Clinical and research implications
Human reproduction is a complex sequence of physiological procedures starting with gamete development, fertilization, implantation, and embryonic development until birth.(12) Currently, with the relaxation of population policies as well as changes in parenting attitudes and lifestyles among young couples, the trend toward “backward” childbearing age is gaining attention. In addition, increased changes in environmental exposures, chromosome abnormalities, lifestyles, and unexplained factors have exacerbated infertility.(27-29) Prevalence of infertility using constructed methods have been extensively studied.(30, 31) Within the same study design and database, we previously utilized a dichotomized method to estimate prevalence of infertility in different populations: the rate was 15.5% in women at risk for pregnancy while 25% in women actively attempting to conceive.(3) Rather than constructed measures of infertility as a parametric indicator, TTP can present the cumulative probability of pregnancy during a certain time period. These cumulative pregnancy prospects will help researchers grasp the comprehensive distribution of human fecundity and decide the appropriate timing for fertility treatment.
TTP can be obtained from various epidemiological designs. The most common method is to ask pregnant women to retrospectively recall their time to conception, which is easy to access with lower costs and has been described well.(9, 13, 32, 33) However, this design may introduce recall bias and neglect infertile couples who did not become pregnant.(34) In contrast, a prospective design has been recognized as the “gold standard” to estimate TTP, as it can recruit eligible couples before their attempts and follow up their exact duration with regular laboratory tests.(5, 10, 12, 35) However, this may be less cost-effective for a national population level and less representative when only focusing on pregnancy planners.(36) The current-duration approach has become popular in recent years as it can obtain TTP from couples who are actively attempting to conceive via a cross-sectional study with less recall bias and better representation from the general population.(1, 13, 32) This study design also has its weakness: 1. This method excludes couples with successful pregnancy within the investigated time, causing low fecundity in the certain sampled population. 2. The TTP in these studies only include couples with the current intention to conceive, who may differ in their pregnancy intention and recognition of fertility status compared with all couples at risk for pregnancy.(11) For our investigation process, we asked women whether they were pregnant within the last 12 months and recorded their TTP for the positive answer. For women who did not conceive within the last 12 months, a simple question was asked to determine whether they were currently attempting to conceive and, if so, for how long they had attempted. Based on this cross-sectional design, our study investigated both women who conceived within the past 12 months and women who did not but had been actively attempting to conceive at the time of the interview. Consequently, we collected data on their time until diagnosis of pregnancy with either a retrospective design (among pregnant women) or the current-duration approach (among women who did not become pregnant but were attempting to conceive). This method could, to some extent, overcome the limitations of previous retrospective designs and simple current-duration approaches with minimal selection bias and enlargement of the represented population.