Seasonal migration and seasonal variation in fecundabillty: Effects on birth rates and birth intervals

Seasonal patterns in conception rates have been documented in several recent studies. In this paper, a mathematical model of the reproductive process is developed through which the impact of such variation in conception rates is assessed. It is found that the effect on fertility can be quite substantial, but that the birth rate when seasonal variation is occurring is approximated well by the birth rate calculated when conception rates are constant at their mean. These results indicate that further documentation of seasonality in conception rates and exploration of the causes of these patterns and their change is an important area for demographic research.     

Fecundability,coital frequency and the viability of Ova

An extension of Barrett and Marshall's model expressing fecundability as a function of coital pattern is proposed. In particular, this extension includes the probability that the ovum remains alive. The extended model has been applied to Barrett and Marshall's data, a series of cycles for which basal body temperature curves and the date of coitus have been recorded. It was thus possible to estimate the daily probabilities of fertilization and, under certain assumptions, the proportion of lost ova. This proportion was estimated to be above 50 per cent, and increases with age. The increase of fecundability with frequency of intercourse is more moderate than that predicted by Barrett and Marshall's model.     

Seasonal Patterns of Reproduction in Matlab,Bangladesh

In Bangladesh twice as many births occur in December as in July. This paper examines the seasonal patterns of the risks of conception, fetal loss and return of menses post partum in a longitudinal study of 2,300 women in 14 villages of Matlab, Bangladesh. Life tables were estimated for each month of entry event and then ‘period’ life tables were constructed with the risks for a given calendar month. Confirming the results of earlier studies, risks of resumption of menses were higher in November and December, regardless of time elapsed since the last birth. Similarly, there are increased conception risks in the period from February to April for all fecundable women. The time of lowest fetal loss and stillbirth risks is in the cool season, though this variation makes only a minor contribution to the overall seasonality of births. The pattern of fecundability estimated from data on coital frequency did not match the pattern estimated from reported conceptions; these discrepancies imply possible seasonal changes in other parameters of fecundability besides intercourse.     

Pregnancy avoidance and coital behavior

This study proposes a behavioral model that identifies determinants of coital activity in the context of pregnancy avoidance and assesses the relationships using weekly panel data collected on 300 rural married women in Sri Lanka in 1988. We discuss the utility of the design, which is similar to that of an epidemiological surveillance system, for the measurement of coital behavior and pregnancy risk perceptions. Perceptions of pregnancy risk, spousal agreement on sexual relations, menstrual and lactational status, and cycle timing, all measured daily, are found to influence significantly the probability of coitus on that day. The findings suggest that substantial gains in studying fertility regulation are likely from closer investigation of the behavioral connections between motivation for pregnancy avoidance and coital incidence. Comparison of such panel data with those of cross-sectional sample surveys also provides insights into the validity of measures of coital and contraceptive behavior.     

Modeling Seasonality in Fecundability,Conceptions, and Births

This paper develops a model of seasonal fluctuations in fecundability, conceptions, and births. We begin with a model of individual fecundability that combines behavioral and biological components, with particular attention to the roles of coital frequency, sperm concentration, fetal loss, and contraception. The individual-level model is then expanded into a model of seasonal fluctuations in births at the population level, which accounts explicitly for seasonal fluctuations in the size of the susceptible population. We illustrate the use of the model by analyzing proposed explanations of birth seasonality that rely on extreme summer heat.     

Fecundability, coital frequency and the viability of Ova

Summary An extension of Barrett and Marshall's model expressing fecundability as a function of coital pattern is proposed. In particular, this extension includes the probability that the ovum remains alive. The extended model has been applied to Barrett and Marshall's data, a series of cycles for which basal body temperature curves and the date of coitus have been recorded. It was thus possible to estimate the daily probabilities of fertilization and, under certain assumptions, the proportion of lost ova. This proportion was estimated to be above 50 per cent, and increases with age. The increase of fecundability with frequency of intercourse is more moderate than that predicted by Barrett and Marshall's model.     

Seasonality parameters for insect populations

Summary The usefulness of a number of parameters is investigated for describing the seasonal variability in insect populations, especially in relation to sample size. It is found that Seasonal Niche Breadth (B) and its maximal value (BM) are useful only for large samples. For smaller samples one tends to find the same values for these parameters whatever the distribution they are supposed to describe. The “standardized” ratio of these two parameters (B/BM) is completely useless. The Season Length (S. L.), defined as 52 minus the longest series of weeks in which the species was absent, or not observed, was found to be quite useful, especially at smaller sample sizes. For species which occur over a large part of the year and have been observed in large numbers, the ratioMin/Max is found to be quite useful. This is the number of individuals during the four weeks at the low point of the season divided by the number observed during the four weeks in the peak of the season. It is suggested that the seasonal distribution be estimated usingS. L. or, when appropriate,Min/Max in order to estimate the Seasonal Standard Deviation (SSD), which assumes a normal seasonal distribution. With thisSSD the 99 per cent range (Seasonal Range) can be calculated to characterize the seasonal distribution. With this Seasonal Range different species or different areas can profitably be compared in their seasonality characteristics even if there is a large difference in sample size.     

The contribution of HIV to fertility decline in rural Zimbabwe, 1985-2000

HIV-related sub-fertility has been reported for those populations in sub-Saharan Africa in which contraceptive use is low. We use data from a retrospective survey in rural Zimbabwe and multivariate logistic regression models to show that recent birth rates and current pregnancy rates are also lower among HIV-positive women than among HIV-negative women in those African populations where contraceptive use is high. The fertility reduction is smaller than where contraceptive use is low because age at first sexual intercourse is later and birth rates at older ages are already low. Nevertheless, total fertility is approximately 8.5 per cent lower and HIV-associated sub-fertility may account for as much as one-quarter of fertility decline in Zimbabwe since the late 1980s. Mechanisms for HIV-associated sub-fertility in rural Zimbabwe include more frequent widowhood and divorce, reduced coital frequency, increased amenorrhoea, and possibly, pelvic inflammatory disease. Miscarriage appears to be a less important factor than elsewhere possibly because syphilis is rare.     

The effects of temperature on human fertility

Monthly birth and temperature data for a variety of states and countries are used to estimate the effect of short-run temperature fluctuations on fertility. Regressions of monthly births on a flexible specification of lagged monthly temperature show that temperature has quantitatively important effects on both seasonal and nonseasonal variation in births. Summer temperature extremes reduce conceptions in the southern United States, explaining a substantial part of the observed seasonal birth pattern. Extreme cold shows no evidence of affecting conceptions. The results also show significant seasonality in births even after accounting for temperature. Controls for monthly temperature do not explain the persistent spring peak in births in northern Europe. This finding suggests that other factors play an important role.     

Writing the names: Marriage style,living arrangements,and first birth interval in a Nepali society

Using data from a Nepali population, this analysis argues that marriage style and postmarital living arrangements affect coital frequency to produce variations in the timing of first birth after marriage. Event history analysis of the first birth interval for 149 women suggests that women’s autonomy in marriage decisions and marriage to cross-cousins accelerate the pace of entry into first birth. Extended-household residence with reduced natal kin contact, on the other hand, significantly lengthens the first birth interval. These findings are consistent with previous arguments in the literature while offering new evidence for the impact of extended-family residence on fertility.     

The effect of marital status at first birth on marital dissolution among adolescent mothers

The sequencing of marriage and first birth was expected to play an important role in the stability of marriage among adolescent mothers. We hypothesized that adolescent women who married prior to conception would have the lowest rates of marital disruption, followed by those who married between conception and birth. Adolescent women who gave birth prior to marriage were expected to suffer the highest rates of marital dissolution. The results provide partial support for our hypotheses. There is little difference in the probability of separation between adolescent mothers who had a postmarital conception and those who had a premarital conception but married before the birth. Having a premarital birth, however, significantly increases the probability of marital dissolution. We also hypothesized that marital status at first birth would have less effect on the probability of marital dissolution for blacks than for whites. This, too, is generally supported by our findings. Among black females, those with a premarital birth are the first to suffer a marital disruption, but by the end of ten years there is little difference in the probability of separation among the three marital status groups. In contrast, among white females, those with a premarital birth are the first to experience a disruption, and this differential persists over all subsequent marriage duration intervals. Thus, the sequencing of marriage relative to birth has similar short term effects for whites and blacks, but the effect for blacks is evident only in the short term. Ten years after the marriage, black adolescent mothers have similar rates of marital stability regardless of the sequencing of marriage. This is consistent with the findings of previous research and with our hypothesis; with the black family pattern of lower rates of marriage, higher rates of illegitimacy and higher divorce rates, the sequencing of marriage has no long lasting consequences on marital stability. Finally, our predicted decline in the effect of marital status at first birth over historical time also finds partial support. For white females there has been a change in the effect of marriage-first birth sequencing on separation over time. In the period encompassed by the women in our study, white adolescent mothers who married subsequent to the birth have been the most likely to experience a separation at all marriage duration intervals, but this differential narrows as age at interview declines. Among black females there has been no change in the effect of a premarital birth over time.(ABSTRACT TRUNCATED AT 400 WORDS)     

Smoking and Variation in the Hispanic Paradox: A Comparison of Low Birthweight Across 33 US States

The Hispanic Paradox in birth outcomes is well documented for the US as a whole, but little work has considered geographic variation underlying the national pattern. This inquiry is important given the rapid growth of the Hispanic population and its geographic dispersion. Using birth records data from 2014 through 2016, we document state variation in birthweight differentials between US-born white women and the three Hispanic populations with the largest numbers of births: US-born Mexican women, foreign-born Mexican women, and foreign-born Central and South American women. Our analyses reveal substantial geographic variation in Hispanic immigrant–white low-birthweight disparities. For example, Hispanic immigrants in Southeastern states and in some states from other regions have reduced risk of low birthweight relative to whites, consistent with a “Hispanic Paradox.” A significant portion of Hispanic immigrants’ birthweight advantage in these states is explained by lower rates of smoking relative to whites. However, Hispanic immigrants have higher rates of low birthweight in California and several other Western states. The different state patterns are largely driven by geographic variation in smoking among whites, rather than geographic differences in Hispanic immigrants’ birthweights. In contrast, US-born Mexicans generally have similar or slightly higher odds of low birthweight than whites across the US. Overall, we show that the Hispanic Paradox in birthweight varies quite dramatically by state, driven by geographic variation in low birthweight among whites associated with white smoking disparities across states.     

Factors Affecting Birth Rates Among Black Women 20--24 Years of Age: A Trend Analysis (January 1972--March 1992)

This research provides a multivariate explanation of monthly (N = 243) birth rate variation for African-American women 20--24 years of age using time series analysis. The research explains both seasonal and intermediate variation in these birth rates. The independent variables were selected for their relationship to: economic understandings of fertility; developing understandings of the relationship of photoperiod to reproductive functioning; routine activity theory; and the relationship of birth rates to quality of life perspectives. The findings of this research are interpreted with respect to important developments and understandings in the quality of life literature. Concepts of 'compensatory validation' and 'contextual compatibility' are derived from quality of life understandings and Rainwater's (1974) concept of 'validating activities'. The previously identified importance of domains of intimacy, sexual activity and family life (Cummins, 1996; Alfonso et al., 1996) are related to empirical findings concerning the conception of children and incorporated into these theoretical concepts. Nine explanatory variables were selected. Theoretical, research and policy implications are summarized.     

Fecundability and coital frequency

Abstract The analysis previouslyreported (Population Studies, 23, 1969) of the risk of conceptionon different days of the menstrual cycleis extended so as to take someaccount of the agesof wives. The risks are estimated from data (basal body temperatures and occurrences of coitus) recorded by 241 married couples not using any contraceptives, to whom at least one child had previously been born. A mathematical formula relating natural fecundability to coital frequency is proposed and considered.     

HIV/AIDS and Urbanization

It is well known that levels of HIV prevalence tend to be appreciably higher inurban areas. This article considers the reasons for this and shows that within world regions that are relatively homogeneous with respect to their experience of HIV/AIDS, variation in the level of urbanization corresponds to about one‐third of variation in estimated HIV prevalence. Furthermore, for populations in the world's worst‐affected area—eastern and southern Africa—there are signs that, partly by differentially raising urban death rates and depressing urban birth rates, HIV/AIDS is slowing the pace of urbanization. Finally, in countries with very high levels of HIV infection and relatively low birth rates, such as in South Africa, the urban sector will soon constitute a “demographic sink”—with death rates exceeding birth rates.     

The risk of conception on different days of the menstrual cycle

Abstract Data on 1898 menstrual cycles, for 241 married women, were analysed by means of a quantal regression programme. The locations of the day of ovulation was determined by the basal body temperature method. Estimates were obtained of the risk of conception from an act of coitus on each day, as measured from the day of ovulation. The relationship between fecundability and coital frequency was also examined.     

A comparison between current Indian fertility and late nineteenth-century Swedish and Finnish fertility

Abstract The paper aims to probe causes of the current high level of the crude birth rate in India. This is accomplished by comparing the current Indian fertility level with that of Sweden and Finland during the last quarter of the nineteenth century. By utilizing marital distribution, age distribution and age-specific marital fertility rates of the three countries, the basic demographic causes of high fertility in India can be established. In the second part, the factors causing age-specific marital fertility rates in India to be lower than nineteenth-century Sweden and Finland are discussed. This is done by comparing the levels of the followingsix variables, operating within marriage, between Indian and Scandinavian cultures: (1) Abstinence - voluntary and involuntary, (2) Frequency of coitus, (3) Incidence of sterility - primary and secondary, (4) Contraception and sterilization, (5) Incidence of foetal mortality and (6) Incidence of induced abortions.     

Estimation Of The Birth Rate For The Congo Through Nonconventional Techniques

This paper attempts to produce a set of alternative estimates of birth rates for the Congo and for its main regions through nonconventional techniques. The main body of data used for this purpose is supplied by the demographic survey undertaken during 1955-57. Two types of information on fertility are produced by this survey: (1) births to women by age during the 12 months preceding the survey and (2) children-ever-born to women by age. The available data, while reliable in certain respects, are deficient in others. Thus, the reports on children born during the year preceding the survey are acceptably accurate for the children who have survived but are insufficient for the children who have died.The task was to make the best possible use of various reliable pieces of information directly or indirectly related to fertility, in order to produce some reasonably acceptable estimate of birth rate. One procedure consisted of correcting the reported birth rate for the omission of infant deaths; another procedure involved the adjustment of the reported birth rate with reference to the mean parity of women. In addition to these, an estimate of birth rate was derived by means of stable population technique from the proportion of children under 5 years of age.Three sets of estimates of birth rate, then, have been produced. They yielded a high agreement for the country as a whole, as well as for the provincial and district levels. These estimates were then compared with the proportion of childless women and the proportion of children-the latter having been derived from the records of "continuous registration of inhabitants," a source of information which is completely independent of the demographic survey of 1955-57. Again, a reasonable agreement has been found among all these estimates.Statistical evidence examined in this paper supports a birth rate of about 45 per 1,000 for the Congo. The analysis reveals a high regional variation in birth rate: from 20 per 1,000 (Bas Uele) to 60 per 1,000 (Kivu). This variation is consistent with the variation in sterility level as evidenced by the proportion of women never having had a live birth. The proportion of childlessness varies from 5 percent (Kivu) to about 40 percent (Bas Uele) for women past child-bearing age.     

Prospective study of birth interval dynamics in rural Bangladesh

A group of 209 married, fecund women in rural Bangladesh were studied prospectively for 24 months from 1969 to 1971 to define some of the biological and sociological factors relating to fertility performance. These women were selected from a larger study population of 112,000 that had been followed with a daily house-to-house vital registration programme since 1966. The selected women were interviewed bi-weekly and were asked questions about menstruation, pregnancy, lactation, husband's occupational absences, and monthly urine tests for pregnancy were taken. The results for 193 non-contracepting women revealed that the seasonal pattern of births previously observed in this population could be associated with a corresponding seasonal pattern of conceptions and that this was due to a seasonal trend in fecundability. The highest conception rates were in the coolest months of the year. Post-partum lactational amenorrhoea was very prolonged, averaging 17 months for women with a surviving child. The appearance of the first post-partum menstrual flow (onset of ovulation) also had a seasonal trend which could not be adequately explained. The median waiting time to conception, once menstruation had resumed was eight months. This interval was influenced by seasonal fluctuations, as well as by the age of women and by husbands' absences. The foetal wastage rate was 15·0 per 100 conceptions, with 62 per cent of the foetal losses occurring during the second month of gestation. Overall, the average birth interval was 33 months, with the prolonged lactational amenorrhoea accounting for almost 45 per cent of this interval.