A Model of Age-specific Fecundability

A new model of the behavioural and physiological causes of age-specific variation in marital fecundability is presented. Total fecundability is decomposed into a series of susceptibility factors (the length of ovarian cycles, the length of the fertile period within each cycle, the probability that a cycle is ovulatory, and the likelihood that an act of unprotected intercourse within the fertile period results in conception) and an exposure factor reflecting the effect of duration of marriage on coital frequency. The impact of intra-uterine mortality on effective fecundability is also modelled. Data on western women, from which standard age curves of fecundability are estimated, suggest that any decline in fecundity between ages 30 and 40 is attributable to changes, not in the ability to conceive, but in the capacity to carry a pregnancy to term. Sensitivity tests suggest that the most important potential sources of inter-population variation in fecundability are intra-uterine death and the incidence of anovulatory cycles.     

Socio-economic correlates of fecundability in a sample of taiwanese women

Estimates of fecundability (monthly probability of conception) in the absence of contraception are derived from the frequency distribution of conceptive delays immediately following marriage, reported by 2,443 married women aged 20 to 39 included in the Taichung (Taiwan) Intensive Fertility Survey of 1962. Average fecundability of women is positively associated with their socio-economic status. These differentials are not accounted for by differences among socio-economic groups with respect to memory and truncation biases (associated with the marriage duration), wife’s age at marriage, or unreported premarital conceptions. A Multiple Classification Analysis suggests that among the socio-economic characteristics, husband’s education, rural background, and modern family type are the more important predictors of fecundability. The importance of genetic factors as opposed to cultural factors in producing these socio-economic differences in fecundability can not be evaluated systematically. Moreover, the relation of a couple’s privacy, their attitude toward family building, and patterns of mate selection to their socioeconomic status would have to be taken into account before the differences in fecundability could be attributed to factors such as nutrition, health, or infections which might directly influence their physiological ability to conceive.     

A method for the estimation of fecundability

A new method for the estimation of the mean and variance of fecundability is described. The data input required for this procedure is the distribution of the interval from marriage to first birth, or from the resumption of the conception risk after contraction to the subsequent birth. The estimates of the mean and variance of fecundability are obtained by fitting a model to the observed interval distribution. To test the method, it is applied to data from five historical populations. The fecundability means in these populations ranged from 0.18 to 0.31 while the co-efficients of variation all had values near 0.56. A short method for the estimation of the mean of fecundability based on the same model, but not requiring a computer, is also presented.     

Effects of various factors on selection for family planning status and natural fecundability: A simulation study

The effect of various factors on selection for family planning status and for natural fecund ability is studied in a simulation that incorporates a beta distribution of fecundability among women. The mean fecundabilities of current spacers, current limiters, current nonusers, and pregnant women are compared. These ratios are influenced by duration of marriage and by desired number of children. Effects of different levels of contraception are measured. Another strategy, spacing the last two wanted births, is also studied. It is found that breast-feeding status has little effect on fecundability selection. The women usually found to have greatest fecundability are not those recently pregnant at durations of marriage five and ten years, unless they wanted fewer than two children, but rather those who are using contraception to limit the number of children.     

Pregnancy outcome and the time required for next conception

Abstract Variations in the time required for next conception by outcome ofthe preceding pregnancy, and the age of woman, are studied for Taiwanese women. Pregnancy interval, defined as the period between the end of one pregnancy and the beginning of the next, is taken as a measure of the time required for the next conception. The averages and variances of pregnancy intervals are estimated from reports of pregnancies occurring in a probability sample of 2,443 married women, aged 20-39, living in Taichung (Taiwan) in 1962. The effects of 'truncation bias' and 'memory bias' on the two moments of pregnancy intervals are estimated indirectly by cross-classifying women according to their ages at interview and their ages at the beginning (or end) of each pregnancy interval. The moments of post-partum amenorrhoea are, then, estimated indirectly from the moments of pregnancy intervals. The effects of truncation bias and memory bias are compensatory in this sample. In the absence of contraception, women, on an average, took longer to conceive following a live birth than following a foetal death. The time taken for next conception increases as women become older irrespective of the outcome of the preceding pregnancy.     

A non‐linear model of fecundability,postpartum amenorrhea,and sterility*

This paper proposes a convolution model of fecundability, controling for the effects of postpartum amenorrhea and unobserved heterogeneity in fecundability. Simulation analysis was used to assess the validity and reliability of estimates derived from the model. Analysis showed that the model captured the mean and standard deviation of age at the onset of sterility in simulated populations where sterility followed either a Gompertz, a gamma, or a lognormal distribution. The model performed well when sterility was specified by either a lognormal or a gamma distribution. The model also accurately estimated fecundability and postpartum amenorrhea. Next, the model was found to fit data from 17th and 18th century French Canadian birth histories. In this French Canadian sample the mean age at sterility was found to be 46.3 years using a gamma model. The decline in fecundability was almost linear after age 30. Thus, fecundability at age 40 had declined to about one‐third of that observed at age 30. Variability in individual fecundability was quite high. For example, women with fecundability one standard deviation above the mean had about 2.3 times as high fecundability as women one standard deviation below the mean.     

Hutterite fecundability by age and parity: Strategies for frailty modeling of event histories

Effective fecundability declines with age and parity. Furthermore, women differ in their effective fecundability: some women have persistently low or high monthly chances of live-birth conception. Estimates are presented concerning the magnitude of these effects in a natural-fertility population: 406 Hutterite women in North America who had 3,206 births, largely in the 1940s and 1950s. The estimates are based on models that incorporate the effects of persistent heterogeneity and that use the full information provided by multiple-spell duration data. In addition, hazards rather than probabilities are modeled, piecewise linear hazard functions are used, and age and parity effects are decomposed systematically. These methods permit the development of more elaborate models of changing fecundability and of heterogeneity in postpartum amenorrhea.     

中国女性初婚模式变动趋势研究

20世纪80年代以来我国女性的初婚模式发生了显著的变化。本文使用人口普查资料、全国1%人口抽样调查数据、人口变动情况抽样调查数据以及IPUMS数据,通过女性平均初婚年龄、曾婚比例、年龄别初婚概率、终身结婚期待率和预期单身寿命等指标探究我国女性自20世纪80年代以来的初婚模式变动情况。研究发现30多年来我国女性平均初婚年龄在波动中上升,到2017年女性平均初婚年龄已经达到25.60岁,而教育程度的提高会推迟女性进入婚姻的时间,接受过高等教育的女性平均初婚年龄明显高于未受过高等教育的女性;另外,通过对各教育程度平均初婚年龄标准化与分解看到随着时间的推移,教育对女性的平均初婚年龄影响作用增大;20-30岁年龄段女性婚姻推迟明显,曾婚比例不断降低,但女性终身未婚比例很低,其中受过高等教育的女性婚姻推迟现象最为明显,但其自身的结婚意愿并未降低,大部分女性只是推迟结婚时间,并不是不结婚。对净婚姻表各指标进行计算发现1982-2010年女性的年龄别初婚概率下降,尤其在20-30岁年龄段下降明显,初婚峰值年龄推迟,结婚年龄集中现象减弱。终身结婚期待率下降速度趋缓,随着女性初婚年龄的推迟,2010年27岁之后的终身结婚期待率要高于1990年与2000年,29-35岁女性的预期单身寿命也较前30年低,较大年龄未婚女性结婚等待时间缩短。     

A dynamic model of the reproductive process

Summary A dynamic deterministic model of the reproductive process is presented. The model describes and analyses the effect of intermediate fertility variables on fertility. The intermediate fertility variables which are the inputs to the model, include the duration of post-partum amenorrhea, fecundability, incidence of spontaneous and induced abortion, contraceptive use and effectiveness, the distribution of age at first marriage and the age specific risks of marital disruption and remarriage. To test the validity of this model, it is fitted to data obtained from reproductive histories of 512 marriages occurring during the first half of the eighteenth century in Canada.     

A vital statistics-based procedure for estimating conception rates

A method for estimating conception rates, using vital statistics data, is developed and applied to data on five-year age groups of California women for 1971. The approach is deterministic and allocates total exposure time to the known pregnancy outcomes of live birth, spontaneous abortion, and induced abortion. The population at risk is defined to exclude women who are known to be sterile or sexually inactive. Early fetal loss, premarital conception, and contraceptive use are taken into account. Estimates are made of the fecundability which would obtain ifno contraception were used.     

The Effects of Breastfeeding and Nutrition on Fecundability in Rural Bangladesh: A Hazards-Model Analysis

Data taken from the ‘Determinants of Natural Fertility’ study, Bangladesh, are used with multivariate hazard models to study variations in fecundability between women, especially the relationship between nutritional status, breastfeeding practices, and the monthly probability of conceiving. It is found that fecundability varies both between women of a given age and, for a particular woman, by age. The variation is related to four variables: separation, which affects coital frequency; age, which represents biological changes; lactation practices; and the duration of amenorrhoea. Nutrition above famine or starvation levels is not a significant determinant of fecundability. The most surprising finding is the effect of the pattern of breastfeeding on fecundability: as a menstruating woman begins to wean her child, her probability of conceiving increases as her serum prolactin, which inhibits ovulation, decreases. Thus, those menstruating women who are most likely to conceive are those who have completely weaned their infants in the very recent past.     

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)     

Proximate determinants of Philippine fertility: 1983 levels and patterns

Data from the 1983 National Demographic Survey are used to analyze the proximate determinants of Philippine fertility in each of the 3 stages of family formation and to identify all of the direct and indirect factors affecting fertility levels and trends. 10,843 ever-married women and 12,771 children were included. The analysis pertains first to the starting patterns of family formation, the age at first birth, and the proximate determinants (age at menarche, age at first marriage/union, conception before first birth, fetal wastage first birth, interval between first marriage and first birth). Further analysis examines birth spacing patterns including the postpartum nonsusceptible period, the exposure interval and stopping patterns. Almost all births occur within marriage, and childbearing begins late at 22.5 years. However, 15.4% of first births are conceived premaritally. The mean age at first birth increases from younger to older cohorts. Urban women were slightly older (23.0 years) at the birth of their first child. Those with education below the 4th grade had first births 3.5 years earlier. Contraceptive use was low at 1.8% before first birth. Younger cohorts were more likely to use birth control and urban wives were more likely to use it than rural wives. 6.4% reported a first pregnancy ending in nonlive births, which were primarily spontaneous abortions (5.2%), stillbirths (1.0%), and induced abortions (.2%). 5.8% report never having been pregnant and 1.1% never having given birth to a live-born child. 20.4% were childless between the ages of 15-24 years, and 4.6% between 25-34 years. Childlessness was slightly higher among urban women (7.1%) than rural women (6.7%). A decreasing age at menarche has appeared; i.e., 13.6 years for the cohort 15-24 years, and 14.0 for the oldest cohort. By age 15, 82.9% had begun menstruating. The mean age at marriage is early at 20.7 years, and older cohorts tended to marry later at 21.4 years. Urban women marry a year later (21.4 years) than rural women. Lower educated women marry 4 years earlier. The mean length between first marriage and first birth was 18.4 months. In the younger cohorts, spacing patterns are shorter. Postpartum susceptibility is short. Return to sexual relations after a birth occurred at 2.8 months. The exposure time required to conceive is fairly long at 16.6 months and is attributed to contraceptive use, since coital frequency is high and temporary separation is infrequent. The average age at last birth is late at 37.6 years.     

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.     

On some probability distributions for forward birth interval

Statisticians from Varanasi, India have proposed 2 mathematical models for forward birth interval which are more realistic than past models. For example, past models have assumed fecundability to be constant throughout a woman's reproductive life. On the other hand, the proposed models assume fecundability changes with parity. Further the statisticians use all types of marital durations making their models usable even for short marriage durations. The 1st model uses a parity specific model to develop a model for forward birth interval regarless of parity. The statisticians also include moments of the 1st and 2nd models as expressed for the rth order raw movement of these models to estimate some of their parameters. These models can be used to conduct prospective surveys of family planning programs since birth intervals can best evaluate the effect of these programs. These models have not yet been advanced enough to follow women over a fixed period, however. They only observe women until the last woman conceives. The statisticians do not apply the models to parity specific data because they were not available. Yet they do run the models with parities 0 and 1. The models reveal that the mean length of forward birth interval is strongly influenced by the conception rate which the woman has at the time of the survey. Thus, if women with a certain parity in a community begin an effective family planning method on the survey day, which in effect would reduce the prevailing conception rate, that method would significantly postpone the next live birth.     

A Multistate model of fecundability and sterility

This paper develops a multistate hazards model for estimating fecundability and sterility from data on waiting times to conception. Important features of the model include separate sterile and nonsterile states, a distinction between preexisting sterility and sterility that begins after initiation of exposure, and log-normally distributed fecundability among nonsterile couples. Application of the model to data on first birth intervals from Taiwan, Sri Lanka, and the Amish shows that heterogeneity in fecundability is statistically significant at most ages, but that preexisting sterility and new sterility are unimportant before age 40. These results suggest that sterility may not be an important determinant of natural fertility until later reproductive ages.     

The initiation of contraception

Using data from the 1970 National Fertility Study, the trend in the initiation of contraception prior to the first pregnancy is examined. This trend is of interest because of its relationship to the tempo of familybuilding. Using data from a recent marriage cohort, it is shown that use before the first pregnancy is related to age at marriage, age at termination of first pregnancy and the probability of having a premarital conception. For women first married during the decade of the 1960’s, there was a substantial increase in the proportion using contraception before the first pregnancy. This trend is found among both white Catholics and white non-Catholics, but not among blacks. An examination of the specific method used by women using contraception before the first pregnancy reveals a shift from reliance on the traditional methods of diaphragm, condom and douche by the earlier birth cohorts to the use of the pill by the more recent cohorts. An appendix examines the reliability of various measures of the interval of first use of contraception.     

An analytic simulation model of human reproduction with demographic and biological components

Abstract This Monte Carlo model for simulating the reproductive history of a cohort of women is described in detail. The model provides for patterns of survival, sterility, formation and dissolution of sexual unions, fecundability, lactation, foetal wastage, family planning practices etc. Natality indices specific for marital status, for duration of marriage and for age, as well as analyses of birth spacing patterns are among the results that may be obtained. In the model, the experimental unit is an individual woman. The complete life history of a woman is generated and recorded before the history of the next woman is generated. The data for the whole cohort are analyzed at the end of the programme. The model includes two kinds of states into which a woman may pass, namely: (1) permanent changes of status such as death, sterility, or becoming a family planner, and (2) temporary states, each with a probability distribution of length of stay. The probabilities of the various events or changes of state may vary from age, parity, and other features of a woman's status or history. Natural fecundability at any age may also vary from woman to woman. In this programme natality patterns and specific indices such as age-specific fertility rates are produced, in a quasi-realistic fashion, by the interplay of the demographic and biological parameters postulated for any cohort. Consequently, the effect of changes in anyone factor can be studied, as well as the interaction resulting from changes in several factors. The purposes and potentials of the model are both substantive and methodological. As an illustration, a series of computer runs attempting to simulate the reproductive patterns of Indian women is presented. These results, as well as some additional ones, indicate some effects of changes in marital patterns, levels of fecundability, duration of post-partum non-susceptibility, age incidence of sterility and foetal wastage. In the final section of the paper, the advantages and possible applications of the model are discussed together with the limitations encountered to date in the efforts to apply the model.     

Net delay of next conception by contraception: A highly simplified case

Abstract A probability model, building on the work of Perrin and Sheps, is presented and applied. The model makes it possible to follow a cohort of women from a pregnancy outcome to next conception. Principal simplifying assumptions are homogeneity among women and constant fecundity. Cases treated include no contraception; a single contraceptive method practised; and a second contraceptive method practised following a first one. Expected durations to next conception may be calculated in relation to the following factors: outcome of previous pregnancy, length of anovulation, natural fecundability, time first contraceptive method is initiated, characteristics of first contraceptive (effectiveness and continuation rate), characteristics of second contraceptive, and gap between termination of first and commencing second contraceptive. By suitably pairing runs of the model, one can construct experiments in which the only differentiating factor is use and non-use of a specified contraceptive, and, by differencing the corresponding mean durations to next conception, compute the net delay of next conception produced by that contraceptive.     

Cohort Trends in Premarital First Births: What Role for the Retreat From Marriage?

We examine cohort trends in premarital first births for U.S. women born between 1920 and 1964. The rise in premarital first births is often argued to be a consequence of the retreat from marriage, with later ages at first marriage resulting in more years of exposure to the risk of a premarital first birth. However, cohort trends in premarital first births may also reflect trends in premarital sexual activity, premarital conceptions, and how premarital conceptions are resolved. We decompose observed cohort trends in premarital first births into components reflecting cohort trends in (1) the age-specific risk of a premarital conception taken to term; (2) the age-specific risk of first marriages not preceded by such a conception, which will influence women’s years of exposure to the risk of a premarital conception; and (3) whether a premarital conception is resolved by entering a first marriage before the resulting first birth (a “shotgun marriage”). For women born between 1920–1924 and 1945–1949, increases in premarital first births were primarily attributable to increases in premarital conceptions. For women born between 1945–1949 and 1960–1964, increases in premarital first births were primarily attributable to declines in responding to premarital conceptions by marrying before the birth. Trends in premarital first births were affected only modestly by the retreat from marriages not preceded by conceptions—a finding that holds for both whites and blacks. These results cast doubt on hypotheses concerning “marriageable” men and instead suggest that increases in premarital first births resulted initially from increases in premarital sex and then later from decreases in responding to a conception by marrying before a first birth.