Abstract
Background: Reports of secondary infertility present induced terminations and spontaneous losses side by side without testing whether they are associated with the outcome to the same degree. We asked whether the association is specific to the type of loss.
Methods: We reanalysed a matched case-control study of secondary infertility, each case matched individually to controls on age, parity and education. Conditional logistic regression entered both exposures as ordered scores; a likelihood-ratio test compared it with the model constraining their coefficients to be equal. Reporting error and unmeasured confounding were quantified by reclassification and E-values.
Results: 83 cases and 165 controls formed 83 matched sets, of which 59 and 62 carried information. Mutually adjusted odds ratios per category increase were 4.09 (95% CI 2.11–8.80) for induced terminations and 7.29 (95% CI 3.87–15.60) for spontaneous losses, and were not equal (p = 0.022). Fitted alone, terminations showed no association (1.08 (95% CI 0.71–1.62)). With random as well as systematic error included, the bias-corrected interval for any induced termination included 1 once controls were assumed to report terminations less completely than cases; with complete reporting among cases, from a completeness of 0.88 among controls (a reporting differential of 0.12).
Conclusions: Spontaneous loss was associated with secondary infertility with and without adjustment, whereas the association with induced termination appeared only with spontaneous losses held constant at fixed parity and was sensitive to modest differential reporting; the two are not interchangeable summaries of pregnancy-loss history.
Keywords
- case-control studies
- pregnancy loss
- abortion, spontaneous
- infertility, female
- conditional logistic regression
- misclassification bias
- matched analysis
Declarations
- Ethics approval
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- Consent to participate
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- Consent for publication
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- Funding
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- Competing interests
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- Author contributions
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- Data availability
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- Registration
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- Acknowledgements
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