The causes of the decline in fecundability with age

Abstract

Demographers have frequently suggested that women's fecundability reaches a maximum some time in their twenties and thereafter remains at a plateau for some years. Recent work by Bendel and Hua (1978) reinforces the author's previously expressed suspicion that this theory is incorrect. The present note is an effort to synthesize current knowledge about the variation with age of the reproductive parameters underlying fecundability. It is suggested (1) that the major cause of the decline in fecundability across ages 20–40 is a decline in coital rates, and (2) that a smaller part of the decline in fecundability is caused by a rise in risk of unrecognized spontaneous abortion. It is concluded that the “biological capacity to conceive” does not vary much during ages 20–40.     

Distributions of coital rates and of fecundability

Abstract

Attempts have been made to fit distributions of conception waits on the assumption that fecundability is distributed as a Pearson Type I beta distribution. Since the fits were unsatisfactory (Majumdar and Sheps, 1970), the question arises whether fecundability is distributed as a Type III. If it were, then the probability of coitus should also be distributed as a Type III, and distributions of coital rates should be better fitted by the negative binomial than by Skellam's distribution. The data available for such fits have been briefly reviewed here and fits attempted. There is some slight evidence in favor of the hypothesis, but more data are needed before the question can be decisively answered.     

Male‐female differences in family size preferences among college students

Abstract

Three dimensions of isolated spousal separation largely determine its fertility impact: (1) length, (2) “completeness” (degree to which it is eliminating fecundability during fecundable months), and (3) its timing as defined by the reproductive state of the wife at its start. While it is obvious that by starting when the wife is in the fecundable state its net delaying of next conception is maximized, it is much less clear with which reproductive state its start should coincide in order to minimize its fertility impact. By means of a simple model, it is shown that for minimum impact a separation should start later in a pregnancy the longer is post‐partum anovulation, the higher is the level of intrauterine mortality, and the shorter is the period of separation. Conditions of natural fertility are assumed.     

Evidence of increasing natural fertility in Taiwan

Abstract

A comparison of the age‐specific marital fertility experience of three successive cohorts in Taiwan indicates that the later cohorts have experienced progressively higher rates of natural fertility at each age from 15–19 to 35–39. Their fecundability is correspondingly higher and post‐partum sterility, shorter. These changes in natural fertility levels occurred almost simultaneously with recorded changes in health and nutrition levels. These findings support the hypothesis that during the process of modernization and the fertility transition, the level of natural fertility increases, potentially as a result of improvements in health and nutrition, and changes in lactation practices. It is the net effect of increases in natural fertility on the one hand and deliberate fertility regulation on the other, that determines the course of fertility over time, especially at early stages of modernization and the fertility transition.     

The age pattern of fecundability: an analysis of French Canadian and Hutterite birth histories

This paper analyzes the age pattern of effective fecundability from populations with no evidence of deliberate fertility control using a new convolution model of fecundability. The analysis is based on a sample of Hutterite birth histories from the mid-20th century, and birth histories of French Canadians from the 17th and 18th centuries. The main findings are as follows: 1) the level of effective fecundability is higher among the French Canadians compared to the Hutterites; 2) effective fecundability peaks at age 20 for the Hutterites, and in the early to mid-20s for the French Canadians; 3) Hutterite effective fecundability declines almost linearly from age 20 to 45, and French Canadian effective fecundability declines slowly from its peak to the early 30s, and more rapidly at older ages; and 4) the duration of postpartum amenorrhea is longer for the French Canadians than for the Hutterites. Because of the shorter periods of postpartum amenorrhea the Hutterites have about the same average number of children as the French Canadians, even though the French Canadians have higher effective fecundability.     

Endometriosis in adolescence: A long-term follow-up fecundability assessment

Objective  

A long-term, follow-up study comparing mild and severe forms of endometriosis and their fecundability, on 28 women diagnosed with endometriosis in adolescence.     

Weight at Birth and Subsequent Fecundability: A Prospective Cohort Study

Objective

To examine the association between a woman''s birth weight and her subsequent fecundability.

Method

In this prospective cohort study, we included 2,773 Danish pregnancy planners enrolled in the internet-based cohort study “Snart-Gravid”, conducted during 2007–2012. Participants were 18–40 years old at study entry, attempting to conceive, and were not receiving fertility treatment. Data on weight at birth were obtained from the Danish Medical Birth Registry and categorized as <2,500 grams, 2,500–2,999 grams, 3,000–3,999 grams, and ≥4,000 grams. In additional analyses, birth weight was categorized according to z-scores for each gestational week at birth. Time-to-pregnancy measured in cycles was used to compute fecundability ratios (FR) and 95% confidence intervals (CI), using a proportional probabilities regression model.

Results

Relative to women with a birth weight of 3,000–3,999 grams, FRs adjusted for gestational age, year of birth, and maternal socio-demographic and medical factors were 0.99 (95% CI: 0.73;1.34), 0.99 (95% CI: 0.87;1.12), and 1.08 (95% CI: 0.94;1.24) for birth weight <2,500 grams, 2,500–2,999 grams, and ≥4,000 grams, respectively. Estimates remained unchanged after further adjustment for markers of the participant''s mother''s fecundability. We obtained similar results when we restricted to women who were born at term, and to women who had attempted to conceive for a maximum of 6 cycles before study entry. Results remained similar when we estimated FRs according to z-scores of birth weight.

Conclusion

Our results indicate that birth weight appears not to be an important determinant of fecundability.     

How does variation in fetal loss affect the distribution of waiting times to conception?

Abstract

Unmeasured variation has long been a concern in analyses of the waiting time to conception. Recent work by Heckman and Walker (1991) and Trussell and Rodriguez (1990) has underscored the fact that statistical considerations alone cannot discriminate among likely models describing the distribution. Here, we specify a single theoretically important source of heterogeneity, namely variability in intrauterine mortality, and assess its effects on the waiting times to a conception which results in a live birth. We find that the effects of variation in fetal loss are confined to the tail of the distribution. Unless variation in fetal loss is extremely large or a substantial proportion of observed waiting times are initiated at late ages, variation in fetal loss does not appear to explain much variation in conception waits. We conclude that heterogeneity in fetal loss does not explain the variation in fecundability that has been observed for first birth intervals. This conclusion supports the hypothesis that at early ages (below age 35) variation in the waiting time to a fertile conception may largely reflect the proportion of nonsusceptible couples in the population. The analyses suggest that for the purposes of testing theoretically motivated models, future efforts should be directed toward examining reproductive experience after age 35 and toward incorporating information on characteristics of the fertile period as it becomes known.     

The causes of the decline in fecundability with age

Demographers have frequently suggested that a woman's fecundability reaches a maximum some time in her 20's and thereafter remains at a plateau for some years. Recent work by Bendel and Hua (1978) reinforces the author's previously expressed suspicion that this theory is incorrect. This work is an effort to synthesize current knowledge about the variation with age of the reproductive parameters underlying fecundability. It is suggested that: 1) the major cause of the decline in fecundability across ages 20-40 is a decline in coital rates, and 2) a smaller part of the decline in fecundability is caused by a rise in the risk of unrecognized spontaneous abortion. It is concluded that the biological capacity to conceive does not vary significantly between ages 20-40.     

A synopsis of <Emphasis Type="Italic">Cohniella</Emphasis> (Orchidaceae,Oncidiinae)

A synopsis of the genus Cohniella is presented with nomenclatural updates with a discusion of diagnostic features and biogeographical data. Thirteen species and one natural hybrid are recognized. Two new species, Cohniella pendula and Cohniella biorbicularis, are described and illustrated. The following four new combinations are proposed: Cohniella binotii, Cohniella brachyphylla, Cohniella cepula, and Cohniella longifolia. Lectotypes are selected for Oncidium subulifolium, Oncidium helicanthum, Oncidium humboldtii, Oncidium jonesianum var. phaeanthum, Oncidium cepula, and Oncidium wittii. We also propose an amended lectotype for Epidendrum cebolleta and several epitypifications and new synonyms. A key to the genera of the Trichocentrum complex is presented (Appendix) as well as keys to the species of Cohniella.     

Effects of cyclic spouse separation on conception times

The lengthening of conceptive delays under conditions of natural fertility by cyclic spouse separation is investigated as a problem in renewal processes. Separations are “complete” if they reduce the conditional monthly probability of conception, or fecundability, to zero, and “incomplete” if they merely reduce it, from f to r say. Earlier work respecting cyclic separation by Menken (1979) is extended with formulas for the mean and variance of conceptive delays conditional on T, the ordinal month of the cycle of separation and nonseparation when the woman passes from anovulation into the fecundable state. A simple expression relating fecundabilities f and r and the conditional proportions P_s(T) and P_s-(T) conceiving during separation and nonseparation is obtained.     

The Cyatheaceae (Polypodiopsida) of Peru

A summary is presented of all Peruvian species of scaly tree ferns (Cyatheaceae). A total of 83 species is native to Peru (Sphaeropteris 1 sp., Alsophila sect. Alsophila 9 spp., Alsophila sect. Gymnosphaera 1 sp., Cyathea 72 spp.), with 13 endemic species in Cyathea and one in Alsophila. The following new species are described and illustrated: Alsophila gastonyi, Cyathea chontilla, C. nephele, C. polliculi, and C. praetermissa. The genera Cnemidaria and Trichipteris are no longer recognized and merged in Cyathea. Necessary combinations are made for Cyathea alatissima, C. nervosa, and C. uleana. Cyathea serpens is presented in detail and its relationship is discussed. An artificial key to the Peruvian species of Cyatheaceae is provided.     

Studies of fertility and fecundity: comparative approaches from demography and epidemiology

The approaches used by demographers and by epidemiologists for studying the fecundity of couples (i.e. their ability to conceive) have converged, whereas they were historically divergent. Demography traditionally focused on fertility - the circumstances in which livebirths occur - with the aim of predicting population changes, in particular. Demographers also rapidly became interested in fecundity, developing concepts such as fecundability and definitive sterility. The measurement of fecundability is based on determination of the time to pregnancy (TTP), the basic tool of the epidemiologist. However, while demographers were developing methods for estimating the distribution of fecundability among couples based on TTP, epidemiologists turned to methods for analysing the role of diverse factors potentially influencing fecundity at the individual level.     

Crystal polymorphism of pharmaceuticals: probing crystal nucleation at the molecular level

Paracetamol, sulfathiazole and l-glutamic acid are presented as examples of pharmaceutical crystal polymorphic systems. The effect of N-acylated sulfathiazole derivatives (3–6) on sulfathiazole crystallisation is discussed, and possible modes of action presented. Methods for the control of the crystal polymorphism of l-glutamic acid which utilise the principles of conformation mimicry and co-operative binding are presented. The preparation of a series of bis-amides of EDTA derived from sulfathiazole, 5-aminoisophthalic acid and 4-hydroxyaniline (i.e. compounds 9a–c) is presented, as is data on the effect of these compounds on the crystallisation of, respectively, sulfathiazole, l-glutamic acid and paracetamol.     

α-Hydroxybenzylphosphonate Modified Oligonucleotides: Synthesis,Properties, and a Novel Route via Monomer Building Blocks

Abstract

In general, α-hydroxybenzylphosphonate modified 2′-deoxyadenosine-thymidine dimer building blocks 1, 2 are utilized for the incorporation into α-hydroxybenzylphosphonate pro-oligonucleotides. For a universal application of our pro-oligonucleotide concept on biologically relevant oligonucleotides a route for the synthesis of modified monomer building blocks 3 was developed and is presented herein.     

Age and fertility: can women wait until their early thirties to try for a first birth?

Postponing the start of childbearing raises the question of fertility postponed versus fertility foregone. One of the limitations of previous studies of 'How late can you wait?' is that any observed decline in the probability of conception with age could be due to a decline in fecundability with age or due to a decline in coital frequency with age or due to both factors. Using data from a multinational longitudinal study conducted to determine the daily probability of conception among healthy subjects, a discrete-time event history model with long-term survivors (sterile population) is used to study the relationship between age and fecundability for childless women, while controlling for the pattern of intercourse within a menstrual cycle. The findings suggest that women can wait until their early thirties to try for a first birth, providing that they are not already sterile, as the magnitude of the decline in fecundability is very modest and of little practical importance.     

New data and new species of Microcoryphia and Zygentoma (Insecta) from Israel

The presented contribution concerns the study of several samples of Microcoryphia and of Zygentoma obtained in Israel; an up to date of the species of these insect Orders reported from Palestine is also presented. Genus Hyperlepisma (Lepismatidae: Ctenolepismatinae) is reported for the first time from the country and one new species H. palaestinensis n. sp., is described from the northern Negev. Another new species this one of the genus Lepidospora s. s. (Nicoletiidae: Coletiniinae), L. (L.) ayyalonica n. sp., is described from the Ayyalon Cave in Western Central Israel, from where several other terrestrial and aquatic endemic arthropods were already known.     

The species of the genus Micrambe Thomson 1863, from South Africa (Coleoptera: Cryptophagidae)

The 22 species of the genus Micrambe Thomson known from South Africa are analyzed and revised. Micrambe danielssoni sp. nov is described. Following a detailed study of the type material in different entomological collections, the status of two taxa is revised and two new synonymies are proposed: Micrambe kolbei Grouvelle 1908, syn. nov. for Micrambe minuta Grouvelle 1908, and Micrambe similis Bruce 1957, syn. nov. for Micrambe peringueyi Grouvelle 1908. A lectotypus is designated for one taxon: Micrambe capensis (Redtenbacher 1867). A key to all the species occurring in South Africa is presented.     

Three-dimensional modeling of the cochlea by use of an arc fitting approach

Purpose: A cochlea modeling approach is presented allowing for a user defined degree of geometry simplification which automatically adjusts to the patient specific anatomy. Model generation can be performed in a straightforward manner due to error estimation prior to the actual generation, thus minimizing modeling time. Therefore, the presented technique is well suited for a wide range of applications including finite element analyses where geometrical simplifications are often inevitable. Methods: The method is presented for n=5 cochleae which were segmented using a custom software for increased accuracy. The linear basilar membrane cross sections are expanded to areas while the scalae contours are reconstructed by a predefined number of arc segments. Prior to model generation, geometrical errors are evaluated locally for each cross section as well as globally for the resulting models and their basal turn profiles. The final combination of all reconditioned features to a 3D volume is performed in Autodesk Inventor using the loft feature. Results: Due to the volume generation based on cubic splines, low errors could be achieved even for low numbers of arc segments and provided cross sections, both of which correspond to a strong degree of model simplification. Model generation could be performed in a time efficient manner. Conclusion: The proposed simplification method was proven to be well suited for the helical cochlea geometry. The generated output data can be imported into commercial software tools for various analyses representing a time efficient way to create cochlea models optimally suited for the desired task.