The season‐of‐birth paradox

Abstract

Vital statistics data show a remarkably consistent seasonality in U.S. birth patterns, with peaks in late summer and winter months, and a valley in the spring. An attitude survey of college students suggests that peaks in the actual birth distribution occur in unpopular months in which to give birth; the valley in the actual birth distribution occurs in popular months. This paradoxical finding is named the Season‐of‐Birth Paradox. Explanations to resolve the paradox include biological and psychological components. A psychological mechanism—named the Misinformed Reproducer Hypothesis—is tested using NSFG data from the 1973–75 and 1979–81 cycles. Results suggest that women stop contracepting with the expectation that they will get pregnant almost immediately. When it takes several months on the average for a successful conception to occur, the actual birth distribution is shifted away from the preferred birth distribution. These results suggest that psychological as well as biological mechanisms underlie the consistent seasonality patterns in U.S. births.     

Are all born equal? Incidence of febrile convulsions by seasons of birth.

To test whether the seasons of birth had an effect on subsequent experience of illness, details were obtained of all Sheffield children born between 1973 and 1977 who were admitted to hospital before their second birthday with a first febrile convulsion. Analysis by date of birth in consecutive 28-day cohorts showed that the incidence of febrile convulsions ranged from 2.5 per thousand live births to 30.2 per thousand in different "month" cohorts. Statistically significant variations were noted in the incidence rates in relation to season and year of birth. The implication is that even large scale epidemiological studies which have been confined to children born in a particular week or month may not be representative of the whole child population.     

Biologists and the promotion of birth control research, 1918–1938

Conclusions In spite of these efforts in the 1920s and 1930s to initiate ongoing research on contraception, the subject of birth control remained a problem of concern primarily to the social activist rather than to the research scientist or practicing physician.⁸⁰ In the 1930s, as has been shown, American scientists turned to the study of other aspects of reproductive physiology, while American physicians, anxious to eliminate the moral and medical dangers of contraception, only reluctantly accepted birth control as falling within their professional domain.As a result, the problem of cheap, effective, and safe contraception was not solved by these earliest attempts. Consideration of the subject was initiated afresh by private philanthropy after World War II, sparked by a new wave of interest in population studies.⁸¹ Summarizing such efforts to support research in the reproductive sciences, a recent Ford Foundation study has noted: To initiate and sustain serious research in the reproductive sciences has required for more than half a century concerted effort by interested individuals and private organizations, mainly from outside the mainstreams of the biomedical research community.⁸² The early laboratory research on chemical contraception described in this paper was but one important outcome of the concerted effort made by reformers in the 1920s to eliminate a variety of social problems thought to derive from excessive fertility. Scientific arguments and expertise were employed to advocate reform as well as to define the appropriate solution to such social problems. Scientists were recruited as advocates for the movement, but they were also employed as researchers in laboratory investigations sponsored by these same reformers.Sponsors of these early laboratory studies noted the difficulty of obtaining first-class investigators.⁸³ The routine analyses necessary for such research, as well as the traditional scientific aversion to applied problems, provide only a partial explanation for this response. The real difficulty lay in recruiting investigators to a field (reproduction and human sexuality) that had previously been taboo. Once opened up — first as socially relevant, and finally as scientifically sound — there was much interest in this area, and the appeal to researchers of the scientific issues surrounding fertility and reproduction soon surpassed that of the reforming value of birth control.A survey of the kinds of experimental investigations sponsored by birth control advocates indicates the range of physiological problems explored by contraceptive research. The most definitive work was done on the efficacy and safety of spermicides, but the potential of other contraceptive methods was also examined. Investigators attempted to develop spermatoxins that would effectively immunize women against sperm, and they also tried to elucidate the mechanism of hormonal control of reproduction. In fact, speculations about the possible hormonal manipulation of fertility were expressed at the Seventh International Birth Control Conference held in Zurich in 1930.⁸⁴ In the 1920s, clinical studies were undertaken to assess the effectiveness of the various birth control methods. Laboratory investigators complemented this work by screening spermicides for safety and testing for their ability to kill sperm. There were a variety of birth control preparations on the market (most of which were sold as feminine hygiene products), but no one really knew whether these were effective or even safe.⁸⁵ Although the physiology of other major organ systems was well advanced, the scientific study of reproductive physiology and contraceptive technology was clearly in its infancy in this period. Routine analyses simply could not be conducted, because the fundamental research establishing baselines had not yet been done. Scientists used this fact to redirect attention to basic research on reproduction.Laboratory research on contraception indicated important unexplored areas for physiological investigation. Social activists, who had encouraged prominent scientists to become interested in both the social value and the genetic implications of birth control, found these investigators revising the goals of their research. The biologists had formed their own network and had begun to seek out funding, reformulating the justification for sponsorship of further investigations. The eugenic motivations underlying these studies, which had initially made them theoretically attractive to biologists, were gradually eroded. Concern with human evolution ceded its place to interest in physiological mechanisms. Crew and others began to note that the use of biological theory to justify essentially political decisions had serious limitations. Biologists had become uncomfortable with those very arguments which had originally captured their interest. Recognition of the potential political abuse wrought by applying scientific principles to society was expressed by Crew just one year after the Zurich meeting. Referring to previous assessments of the role of sex in reproduction, he generalized: In the past the biologist has justified feudalism, Manchester Liberalism, socialism and every other type of social organization and political programme by reference to selected biological phenomena.⁸⁶ By 1932 Crew had also begun to question the biological logic of regulated breeding, and had made it clear to his American sponsor that there was no simple correspondence between the practice of birth control and the genetic improvement of the human race.⁸⁷ Biologists further began to recognize, however, that although the hopeful genetic solution to human problems was probably an illusion, contraception still remained one tangible means to alleviate, human misery. Some laboratory scientists, like Crew, acknowledged the applicability of their own particular skills to this problem. For a few brief years, social needs and scientific goals were mutually supportive and closely intertwined. But as laboratory researchers gained interest in the study of reproduction and established their own priorities in this field, they temporarily withdrew from the arena of debate over birth control as an important mechanism for social reform.With the rise of Hitler, the genetic arguments for birth control rapidly lost their appeal. But by that time the scientific problem of how to achieve effective contraception had entered the professional consciousness. Both physicians and scientists began to be aware of birth control as a subject within their domain of expertise, although outside the principal focus of their research. Scientific discussion of birth control permanently altered from a question of justification to a problem of method: How could one achieve reliable and safe contraception? This had been Sanger's and Dickinson's goal from the beginning. Laboratory scientists had indeed been persuaded to undertake this work; this research had in turn affected biologists' perceptions of the whole field of reproductive physiology, encouraging further study of reproductive mechanisms. The promise of new knowledge provided for continued funding of this research, despite the caution by scientists that the social benefits would not be as immediate or as far-reaching as advocates and they themselves had first argued.The activities of birth control activists and their supporting agencies, and the financial backing of private contributors and foundations, notably the Rockefeller philanthropies, provided an important new stimulus to the development of research on the biology of reproduction in the late 1920s and early 1930s. Biologists were able to claim an enlarged realm of issues for scientific study through their activities as advocates and as investigators for the birth control movement. At the same time, they promised as-yet-undiscovered possibilities for regulating human reproduction once its physiology was understood. The knowledge and control that they promised lay in understanding the whole reproductive cycle — not merely in evaluating the toxic effects of presumptive spermicides.Chemical spermicides never summoned the interest of scientists as the contraceptive pill was to do, yet that research did reinforce the widespread perceptions of scientific research as essential to social reform. Spermicide investigations focused research efforts in reproductive biology by challenging traditional taboos, defining problems for further study, and providing laboratory investigators the opportunity to assert the social and scientific value of their own skills. Crew echoed this attitude as he observed in 1934:Man has turned from the adventurous conquest of his environment to the conquest of himself. To-day is the day of biological invention, eagerly used for the control of the undesirable and the unwanted. Sex and reproduction are no longer hedged around by myth and taboo; they are no longer accepted as mysteries that defy understanding. They are matters inviting examination and explanation; they are regarded as expressions of physico-chemical forces, the nature of which is to be displayed. It is accepted that when knowledge is sufficient, control will be absolute, and, though knowledge is not yet sufficient, readers ... must be persuaded to the view that this will not always be so.⁸⁸ The synergism between reproductive biology and social needs has been temporary and sporadic, but recurring, since the 1920s. Scientific research programs have clearly been influenced by issues raised by public debate. Nevertheless, reproductive biologists have continued to assert their own professional goals. For the most part, they have rejected problems without inherent scientific interest and have spurned applied research except as it has had a direct bearing on current research themes. This attitude, apparent among American and British investigators in the 1930s, created the intellectual context for the invention and acceptance of the technically sophisticated oral contraceptive pill. It did not foster the production and improvement of the simple, safe, effective, and cheap vaginal contraceptive desired by early birth control advocates.     

When will the stork arrive? Patterns of birth seasonality in neotropical primates

We review and discuss the ultimate and proximate causes of birth seasonality in Neotropical primates and the seasonal patterns shown by each genus within this group. Our review of the literature shows that most New World monkey populations studied so far show some degree of birth seasonality. Photoperiod is the most important proximate cue used by populations living at relatively high latitudes to time their reproductive events, but almost nothing is known about the proximate factors used by those near the equator. The findings are consistent with the hypothesis that food availability is the most important ultimate cause of birth seasonality. Predation seems to promote birth synchrony in some species (e.g., squirrel monkeys). Multiple regression ANCOVA was used to estimate how the degree of birth seasonality is affected by ecological and life history variables. The ANCOVA model shows that three factors affect the degree of birth seasonality: diet, latitude, and body size. Folivores (howlers) are less seasonal than frugivores and insectivores. The degree of seasonality increases with latitude and shows a humped relationship with body size, peaking at 1.66 kg body mass. This last relationship was expected since small bodied species have to pay a cost (in terms of time lost) by being seasonal on a yearly basis, and large species are buffered against fluctuations in food availability due to their large body mass. To understand which of three alternative birth strategies is followed by each species (reduce energy stress during peak lactation, wean infants during peak food availability, or store reserves during peak energy availability), we compared the location of the birth peak in relation to the peak in food-availability for those populations from which data were available. Most species conform to the typical pattern of births concentrated before the peak in food availability, allowing peak lactation (small-sized species) or weaning (capuchins) to take place before the start of the lean season. The pattern of births of the atelines is consistent with the weaning hypothesis. However, since they give birth during the lean season, this pattern is also consistent with an alternative strategy.     

How do tenascins influence the birth and life of a malignant cell?

Tenascins are large glycoproteins found in embryonic and adult extracellular matrices. Of the four family members, two have been shown to be overexpressed in the microenvironment of solid tumours: tenascin-C and tenascin-W. The regular presence of these proteins in tumours suggests a role in tumourigenesis, which has been investigated intensively for tenascin-C and recently for tenascin-W as well. In this review, we follow a malignant cell starting from its birth through its potential metastatic journey and describe how tenascin-C and tenascin-W contribute to these successive steps of tumourigenesis. We consider the importance of the mechanical aspect in tenascin signalling. Furthermore, we discuss studies describing tenascin-C as an important component of stem cell niches and present examples reporting its role in cancer therapy resistance.     

Evolution of birth intervals in a french rural population (1840–1950)

This study presents the first results concerning the evolution of the birth intervals since 1840 among a rural population of the centre of France, where birth control appeared as far back as the end of the 18th Century. We observe the absence of variation of the average interval between successive briths in relation to the period of marriage, family size and socio-economic status of spouses. This confirms that child spacing does not play a part in the constitution of family. The average intervals between marriage and first birth are associated with most of the variables, essentially due to the marrying age and to a lesser extent to the family size. This rural population, in spite of a modern profile by the widespread practice of birth control, still presents traditional attitudes in the structure of its fertility as is indicated by the absence of birth spreading and the important part which seems to be played by the marriage age of women.     

Can mothers judge the size of their newborn? Assessing the determinants of a mother's perception of a baby's size at birth

Birth weight is known to be closely related to child health, although as many infants in developing countries are not weighed at birth and thus will not have a recorded birth weight it is difficult to use birth weight when analysing the determinants of child illness. It is common to use a proxy for birth weight instead, namely the mother's perception of the baby's size at birth. Using DHS surveys in Cambodia, Kazakhstan and Malawi the responses to this question were assessed to indicate the relationship between birth weight and mother's perception. The determinants of perception were investigated using multilevel ordinal regression to gauge if they are different for infants with and without a recorded birth weight, and to consider if there are societal or community influences on perception of size. The results indicate that mother's perception is closely linked to birth weight, although there are other influences on the classification of infants into size groups. On average, a girl of the same birth weight as a boy will be classified into a smaller size category. Likewise, infants who died by the time of the survey will be classified as smaller than similarly heavy infants who are still alive. There are significant variations in size perception between sampling districts and clusters, indicating that mothers mainly judge their child for size against a national norm. However, there is also evidence that the size of infants in the community around the newborn also has an effect on the final size perception classification. Overall the results indicate that mother's perception of size is a good proxy for birth weight in large nationally representative surveys, although care should be taken to control for societal influences on perception.     

Does troop size of wild Japanese macaques influence birth rate and infant mortality in the absence of predators?

For the wild Japanese macaques of Yakushima and Kinkazan Islands, we analyzed the relationship between the troop size or the number of adult females of each troop, infant/adult female ratio (IFR; crude birth rate), and infant mortality (IM) in habitats with no predators. In Yakushima, IFR was positively correlated to troop size and the number of adult females. In Kinkazan, however, IFR tended to decrease with the number of adult females. This difference may be due to the difference in troop size; i.e. in Yakushima, where troop size was small, IFR may increase with that of troop size, because a relatively larger troop is likely to the advantage in intertroop competition. In Kinkazan, where troop size was large, however, IFR is likely to decrease with troop size, because intratroop competition may increase. Thus, the present data roughly supportWrangham's model of the social structure of female-bonded primates, and suggests that there is an optimal troop size for birth rate (BR). On the other hand, there was no clear correlation between IM and the troop size or number of adult females of each troop.     

A birth–death model of ageing: from individual-based dynamics to evolutive differential inclusions

Journal of Mathematical Biology - Ageing’s sensitivity to natural selection has long been discussed because of its apparent negative effect on an individual’s fitness. Thanks to the...     

From the history of Croatian dentistry: 100th anniversary of the birth of Professor Sime Kordić (1909-1984)

The aim of this study was to pay tribute to the memory of extraordinary Croatian Professor of children's and preventive dentistry Sime Kordi? on the occasion of the 100th anniversary of his birth. The biography and work of this scientist and teacher were presented, as well as his researches about the history of dentistry and medicine in our coastal cities. On the bases of his published papers, we analyzed a significant role of Professor Sime Kordi? for the development of dental medicine and promotion of health culture in Croatia.     

Comparison of black–white disparities in preterm birth between Canada and the United States

Background:

A higher risk of preterm birth among black women than among white women is well established in the United States. We compared differences in preterm birth between non-Hispanic black and white women in Canada and the US, hypothesizing that disparities would be less extreme in Canada given the different historical experiences of black populations and Canada’s universal health care system.

Methods:

Using data on singleton live births in Canada and the US for 2004–2006, we estimated crude and adjusted risk ratios and risk differences in preterm birth (< 37 wk) and very preterm birth (< 32 wk) among non-Hispanic black versus non-Hispanic white women in each country. Adjusted models for the US were standardized to the covariate distribution of the Canadian cohort.

Results:

In Canada, 8.9% and 5.9% of infants born to black and white mothers, respectively, were preterm; the corresponding figures in the US were 12.7% and 8.0%. Crude risk ratios for preterm birth among black women relative to white women were 1.49 (95% confidence interval [CI] 1.32 to 1.66) in Canada and 1.57 (95% CI 1.56 to 1.58) in the US (p value for heterogeneity [p_H] = 0.3). The crude risk differences for preterm birth were 2.94 (95% CI 1.91 to 3.96) in Canada and 4.63 (95% CI 4.56 to 4.70) in the US (p_H = 0.003). Adjusted risk ratios for preterm birth (p_H = 0.1) were slightly higher in Canada than in the US, whereas adjusted risk differences were similar in both countries. Similar patterns were observed for racial disparities in very preterm birth.

Interpretation:

Relative disparities in preterm birth and very preterm birth between non-Hispanic black and white women were similar in magnitude in Canada and the US. Absolute disparities were smaller in Canada, which reflects a lower overall risk of preterm birth in Canada than in the US in both black and white populations.In the United States, a higher risk of preterm birth among black women than among white women is well established.^1–3 This racial disparity is of great concern because preterm birth is a leading cause of perinatal mortality and is predictive of developmental problems and adverse health outcomes later in life.⁴ The underlying causes of the racial disparity in preterm birth in the US are not well understood, although research has suggested contributing roles for a wide range of factors, including socioeconomic disadvantage,⁵ poor neighbourhood conditions (e.g., poverty, crime),^5,6 lack of access to health care,⁷ psychosocial stress,⁸ racial discrimination⁹ and adverse health behaviours.¹⁰Rates of preterm birth have consistently been lower in Canada than in the US.^11,12 However, in contrast to the US, little is known about differences in rates by race or ethnicity in Canada. There is evidence that African-born and Caribbean-born women in the provinces of Quebec and Ontario have higher rates of preterm birth than Canadian-born women.^13–15 Although the magnitude of these differences is smaller than the disparity in preterm births between black and white women in the US,¹⁶ foreign-born black women in the US have been found to be at lower risk of preterm birth than US-born black women.^17,18In both Canada and the US, socioeconomic conditions at both individual and neighbourhood levels are important predictors of preterm birth.^19–21 Although the income gap between black and white people is markedly smaller in Canada than in the US,²² black populations in both countries have lower education levels, higher unemployment rates and a greater likelihood of living in low-quality neighbourhoods compared with white populations.²³ Canada and the US share similar social and economic influences, yet the historical experiences of black populations and the social welfare systems (e.g., universal health care) are quite different in the 2 countries. Black people constitute about 13% of the total US population, but only about 3% of the Canadian population.^24,25 The overwhelming majority of Canada’s black population are immigrants who entered the country after 1960 and their descendants, whereas more than 85% of black Americans can trace their ancestry 3 or more generations in the US, with most being descendants of slaves.²²The objectives of our study are twofold. First, using data from a new cohort linking birth registrations with information from the 2006 Canadian long-form census, we present Canada-wide estimates of differences in preterm birth rates between black and white populations. Second, we use comparable methodology to compare racial differences in preterm birth rates between Canada and the US. Given different historical experiences of black populations in the 2 countries, as well as Canada’s commitment to universal health care and its general perception as a more egalitarian society than the US,²² we hypothesized that we would observe smaller racial disparities in the rates in Canada than in the US.     

Impact of maternal age on birth outcomes: a population-based study of primiparous Brazilian women in the city of São Paulo

The aim of this study is to analyse the impact of maternal age at first birth on low birth weight, preterm birth and low Apgar scores at one minute and at five minutes among live births delivered to primiparous Brazilian women in the city of S?o Paulo. Analyses were based on 73,820 birth records from the 1998 birth cohort. Logistic regression was used to assess the association between maternal age and each outcome variable, controlling for the following risk factors: delivery mode, plurality, sex, maternal education, number of prior losses, prenatal care, race, parity and community development. Maternal ages below 20 and above 30 years were significantly associated with the risks of low birth weight and preterm birth, but no association was found between maternal ages and Apgar score, with the exception that ages 15-19 reduced the odds of a low one-minute score. Even though this result seems to be inconsistent, low birth weight, preterm birth and low Apgar scores measure different dimensions of newborn well-being, and the association of each measure with maternal age is expected to diverge.