesarean section. These mechanisms seem plausible in the light of the associations between Caesarean section and subsequent ectopic pregnancy, placenta praevia and placental abruption (Hemminki and Merilainen, 1996).
We have not addressed the indications for Caesarean section or specific intrapartum and post-partum complications and these factors warrant further research. It is also important that potential associations between Caesarean section and other reproductive outcomes are considered. This study evaluates subfertility from the perspective of women who succeed in having an ongoing pregnancy and therefore may underestimate the true magnitude of association (Weinberg et al., 1994). It is possible that some women will choose not to have a further pregnancy or will fail to achieve any further pregnancy following Caesarean section. It is important that these and further additional outcomes such as miscarriage and ectopic pregnancy are fully evaluated. The questionnaires used for the study addressed subfertility of 1 and 3 year durations, which precludes month by month survival type analysis and the calculation of fecundability ratios. The 1 year threshold level is the traditional threshold for referral to specialist fertility services and provides useful clinical information; however, future studies should record data on months to conception in order to increase statistical power for detailed evaluation of potential associations (Baird et al., 1986
; Weinberg et al., 1994
).
The findings of the present study are in agreement with previous studies in showing an association between a history of subfertility and subsequent Caesarean section. Further studies are warranted to confirm the new finding that Caesarean section may have a negative influence on future fertility. Reliable evidence on long-term consequences of Caesarean section is essential if women are to be offered informed choice with regards to mode of delivery. The justification for higher Caesarean section rates among women with a history of subfertility warrants critical review in order to establish whether operative deliveries are clinically indicated or simply the result of parental and obstetrical anxiety.
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Acknowledgements |
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Notes |
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References |
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Submitted on January 10, 2001; resubmitted on January 30, 2002; accepted on March 8, 2002.