Biological and behavioral determinants of fertility in Tierra del Fuego

The reproductive history of 182 women in postreproductive life or near menopause from the Chilean part of Tierra del Fuego was traced back by means of familial interviews. These postmenopausal women represent the population since almost the beginning of the settlement, and their reproductive years were spent on the island. Path analysis was applied to analyze fertility determinants of these women and to propose a complex model of interconnections among factors. The reproductive history of these women is characterized by a long fertile span, a short childbearing period, and low fertility. Age at menarche is relatively late, and the age of the women at first birth is mainly determined by their late age at marriage. The use of contraception is related to both spacing and stopping behaviors. The late age of women at marriage, the rhythm of conception, and practices of contraception are proposed as the main determinants of fertility in Tierra del Fuego.     

Birth spacing in Pakistan

Life table analysis was applied to data from the 1975 Pakistan Fertility Survey to identify child spacing differentials between population subgroups. Women in urban areas had shorter birth intervals than their rural counterparts from parities 1-6; only after parity 7 was this differential reversed. Similarly, women with some education had shorter birth intervals at the earlier parities than uneducated women. While overall family size is relatively homogeneous in Pakistan, women of more modern backgrounds seem to space their children more closely than traditional women. Age at marriage appears to play an important role not only in determining the length of the 1st interval, but also that of subsequent intervals. An unexpected finding was that ever users of contraception had distinctly more rapid spacing of their births than never users. The median interval to 1st birth was shortest in North West Frontier Province, but similar in Punjab and Sind. Multiple classification analysis revealed that some differentials in child spacing by education, residence, and province persisted even after other variables were controlled. Cohort of mother had an independent effect, with younger cohorts having shorter birth intervals. However, the variable that had the strongest effect on length of interval (aside from the 1st interval) was breastfeeding duration. It is likely that increasing urbanization and improved levels of education among women will lead to high levels of marital fertility associated with shorter birth intervals. Even though these trends tend to increase the age at marriage, they are associated with shorter durations of breastfeeding. In the longer term, greater use of contraception among women in the modern sector may partially counteract the fertility increasing effect of reduced birth intervals.     

Smoking in pregnancy and development into early adulthood.

Follow up analyses of data on the 1958 cohort from the national child development study have shown that the association of smoking in pregnancy with physical and intellectual development diverges between the sexes at age 16, no longer being significantly associated with height in girls. These studies, however, have emphasised that the differences in outcome are small and may be explained by other factors. The analyses have taken account of birth weight and have therefore examined the effects of smoking on subsequent development in addition to this variable. To assess the importance of smoking on development in early adult life and whether the effect is independent of birth weight data from the 1958 cohort at age 23 were analysed. Only weak evidence for a relation between smoking in pregnancy and self reported height of the offspring was apparent once social class, size of family, mothers'' height, and birth weight for gestational age were taken into account. After omission of birth weight from the analyses, however, the average difference in height between subjects whose mothers smoked 20 cigarettes a day or more during the second half of pregnancy and those whose mothers did not was 0.93 cm in men and 1.83 cm in women. A strong association was also evident with the highest qualification achieved by subjects at this age, suggesting a long term relation between smoking in pregnancy and the intellectual development of the offspring.     

Why do women stop reproducing before menopause? A life-history approach to age at last birth

Evolutionary biologists have long considered menopause to be a fundamental puzzle in understanding human fertility behaviour, as post-menopausal women are no longer physiologically capable of direct reproduction. Menopause typically occurs between 45 and 55 years of age, but across cultures and history, women often stop reproducing many years before menopause. Unlike age at first reproduction or even birth spacing, a woman nearing the end of her reproductive cycle is able to reflect upon the offspring she already has—their numbers and phenotypic qualities, including sexes. This paper reviews demographic data on age at last birth both across and within societies, and also presents a case study of age at last birth in rural Bangladeshi women. In this Bangladeshi sample, age at last birth preceded age at menopause by an average of 11 years, with marked variation around that mean, even during a period of high fertility. Moreover, age at last birth was not strongly related to age at menopause. Our literature review and case study provide evidence that stopping behaviour needs to be more closely examined as an important part of human reproductive strategies and life-history theory. Menopause may be a final marker of permanent reproductive cessation, but it is only one piece of the evolutionary puzzle.     

Social-demographic influence on first birth interval in China, 1980-1992

This study examines the delay between first marriage and first live birth in China among a sample of women who married between 1980 and 1992. Most couples in China only use contraception after the first child is born. Most sample women had their first child within 2 years of marriage. However, there are significant rural-urban differences in the first birth interval, indicating that there was most probably deliberate fertility regulation after marriage among many urban couples. Survival analysis shows that place of residence, level of education, age at first marriage and marriage cohort affect the first birth interval.     

Fertility of couples following cessation of contraception

A retrospective study of the delay in conception after stopping different methods of contraception is undertaken among 5108 pregnant women residing in a departement in the Paris region. Particular attention is paid to the delay that sometimes separates the cessation of contraception and the beginning of intercourse without precautions. The counfounding factors considered are: the observation of a waiting period following cessation of contraception; the existence of a strategy, i.e., a coital pattern adopted by certain couples who have intercourse more often during the middle of the cycle, hoping to conceive more rapidly; the regularity of the menstrual cycle and the woman's age at the beginning of unprotected intercourse. The statistical techniques include chi-square analysis, analysis of variance correlation, partial multiple regression and actuarial rates of infecundity. The mean age of women at their last period was 26.3 years; 37.6% were primagravida; 87.3% worked outside the home and 36.6% were smokers. 72% of the women used oral contraceptives; 14% the IUD, and 14% other methods. Among all factors studied, 4 are strongly related both to fertility and to mode of contraception: the waiting period, coital strategy, regularity of the menstrual cycle and the woman's age. Shorter average time required to conceive (TRC) and greater fecundability are associated with the observation of a waiting period, no coital strategy, and regular cycles. The woman's age at the time of unprotected intercourse shows no correlation with TRC; it does, however, show a correlation with fecundability, which increases with age to reach a maximum in the age group 25-29 and then decreases progressively. These 4 factors are also related to the mode of contraception. Those using hormonal contraception were more likely to observe a waiting period. However, they report a coital strategy less frequently, their usual cycle is more often irregular and they are younger than IUD or other method users. Results demonstrate a decreased fertility for couples after the pill and, to a lesser degree, after IUD use, but only among those who did not take other precautions (waiting period). For couples observing a waiting period, there is no decrease of fertility following the initial exposure to the risk of conception. This comparison of return of fertility following cessation of different contraceptive methods shows that raw data must be interpreted with caution. It is essential to take into account the possibility of delay in becoming exposed to the risk of conception while various confounding factors must also be considerd.     

Sex ratio at birth, polygyny, and fertility: a cross-national study

The sex ratio at birth may reflect frequency of intercourse that affects the timing of conception. If so, cross-national variation in polygyny and fertility might account for country differences in secondary sex ratios. Consistent with the timing of intercourse hypothesis, the birth sex ratios of 148 countries declined with total fertility rates and polygyny intensity, and increased with contraception use in correlational analysis. Regression analysis confirmed that polygyny was a negative predictor of the sex ratio (and contraception was a positive predictor), with level of economic development and mother's age controlled, but the effects disappeared with total fertility added to the equation. The sex ratio evidently declines with increases in fertility because more children are born at a later birth order when frequency of intercourse is lower.     

Cigarette smoking and human papillomavirus in patients with reported cervical cytological abnormality.

OBJECTIVE--To assess the relation between two risk factors for cervical neoplasia: smoking and infection with oncogenic human papillomavirus. It has been suggested that smoking causes a local immunological defect, which could facilitate the infection and persistence of human papillomavirus. DESIGN--Cross sectional epidemiological study. Completion of a structured questionnaire by the patients, analysis of cervical scrapes for human papillomavirus, and morphological examination of biopsy specimens. SETTING--Outpatient gynaecological clinic. SUBJECTS--181 women with a report of cervical cytological abnormality. MAIN OUTCOME MEASURES--Prevalence of infection with oncogenic human papillomavirus and smoking habits. RESULTS--Oncogenic human papillomavirus was found in the cervix of 26 (41%) of the 63 women who did not smoke, 22 (58%) of the 38 who smoked 1-10 cigarettes a day, 28 (61%) of the 46 who smoked 11-20 cigarettes a day, and 26 (76%) of the 34 who smoked > or = 21 cigarettes a day. The prevalence of the virus thus increased in accordance with the number of cigarettes smoked (p = 0.001). This relation remained after adjustment for age at first intercourse and lifetime number of sexual partners. Of the 63 non-smokers, 23 had previously smoked at least 10 cigarettes a day at some time. Of these 23 women, 14 (61%) had oncogenic human papillomavirus in their cervix. Of the 40 women who had never smoked at least 10 cigarettes a day, 12 (30%) had the virus. The prevalence of oncogenic human papillomavirus in non-smokers therefore depended on previous smoking habits (p = 0.03). CONCLUSION--The dose dependent effect of cigarette smoking on the occurrence of oncogenic human papillomavirus favours a causal relation between these risk factors for cervical neoplasia.     

Reproductive Responses to Economic Uncertainty

In the face of economic and political changes following the end of the Soviet Union, total fertility rates fell significantly across the post-Soviet world. In this study we examine the dramatic fertility transition in one community in which the total fertility rate fell from approximately five children per woman before 1993 to just over one child per woman a decade later. We apply hypotheses derived from evolutionary ecology and demography to the question of fertility transition in the post-Soviet period, focusing on an indigenous community (Ust’-Avam) in the Taimyr Region, northern Russia. We employ a mixed parametric accelerated failure-time model that allows comparison of age at first birth, interbirth interval, and reproductive postponement or cessation prior to and following 1993. We find that short-term reproductive delay alone does not explain the dramatic drop in fertility in Ust’-Avam. Age at first birth remains constant. Interbirth intervals increase moderately. The estimated fraction of women who have ceased or indefinitely postponed reproducing doubles (for parities 2 through 4) or triples (for nulliparous women). We caution against assuming that environmental harshness necessarily leads to earlier and more rapid reproduction. An evolutionary theory of fertility responses to acute environmental shocks remains relatively undeveloped. In such contexts it is possible that selection favors a conservative reproductive strategy while more information is learned about the new environment. When investigating fertility responses to environmental stressors we suggest researchers examine postponement and stopping behavior in addition to changes in age at first birth and interbirth interval.     

Continuing fertility transitions in a plural society: ethnic trends and differentials in Peninsular Malaysia

Fertility in Peninsular Malaysia has declined continuously from the late 1950s, reaching a total fertility rate of 3735 in 1983. All ethnic groups in Malaysia have contributed to this modern demographic transition but the rate of change has been most rapid for Chinese and Indians, Malay fertility having reached a plateau in the early 1980s. The effect of age structure, marital patterns and marital fertility (by parity) on the fertility declines for each ethnic community are analyzed. There has been a tendency, in each ethnic group, for the age distribution within the group of reproductive-age women to grow younger, reflecting the entry into the younger reproductive ages of the large birth cohorts of the 1950s and early 1960s. The effect of this on crude birth rates is hard to determine, because rising age at marriage and increasing use of contraception meant that fertility was increasingly concentrated in the more central reproductive ages. By the 1990s, the earlier declines in fertility will bring about a decline in the proportion of the total population made up of females in the main reproductive ages. After that point, further declines in fertility will be reflected in a sharper decline in the crude birth rate and hence the rate of population increase. Between 1947 and 1980, the age at marriage changed dramatically for females of all ethnic groups. The transition to higher age at marriage for Chinese was completed earlier, and since 1970 has risen by only a year. For Malays and Indians, the rise began later, proceeded faster and continued right up to 1980 when the medium ages at 1st marriage were Malays 22, Indians 23, Chinese 24 years. In 1980, Malay women on average were marrying 5 years later, and Indian women 6 years later than had their mothers' generation in 1947. The proportion never-married among Malay and Indian women aged 20-24 rose from 1/10 to 1/2 over this period; relatively greater changes are evident at ages 25-29. Other factors are the almost complete shift from parent-arranged to self-arranged marriages. Family size desired has decreased for all groups and the decline in breastfeeding has been offset by the sharp increase in the practice of contraception. Continuation of these trends would lead to replacement-level fertility for Malaysian Chinese and Indians by the year 2000. Malay fertility is likely to continue to decline but at a more moderate pace.     

A synthetic biosocial model of fertility transition: Testing the relative contribution of embodied capital theory,changing cultural norms,and women's labor force participation

This article presents a biosocial model of fertility decline, which integrates ecological‐economic and informational‐cultural hypotheses of fertility transition in a unified theoretical framework. The model is then applied to empirical data collected among 500 women from San Borja, Bolivia, a population undergoing fertility transition. Using a combination of event history analysis, multiple regression, and structural equation modeling, we examine the pathways by which education responds to birth cohort, parental education and network ties, and how age at first birth and total fertility, in turn, respond to birth cohort, social network ties, education, expectations about parental investment, work, and contraceptive use. We find that in addition to secular trends in education, respondent's education is associated with the education of parents, the investment she received from them, and the education of older siblings. Total fertility has dropped over time, partly in response to increased education; moreover, the behavior of other women in a woman's social network predicts both initiation of reproduction and total fertility, while expected parental investment in offspring negatively predicts total fertility. Involvement in paid work that is incompatible with childcare is associated with a later age of first reproduction, but not subsequent fertility. Contraceptive use partially mediates the effect of education and birth cohort on total fertility, but is not a mediator of the effect of social network or expected parental investment on total fertility. Overall, the empirical results provide support for a biosocial model of fertility decline, particularly the embodied capital and cultural pathways. Am J Phys Anthropol 154:322–333, 2014. © 2014 Wiley Periodicals, Inc.     

Determinants of natural fertility in Sudan

The population of Sudan (North) is at a very early stage of fertility transition and experiences high, stable fertility at a close to natural level. The high observed fertility is found to be a function of the high proportion of married women and ineffective contraceptive procedures. The data used in this study were drawn from the 1979 Sudan Fertility Survey (SUDFS), in which 3115 ever-married women 50 years old from 12,028 households were interviewed. 90% of Sudanese women breast fed for at least 6 months and 80% for at least 12. The main inhibitor of fertility is perceived to be lactational amenorrhea averaging about 11.8 months, which is reported to be high, although among younger women the duration of amenorrhea is shorter due to earlier introduction of supplementary foods. An average of 5.2 months of postpartum sexual abstinence is evidenced, but this is shorter than the period of amennorhea, and therefore has no effect on the birth interval. Neither does marital instability or mean length of separation, which are both close to nonexistent. Sudanese women are comparatively very infecund. 22% gave birth within the 1st year of marriage, 62% within the 2nd, and 83% the 3rd. The proportion of ever-users of contraception is high (e.g. 15.4% among the 25-34 among category), but current use was low (8.1% for the same). The use of contraception is responsible for a reduction of 4.2% of the fertility per married woman. The duration of temporary separation between spouses, due to temporary or seasonal migration of husbands, and the pathological causes of high primary sterility, which contribute to the low fecundability, need further investigation.     

Craniosynostosis and maternal smoking

BACKGROUND: Several previous studies suggested increased risk of craniosynostosis among infants born to women who smoked. METHODS: This study used data from the National Birth Defects Prevention Study, a multi‐state, population‐based case‐control study of infants delivered from 1997–2003. Nonmalformed, liveborn controls were selected randomly from birth certificates or birth hospitals. Data from maternal telephone interviews were available for 531 cases and 5008 controls. RESULTS: Smoking during the first month of pregnancy was not associated with craniosynostosis. Smoking later in pregnancy was associated with increased risk, but only among mothers who smoked at least one pack/day. For example, during the second trimester, the odds ratio for smoking <5 cigarettes/day was 1.0 (95% confidence interval [CI] 0.6, 1.8), but the odds ratio (OR) for smoking 15 or more cigarettes/day was 1.6 (95% CI 0.9, 2.8), after adjustment for maternal age, education, race‐ethnicity, sub‐fertility, parity, folic acid supplement intake, body mass index, and study center. Among women who did not smoke, adjusted odds ratios suggested that secondhand smoke exposure at home, but not at work/school, was associated with modestly increased risk; the OR for home exposure was 1.3 (95% CI 0.9, 1.9). Results followed a similar pattern for some, but not all, specific suture types, but numbers for some groupings were small. CONCLUSIONS: The results suggest moderately increased risk of craniosynostosis among mothers who were the heaviest smokers and who continued to smoke after the first trimester. Results are somewhat equivocal, given that most confidence intervals included one. Birth Defects Research (Part A), 2008. © 2007 Wiley‐Liss, Inc.     

Rapid fertility decline in Iran: analysis of intermediate variables

The remarkable decline in fertility in Iran, which saw the total fertility rate fall from 7 children per woman in 1986 to 2 in 2000, has received only limited analysis in the demographic literature. Using the 2000 Iran Demographic and Health Survey and Bongaarts' age-specific fertility model, this paper examines the role of the major proximate determinants of fertility in bringing about the rapid decrease in fertility in Iran. The analysis indicates that contraception had the largest effect on fertility, accounting for 61% of the reduction in fertility from its theoretical maximum. The fertility-inhibiting effect of marriage patterns accounted for an additional 31% reduction, and was most important among the young. Further analysis of contraceptive behaviour suggests that the current period fertility rate of 2.0 children per woman is an outcome of a synchronization of delaying and spacing of births among younger women with stopping of childbearing among women in the middle and late reproductive ages. The policy implications of the results are discussed.     

An evolutionary model of stature, age at first birth and reproductive success in Gambian women

We have built a model to predict optimal age at first birth for women in a natural fertility population. The only existing fully evolutionary model, based on Ache hunter-gatherers, argues that as women gain weight, their fertility (rate of giving birth) increases-thus age at first birth represents a trade-off between time allocated to weight gain and greater fertility when mature. We identify the life-history implications of female age at first birth in a Gambian population, using uniquely detailed longitudinal data collected from 1950 to date. We use height rather than weight as an indicator of growth as it is more strongly correlated with age at first birth. Stature does not greatly influence fertility in this population but has a significant effect on offspring mortality. We model age at first reproduction as a trade-off between the time spent growing and reduced infant mortality after maturation. Parameters derived from this population are fitted to show that the predicted optimal mean age of first birth, which maximizes reproductive success, is 18 years, very close to that observed. The reaction norm associated with variation in growth rate during childhood also satisfactorily predicts the variation in age at first birth.     

Kin influences on fertility in Thailand: Effects and mechanisms

It has been suggested that human mothers are cooperative breeders, as they need help from others to successfully raise offspring. Studies working under this framework have found correlations between the presence of kin and both child survival and female fertility rates. This study seeks to understand the proximate mechanisms by which kin influence fertility using data from the 1987 Thailand Demographic and Health Survey (DHS), a nationally representative sample of 6775 women. Kin influence is measured by the length of time couples live with the husband's or wife's parents after marriage. Event history analysis, multilevel modeling and structural equation modeling are used to investigate both fertility outcomes and potential pathways through which postnuptial residence may influence fertility outcomes, including employment status, maternal and child outcomes, contraceptive use, breastfeeding duration, and age at marriage. We show that living virilocally (with husband's kin after marriage) increases total fertility by shortening time from marriage to first birth, and increasing the likelihood of progression to each subsequent birth. These effects are mediated through correlations between virilocal residence and earlier age at marriage as well as delayed initiation of contraceptive use. We find no influence of husband's kin on maternal or child outcomes. Living uxorilocally (with wife's kin after marriage) also reduces age at marriage, shortens time from marriage to first birth and (marginally) improves child survivorship, but has no effect on other child and maternal outcomes or progression to subsequent births and results in a similar number of living children as women living neolocally.     

The influence of female education,labor force participation,and age at marriage on fertility behavior in Bangladesh

Abstract

This paper studies the relationship between three aspects of female status (education, work experience, and age at marriage) and the use of contraception and fertility in Bangladesh. Education is found to be the variable most strongly correlated with use of contraception and is also one of the significant variables explaining fertility behavior. The most important factor explaining fertility behavior is age at marriage. The higher the age at marriage, the lower the fertility, when all other factors are held constant. Work experience has very little or no effect on current use of contraception and fertility.     

Changes in contraceptive use and fertility: El Salvador, 1978-88

In El Salvador from 1978 to 1988, contraceptive use among married women 15-44 years of age increased from 34% to 47%, and the total fertility rate declined from 6.3 to 4.6 children per woman. Most of this change took place from 1978 to 1985. Sterilization is the most prevalent method used, but nearly one-half of the women who are sterilized did not use any contraception before their operation. Few young couples use reversible methods of contraception to space births or delay the start of childbearing. On average, women wait 8 years after marriage and have nearly three children before they use contraception.     

Menstrual Cycle Phase Does Not Predict Political Conservatism

Recent authors have reported a relationship between women''s fertility status, as indexed by menstrual cycle phase, and conservatism in moral, social and political values. We conducted a survey to test for the existence of a relationship between menstrual cycle day and conservatism.2213 women reporting regular menstrual cycles provided data about their political views. Of these women, 2208 provided information about their cycle date, 1260 provided additional evidence of reliability in self-reported cycle date, and of these, 750 also indicated an absence of hormonal disruptors such as recent hormonal contraception use, breastfeeding or pregnancy. Cycle day was used to estimate day-specific fertility rate (probability of conception); political conservatism was measured via direct self-report and via responses to the "Moral Foundations” questionnaire. We also recorded relationship status, which has been reported to interact with menstrual cycle phase in determining political preferences.We found no evidence of a relationship between estimated cyclical fertility changes and conservatism, and no evidence of an interaction between relationship status and cyclical fertility in determining political attitudes. Our findings were robust to multiple inclusion/exclusion criteria and to different methods of estimating fertility and measuring conservatism. In summary, the relationship between cycle-linked reproductive parameters and conservatism may be weaker or less reliable than previously thought.     

Fertility and family planning in Accra

A sample of 1000 currently married women aged 25-39 and living in 3 geographic areas of the Ghanaian capital were interviewed about their contraceptive knowledge and practice. Slightly over 90% of the respondents said that they had heard of ways to delay or prevent pregnancy. There was no difference between the 3 areas, but more of those aged 30-34 had heard about contraception. The chances that a respondent had heard about contraception increased with the level of education. Catholics were less likely to have heard of any contraceptive methods than Protestants. Also, women engaged in traditional informal occupations were the least likely to have heard of any contraceptive methods. Overall, education emerged as the most important differentiating factor. The pill, condom, and IUD were the best known methods. Regarding actual contraceptive practice, only 41% of the respondents were currently using some form of contraception, with about 19% using modern and 27% traditional methods. Results of contraceptive usage by user characteristics show that the majority of women in 2/3 of the areas rely on a mixture of traditional and modern methods; that the % of users (any method) increases with age up to 30-34 years, after which it decreases. The % of ever-users of any type of contraception in this study is highest for the highest education levels, at 84% for women with 2ndary and post 2ndary education. With the exception of the IUD, douche and folklore methods, better educated women have higher ever-use %s for both traditional and modern methods. While the %s of users increases with education for foam, diaphragm and withdrawal; the reverse is observed for the pill and IUD, probably due to fear of negative side effects. Overall, there are no significant variations in contraceptive use by migration status; but the % of non-users is highest among recent migrants who are also the least likely to have ever used modern methods. Fertility differentials across ethnic groups have been observed in Ghana. The Akan have the highest fertility, followed by the Ewe and Ga-Adangbe with moderate fertility, and the northern ethnic groups with the lowest fertility. The Akan also have the highest % of ever users of modern contraceptives, an inconsistency which may be explained by their recent recognition of the burdens of high fertility and their attempts to control it. Religious differences are pronounced, especially between the traditional category on the 1 hand, and Christians and Muslims on the other. Results of an analysis of variance suggests that family size is the strongest motivation for adopting modern contraception, and that it may be used for limiting rather than spacing births.