The Danish Epidemiology Science Centre, Aarhus University at the Department of Epidemiology and Social Medicine, Vennelyst Boulevard 6, DK 8000 Aarhus C, Denmark. E-mail: OB{at}soci.au.dk
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Abstract |
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Methods By means of computer simulations we estimated bias caused by differences in persistence in pregnancy attempts. We investigated whether the assumptions made in the simulation were realistic by using empirical data from a European study.
Results The mean waiting time to pregnancy and other estimates of subfecundity (or infertility) strongly depend upon the persistence of couples in pursuing a pregnancy. We show that even moderate changes in the planning behaviour considerably modify the waiting time distribution. Empirical data confirm that persistence in trying to become pregnant is age-related.
Conclusions Persistence in pregnancy attempts affects outcome measures of subfecundity in studies based upon TTP in pregnant women. It is likely that the length of time during which couples keep trying to become pregnant is influenced by a number of factors which would probably change over time or be different between populations to be compared.
Keywords Time-to-pregnancy, fecundity, epidemiology, bias
Accepted 13 April 2000
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Introduction |
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In principle, comparable persistence in trying to become pregnant between regions and over time is required for a monitoring system to provide unbiased results based upon TTP measures, which is a strong assumption. This study provides a tool to estimate how crucial this assumption may be for detecting geographical and temporal variations in fecundity based upon pregnancy samples.
We observed that older women on average conceived faster than younger women in pregnancy-based TTP studies,9 while the opposite was seen when using survey data including unsuccessful waiting times.10 We analyse if a differential persistence in the pregnancy attempt according to age could be a possible explanation.
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Methods |
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We used the following outcome measures:
Simulations are not needed if all couples within each subgroup are given the same fecundability. A closed formula is then available to identify the determinants of the mean TTP in populations with different patterns of giving up (see Appendix).
Data from The European Studies of Infertility and Subfecundity9 are used to illustrate TTP measures in pregnancy samples and the pattern of stopping a pregnancy attempt according to age in data with censored waiting times. The empirical data on persistence in pregnancy attempts were obtained from a population-based survey on fecundity in Europe.10 Kaplan-Meier estimates were calculated by censoring waiting times at the time of conception or interview.
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Results |
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Discussion |
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The wish to continue a pregnancy attempt may differ over time, between populations, or between subgroups defined according to a given determinant. Obviously, a change of partner or a divorce usually terminates a pregnancy attempt. Such events are probably rare shortly after the beginning of a waiting time to pregnancy, but will be more frequent over longer waiting times. Changes in income, housing and job situations could play a role. The importance of these potential determinants of duration of the pregnancy attempt will partly depend upon the prior desire to have a child, which may be a function of parity and age.
Whether the described source of bias actually explains the observation of a shorter TTP among older women in pregnancy-based samples is not known, but it is a possible explanation and, as shown in Figure 1, differential persistence does exist. The older the couple, the more difficult it may be to make room for a pregnancy, and many will have children already. Longer TTP in young couples could, of course, also be due to a declining fecundity for younger birth cohorts.11,12
It should be well accepted that TTP does not provide estimates of fecundity. Still, it may be a useful measure for identifying determinants of subfecundity,4,13,14 if these determinants have no impact on the persistence in trying to become pregnant. Caution is especially needed when TTP are compared over long time periods since time in itself is likely to change both proximal and distal determinants in pregnancy planning behaviour.
In affluent societies with low fertility and good opportunities for treatment, an increasing persistency in trying to achieve a pregnancy is probably to be expected, and this may lead to an increase in TTP, even if fecundity remains unchanged over time.
In this paper we have used simple TTP measures rather than analysing the waiting time by survival techniques since in pregnancy-based TTP studies only censored observations are available.
A monitoring system based upon TTP may be too prone to bias to produce valid results, especially over longer time periods and when comparing different populations. The difference in TTP between the Danish and Italian regions may partly be explained by differences in persistence in attempting pregnancy, as shown in Figure 1. Experience shows, however, that most determinants of subfecundity usually are detectable in TTP studies. In the future, researchers should try to incorporate information on some of the known determinants of the wish to pursue a pregnancy attempt.
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Appendix |
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Formula (8) shows that the mean TTP is a function of fecundity and of the compliance to pregnancy planning (how early couples start giving up and how many do so). When giving up occurs, the mean TTP decreases. If, e.g. older couples more easily give up a pregnancy attempt, the remaining couples will have a lower mean TTP even without differences in the underlying fecundity distribution in the compared groups. The TTP could still appear to be lower even if fecundity in older women is reduced. If a large proportion of couples only persists in trying to become pregnant for a few months, the resulting mean TTP will overestimate fecundity.
In case all women keep trying for the duration of the follow-up, the mean TTP is given by:
![]() | (1) |
Where q = (1 p);
p = fecundity and
m = duration of follow-up
In general, we have that
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and
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So,
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Expression (1) is then equivalent to:
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which becomes, after expanding (5) by computing the derivative:
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In case a fraction of the women give up trying, the mean TTP is given by a more complicated expression:
![]() | (7) |
s= month in which giving up starts for a fraction (r) of the couples
t= (1 r); r = rate of giving up per cycle
q= (1 p); p = fecundity
m= total number of months of follow-up
Expression (7) can also be written, by following analogous steps as in (2) and (3), in the following way:
| (8) |
When m is large or (qt) is small enough as to approximate 0 when at power m, the formula can be simplified to:
![]() | (9) |
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Acknowledgments |
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We thank Inger Schaumburg (Denmark) and Luigi Bisanti (Italy) for their kind permission for using data from The European Studies on Infertility and Subfecundity.
This study was partly supported by the Danish Medical Research Council (jr. No. 1216631). The activities of the Danish Epidemiology Science Centre are funded by a grant from the Danish National Research Foundation.
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References |
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