Time Trends In Fertility Research Articles

Selection bias due to parity-conditioning in studies of time trends in fertility.

Studies of couple fertility over time have often examined study populations with broad age ranges at a cross-section of time. An increase in fertility has been observed in studies that followed episodes of fertility events either prospectively among nulliparous women or retrospectively among parous women. Fertility has a biological effect on parity. If defined at a cross-section of time, parity will also be affected by year of birth, and thus becomes a collider. Conditioning (stratifying, restricting, or adjusting) on a collider may cause selection bias in the studied association. A study with prospective follow-up was taken as the model to assess the validity of fertility studies. We demonstrate the potential for selection bias using causal graphs and nationwide birth statistics and other demographic data. We tested the existence of parity-conditioning bias in data including both parous and nulliparous women. We also used a simulation approach to assess the strength of the bias in populations with prior at-risk cycles. Finally, we evaluated the potential for selection bias due to conditioning on parity in various sampling frames. Analyses indicate that the observed increase in fertility over time can be entirely explained by selection bias due to parity-conditioning. Heterogeneity in fertility and differential success in prior at-risk cycles are the ultimate factors behind the selection bias. The potential for selection bias due to parity-conditioning varies by sampling frame. A prospective multidecade study with representative sampling of birth cohorts and follow-up from menarche to menopause would bypass the described bias.

Bias in cross-sectional cohort studies on time trends in fertility

ObjectivesDifferent designs have been used in studies on time trends in fertility, that is, the capacity to conceive and deliver a baby. In some designs, the study populations do not...

Has Human Fertility Declined Over Time?

Reports of decreased semen quality over time have raised concerns about possible reductions in human fertility. Studies of couple fertility have produced conflicting results. We evaluate how changes in the availability and use of effective contraception and induced abortion might bias the direct study of time trends in couple fertility. We assess the potential for bias in the context of 2 common study designs: (1) a study of time-to-pregnancy that estimates fecundability (excluding unintended pregnancies) and (2) a study of infertility rates that categorizes couples as fertile or infertile (including couples with unintended pregnancies as fertile). In time-to-pregnancy studies, bias alone could produce more than a 2-fold apparent increase in fecundability over recent decades. In studies of infertility rates, the bias works in the opposite direction: a 30% underestimation of infertility during earlier decades could produce an apparent decrease in fertility over time. Over the past 5 decades, changes in social factors that affect the rate and fate of unintended pregnancies could substantially bias time trends in fertility. These biases may explain the conflicting reports in the literature. Except in rare settings in which the factors affecting reproductive choices have not changed, it is probably impossible to identify biologic changes in fertility over recent decades.

The spatial dynamics of U.S. fertility, 1970-1990.

"The fertility downturn in the U.S. since the baby boom has been accompanied by a growing divergence in regional fertility rates. This paper examines the spatial implications of recent fertility trends. Two interrelated questions are posed. First, how and why have the time trends in fertility varied spatially? Second, how have the regional patterns of fertility changed over time? These questions are investigated using a continuous spatio-temporal model of U.S. fertility built using the Dual Expansion Method. Results indicate that the pace of fertility decline has been the most rapid in the Northeast and the slowest in the West. Further, the traditional North-South distinctions in fertility rates are being replaced by East-West patterns. These changes tentatively suggest that regional ethnic composition is becoming an increasingly important factor in accounting for the spatial variation of U.S. fertility."

Period Paramount? A Critique of the Cohort Approach to Fertility

The aim of this discussion of cohort and period fertility estimation was to clarify the issue and resolve the conflict between 2 approaches. The typical view of these approaches is that fluctuations in period fertility represented in the total fertility rate (TFR) are due to the timing and pace of cohort fertility; these TFR changes distort. TFR appears high when the pace of cohort fertility increases. This tempo component of fertility must be removed through adjustment in cohort and period measures. Intercohort change reflects the true time path of fertility. The conventional view has been comprehensively discussed by such persons as Hajnal and others who have pointed out that age specific fertility rates are more consistent in period than in cohort analysis. The weight of statistical evidence supports the notion that all birth and marriage cohorts react as a single unit to the factors that determine the general level of fertility at that time. The cohort approach continues to be used because it offers more options for interpretation complements period analysis and has stronger arguments for its use. The resilience in the use of the cohort approach was examined historically by focusing on the demographic role of the cohort perspective and identifying the parts of current thinking on the demography of the period/cohort issue. Ryder has concertedly argued that fertility is affected by past reproductive history or life cycle stage that cohorts are distinctive that theories of fertility use a cohort framework that time series of cohort TFRs are smoother that the cohort framework is a natural approach to behavior and that changes in cohort fertility reflect changes in period amount. Several other views are indicated but the 6 arguments are discussed individually at some length. The problems with period fertility are indicated and an alternative approach to fertility measurement is presented: the period parity progression approach not as an index for a period but as the schedule of duration specific probabilities. The data issues and the role for the cohort method are discussed. The conclusion is that the confusion that has arisen is 2-fold: confusion between time trends in fertility and time trends in reproductivity. Confusion has also arisen due to Hajnals solution to short-term fluctuations in period fertility.

Effect of Age on Fertility of Bulls in Artificial Insemination

Summary The effect of age on changes in fertility of Holstein and Guernsey bulls was examined in the 60–90 day nonreturn records of an artificial breeding stud. Bulls used for 1,000 or more services per year in the period from 1947 through 1959 were included. Data from the terminal year of service were excluded for each bull culled either because of low fertility or poor semen quality. This exclusion eliminated records on bulls whose fertility dropped markedly in a short period of time. Ninety-one Holstein and 66 Guernsey bulls had one or more records which met the criteria established for inclusion in the analysis. The two groups contributed 322 and 218 bull-years, respectively. In each year, the deviation of the nonreturn rate of each bull from the stud average in percentage points for his breed was calculated. These deviations were used in the analysis of age effects with the intention of reducing the effect of time trends in fertility. Analysis of variance indicated a highly significant difference among age groups (P