The seasonality of conception

Individual data on menstrual cycles of noncontracepting women living in Western countries were used in order to verify whether the biological seasonality of conception persists after sexual behavior is controlled for. Episodes of intercourse were recorded daily, and the time of ovulation was detected by a marker We find that the seasonality of conception changes with woman's age and frequency of episodes of sexual intercourse. In particular; for women aged 27-31 having only one act of intercourse during the six most fertile days of the menstrual cycle, the seasonality of fecundability is stronger In this age group in the Northern Hemisphere, if seasonality of acts of sexual intercourse is controlled, the monthly distribution of probability of conception is bimodal, with two maxima (September and January) and two minima (December and March). When unobserved characteristics of the couples are considered, this seasonal pattern of conception persists.     

Incorporating heterogeneous intercourse records into time to pregnancy models

Information on the timing of intercourse relative to ovulation can be incorporated into time to pregnancy models to improve the power to detect covariate effects, to estimate the day-specific conception probabilities, and to distinguish between biological and behavioral effects on fecundability, and therefore the probability of conception in a menstrual cycle. In this paper, Bayesian methods are proposed for joint modeling of intercourse behavior and biologic fecundability. The model accommodates a sterile subpopulation of couples, general covariate effects, and heterogeneity among fecund couples in menstrual cycle viability and in frequency of unprotected intercourse. Methods are described for incorporating cycles with varying amounts of intercourse information into a single analysis. A Markov chain Monte Carlo algorithm is outlined for estimation of the posterior distributions of the unknowns. Themethods are applied to data from a North Carolina study of couples attempting pregnancy.     

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.     

生育率变动的综合分析

生育率的变动是各种因素综合影响的结果，这些因素直接和间接作用于生育过程的四个主要环节—性交、受孕、妊娠、分娩，最终影响着生育率的变动。     

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.     

Mirror Rubbing: A Critical Genealogy of Pre-Modern Chinese Female Same-Sex Eroticism

This article offers a critical genealogy of pre-modern Chinese female same-sex relationships. Through the analysis of the primary source materials in history, fiction, and drama, the author shows that female homosexuality is silenced and suppressed. To Confucianism, female same-sex relationships threaten to exclude men from accessing female sex and keep women away from participating in extending the family line. Even the Daoist theory of sex can be used to discriminate against female homosexuality by denying women the ability to initiate and maintain the cycle of yin-yang interaction in sexual intercourse. There are 2 recurring themes in the male writers' imaginings of female same-sex eroticism. First, heterosexuality is the preferred sexual order, and female same-sex desire arises due to the lack of sexual access to men. Second, heterosexual relationships and intercourse are the norm that female homosexuality aspires to imitate.     

Variations in Natural Fertility: The Effect of Lactation and Other Determinants

The analysis focuses on average waiting times to a fertile conception, as derived from non-contraceptive exposure in second and higher-order birth intervals. Life-table estimates are derived from exposure in the two-year periods preceding the survey, for 20 surveys in Africa, Asia and Latin America undertaken as part of the WFS programme. Differences in the waiting time to conception are examined as a function of the duration of lactation and post partum abstinence. In addition, the extent to which variations in waiting times are produced by country and regional effects, and effects due to age, duration of marriage and parity are examined. The analysis points out the dangers of deriving estimates of natural fertility from the sub-group of women who never breastfed.     

Postamenorrheic versus postpartum strategies of contraception

The postpartum strategy of inserting an intra-uterine device shortly after a birth essentially eliminates the risk of conceiving again before starting contraception but maximizes the overlap between wearing time and postpartum anovulation when protection is not needed. In contrast, the postamenorrheic strategy of delaying insertion until right after the woman’s first menses all but removes overlap with anovulation but allows a chance of conception before start of contraception because sometimes an ovulation precedes the first menstruation. In this paper some algebra is developed and utilized to see which of the two strategies delays the next conception longer. The postamenorrheic strategy is found to have a slight advantage over the postpartum approach for a wide range of fecundability levels, lengths of anovulation, and rates of continuation of IUD. However this slight advantage presupposes that insertions are taking place at the exact time prescribed. When a progressively larger factor of procrastination is introduced, the advantage rapidly passes from the postamenorrheic to the postpartum approach. An explanation for the differing sensitivity of the two insertion strategies with respect to procrastination is derived from the results of an earlier analysis.     

The length of the human fertile period

Estimates of the. length of the human fertile period (that interval within the menstrual cycle during which insemination is associated with a non-zero probability of conception) are necessarily indirect. Moreover, those estimates do not seem in close agreement with one another. For instance, it has been suggested that:     

The length of the human fertile period

Abstract Estimates of the. length of the human fertile period (that interval within the menstrual cycle during which insemination is associated with a non-zero probability of conception) are necessarily indirect. Moreover, those estimates do not seem in close agreement with one another. For instance, it has been suggested that:     

Estimates of fecundability from a truncated distribution of conception times

A population consisting of women who have conceived before a time t ignores all women whose conception time exceeds t. Such a population is considered as a truncated population, and the samples are called truncated samples. Under the assumption that fecundability among women varies according to a Beta distribution (with parameters a and b), the distribution of conception times in a truncated population can be considered as truncated Type I geometric. This paper presents an algorithm to obtain the moment and maximum likelihood estimates of a and b from the truncated samples. Large sample properties of the estimators are also studied. Examples using the Hutterite and the Princeton Fertility Survey data are given.     

A modification of a continuous time model for first conception

The interval between marriage and the first conception leading to a live birth plays an important role in the determination of fertility components. Several probability models (Potter and Parker, 1964; Singh, 1961, 1964, 1967) based on varying sets of assumptions relating to this interval have been propounded in the recent past. All of them are based on the assumption that the females under study are susceptible to conception at the time of marriage. However, in certain situations, where some of the females already pregnant at the time of marriage report to have conceived within a short interval following marriage, this condition is not satisfied and these models become unsuitable. A probability distribution which is an inflated form of the continuous model proposed by Singh, for the time of the first conception leading to a live birth, is presented in this paper. It describes reasonably well the data on first conception times in the context of premarital conceptions. Simple expressions for the maximum likelihood estimators of the parameters involved in the model are obtained and a method for finding the asymptotic covariance matrix of the estimators is outlined. The model is applied to four sets of data.     

Age-old methods of contraception

Folk methods of contraception have been used in Philippines from immemorial times, especially in the rural areas. Some methods seem to be practical while others are mystical. The popularity of methods are due to their low cost and the trust women in the community feel for the traditional healer. Herbs are the most commonly method used. For external application, pounded and heated leaves are placed inside a bag. Traditional practitioners believe that when the "charm" is worn during intercourse, the semen will "dropout" of the vagina. For internal application a beverage is prepared with seeds of the plant and drunk before sexual intercourse. For contraceptive purposes other herbs used in rural communities could induce abortion, which is illegal in Philippines. A projected study by the University of the Philippines' College of Medicine will try to establish the pharmacological effectiveness of local plants to determine the validity of some methods of family planning reported by medical students who have talked to rural herbalists and patients. The one which seems most relevant for this study is the herb Kibatalia Gitingesis, which is supposed to contain "progesteronal-like principles." Other methods of contraception are abdominal massage, which involves manual manipulation of the uterus, chemicals, like asoge (mercury), salt, and tablets, and the use of inanimate objects, such as the bato-balani or magnetic stone. If herbs and other folk methods of contraception are proven to be effective, the possibility to have them incorporated into the National Population Program has to be considered. The untrained hilots (nurse midwives) could become trained program workers, and because of their influence in the community the number of acceptors would increase.     

Premarital first births: The influence of the timing of sexual onset versus post-onset risks in the United States

Motivated by long-standing debates between abstinence proponents and sceptics, we examine how socio-economic factors influence premarital first births via: (i) age at first sexual intercourse and (ii) the risk of a premarital first birth following the onset of sexual activity. Factors associated with an earlier age at first intercourse will imply more premarital first births owing to increased exposure to risk, but many of these same factors will also be associated with higher risks of a premarital first birth following onset. Our analyses confirm previous findings that women from disadvantaged backgrounds are younger at first intercourse and have higher premarital first-birth risks than women from more advantaged backgrounds. However, differences in onset timing have a strikingly smaller influence on premarital first-birth probabilities than do differences in post-onset risks. Our findings thus suggest that premarital first births result primarily from differences in post-onset risk behaviours as opposed to differences in onset timing.     

Sexual violence at first intercourse against women in Moshi, northern Tanzania: prevalence, risk factors, and consequences

To explore the relationship between sexual violence at first intercourse and later sexually transmitted infections (STIs) in Moshi, Tanzania, we analysed data from a representative household survey that comprised face-to-face interviews with 1,835 women and tests for six STIs on biological samples from 1,235 of these women. Overall, 10.9 per cent report forced first intercourse and 15.3 per cent report unwanted first intercourse. Unadjusted analysis shows a relationship between forced first intercourse and STIs (OR: 1.72, 95 per cent CI: 1.19-2.51). Life-course variables mediate this relationship. Significant predictors of having an STI include older age, more sexual partners, and a partner who has children with other women. Coerced first intercourse appears to be associated with changes in the life course of women and with a heightened risk of contracting an STI.     

社会网络视角下中国农村成年男性初婚风险的影响因素分析

在婚姻挤压背景下,男性拥有较多的社会网络资源是否意味着他们有更多结识异性和获得应急性经济支持的机会,从而增加了其初婚的概率?利用2008年西安交通大学人口与发展研究所在安徽X县调查的数据,从社会网络角度出发,采用事件史分析方法分析影响18~50岁农村男性初婚风险水平的因素。分析结果表明,当前农村男性的初婚年龄主要集中在22~27岁之间,27岁以上仍未结婚的男性,其成婚的概率急剧下降,成为婚姻市场的弱势群体;男性自身的特征和资源拥有状况,包括婚前社会网络、个人经济和非经济特征、家庭和社区因素,往往决定着个人的初婚风险水平,一般来说,男性自身特征和资源拥有状况越差,结婚的可能性越小。     

The contraceptive potential of early versus delayed insertion of the intrauterine device

One argument for a postpartum program of contraception is minimizing chances that the mother will conceive again before she can accept contraception. However early insertion increases the extent of overlap between retention of the device and anovulation when the woman is protected anyway while at the same time reducing the average span of wearing time coinciding with fecundable months when protection is needed. Thus it is not clear that early insertion yields the maximal postponement of next conception. Delaying insertion three or even six months might accomplish more. To investigate this matter, a model has been developed which incorporates the following factors: time of insertion, distribution of anovulatory length, natural fecundability and rates of effectiveness and continuation of IUD. It turns out that predicated upon the distribution of anovulatory length regarded as most realistic, the penalty of early insertion in terms of reduced postponement of next conception proves consistently small.     

Factors Associated With Emotional Satisfaction During First Anal Intercourse in a Sample of YMSM

ABSTRACT

We sought to determine, in a sample of 2,813 HIV seronegative young men who had sex with other men age 18–24, whether physical satisfaction would explain emotional satisfaction during first anal intercourse and whether emotional satisfaction would impact having sex with a partner a second time. Emotional satisfaction was explained mostly by physical satisfaction during the event, but partner type also had direct impact on emotional satisfaction. Our findings suggest that first anal intercourse experiences in young men are both emotionally and physically satisfying and may impact subsequent sexual behavior and partner decision-making.