Effect of fertility on secondary sex ratio and twinning rate in Sweden, 1749-1870.

We analyzed the effect of total fertility rate (TFR) and crude birth rate (CBR) on the number of males per 100 females at birth, also called the secondary sex ratio (SR), and on the twinning rate (TWR). Earlier studies have noted regional variations in TWR and racial differences in the SR. Statistical analyses have shown that comparisons between SRs demand large data sets because random fluctuations in moderate data are marked.

Repeated multiple maternities in triplet families.

In earlier studies, scientists have attempted to identify genetic and environmental factors affecting the rate of multiple maternities among humans. We contribute to these studies by analysing the frequencies of multiple maternities in sibships containing triplets. Use of the Hellin transformation is included in evaluation of the triplet rate.

Birth weight among single and multiple births on the Åland islands.

Numerous papers have investigated the distribution of birth weight. This interest arises from the association between birth weight and the future health condition of the child. Birth weight distribution commonly differs slightly from the Gaussian distribution.

The international biological program/human adaptability studies among the Skolt Sami in Finland (1966-1970).

Background: The population is increasingly lighter pigmented moving in a northward direction in Europe until reaching the Arctic Circle, where the Samis (Lapps) are clearly more pigmented.

Methods: In 1966-1970, we investigated a total of 689 subjects in the villages of Sevettijärvi and Nellim, including persons with mixed Sami and Finnish heritage; of these, 487 (242 males, 245 females) had both parents classified as Skolt Sami. For estimation of the colour of the iris and hair, international scales were used.

Temporal trends in the secondary sex ratio in Nordic countries.

Attempts have been made to identify factors influencing the number of males per 100 females at birth, also called the secondary sex ratio. It has been proposed to vary inversely with the frequency of prenatal losses, but available data lend at best only weak support for this hypothesis. Statistical analyses have shown that comparisons between secondary sex ratios demand large data sets.

Secondary sex ratio in multiple births.

Attempts have been made to identify factors influencing the sex ratio at birth (number of males per 100 females). Statistical analyses have shown that comparisons between sex ratios demand large data sets. The secondary sex ratio has been believed to vary inversely with the frequency of prenatal losses.

Spatial variation in the twinning rate in Sweden, 1751-1850.

Strong geographical variations in the twinning rate have been presented in the literature. In general, the rate is high among people of African origin, intermediate among Europeans and low among most Asiatic populations. In Europe, a progressive increase has occurred in the twinning rate from south to north, with a minimum around the Basque provinces.

Temporal and regional variations in the seasonality of births in Aland (Finland), 1653-1950.

Monthly numbers of births are influenced by the length of the month and, thus, rates per day should be used. Comparisons of seasonality in data sets of different sizes must be based on standardized indices. Although strong seasonality exists, a poor model may incorrectly yield low seasonality measures.

Statistical analyses of Hellin's law.

During the history of research on multiple maternities, Hellin's law has played a central role as a rule of thumb. It is mathematically simple and approximately correct, but shows discrepancies that are difficult to explain or to eliminate. It has been mathematically proven that Hellin's law does not hold as a general rule.

On the history of Hellin's law.

Theorems, proofs, laws and rules are commonly named according to the presumed investigator, but often earlier investigators have contributed substantially to the findings. One example of this is Hellin's law, which was named after Hellin, although he was not the first to discover it. In research on twinning and higher multiple maternities, the law has played a central role because it is approximately correct, despite showing discrepancies that are difficult to explain or eliminate.

Maternal age and temporal effects on stillbirth rates.

Stillbirth rates among single and multiple births show markedly decreasing temporal trends. In addition, several studies have demonstrated that the stillbirth rates are dependent on maternal age, in general, showing a U- or J-shaped association with maternal age. In this study, the temporal trends in and the effect of maternal age on the stillbirth rate were considered simultaneously.

Temporal, seasonal, and regional differences in births and deaths in Aland (Finland).

A good knowledge of the seasonal variation during normal years is of fundamental importance for analyses of the effects of wars, famines, epidemics, or similar privations on births and deaths. In this study we consider data from the Aland Islands (Finland) for 1650-1950. During the period 1650-1793 there are subperiods with missing data for all parishes, and consequently the total data for the Aland Islands for this period have to be estimated using available data.

Correlations between live and stillbirth outcomes in twin pairs.

A relationship has been proposed to exist between individual outcomes (live or stillbirth) of twins in the same set. Here, we analyze this association between live births and stillbirths among individuals in different twin pairs. When national birth registers are analyzed, individuals in opposite-sex twin sets can be identified and the correlation between individual outcomes estimated.

Sex ratio in sibships with twins.

In national birth registers of Caucasians, the secondary sex ratio, that is, the number of boys per 100 girls at birth, is almost constant at 106. Variations other than random variation have been noted, and attention is being paid to identifying presumptive influential factors. Studies of the influence of different factors have, however, yielded meagre results.

Temporal trends in the rates of multiple maternities in England and Wales.

After a long continuous decrease, the twinning and higher multifetal rates in many developed countries have increased during the last 2 to 3 decades. This change has been attributed to delayed childbearing and to increased use of subfertility treatments, particularly in women over 35 years of age. In this study we analyze how these new trends depend on changes in the effect of maternal age on the rates of multiple maternities.

Estimation of the stillbirth rate in twin pairs according to zygosity.

It is common opinion that the stillbirth rate is higher among monozygotic (MZ) than among dizygotic (DZ) twins. This is supported by the fact that stillbirth rates are higher among same-sexed than among opposite-sexed twins, and the relatively high stillbirth rates among twins of young mothers. In this study we present a method to estimate the stillbirth rates for MZ and DZ twins and identify the difference.

Weinberg's differential rule reconsidered.

A central problem in research on twins is the estimation of the rates of monozygotic and dizygotic twin maternities. The estimation usually follows Weinberg's differential rule. According to this rule, the rate of dizygotic twinning is twice the rate of twin maternities in which the twins are of opposite sex.

Stillbirths in multiple births: test of independence.

The stillbirth rate in twins is a more sensitive indicator of environmental hazards than the stillbirth rate in singletons. Medical care or other socioeconomic factors may be more influential for perinatal survival in twin than in single deliveries. Studies have indicated that stillbirths among children in a set of multiple maternities are not independent.

Factors influencing the stillbirth rates in single and multiple births in Sweden, 1869 to 1967.

Temporal variations in the stillbirth rate among singletons, twins and triplets in Sweden between 1869 and 1967 were studied. Both among single and multiple births there were marked secular decreasing trends in the stillbirth rates. Based on our long time series since 1869, this study confirms that among twins and triplets the stillbirth rate was higher among same-sexed than among opposite-sexed sets.

Stillbirth rates in singletons, twins and triplets in Sweden, 1869 to 2001.

The temporal variation in the stillbirth rates (SBR), measured as the number of stillborn per 1000 total births, among singletons, twins and triplets was studied on Swedish birth data for the period 1869 to 2001 and comparisons with data from other populations were made. Among both single and multiple births there were marked, almost monotonously decreasing trends in the stillbirth rates. Among singletons the stillbirth rate decreased from 29.

Variations in the maternal age effect on twinning rates: the Nordic experience.

Maternal age is the most important nongenetic factor influencing the twinning rate. Every study of the effect of other factors should consider the age distribution of the mothers. Besides standardizing techniques, the age-specific twinning rates are analyzed.

The convergence of the regional twinning rates in Sweden, 1751-1960.

Sweden has, as a whole nation, the oldest continuous population register of births, including twin and higher multiple maternities, starting in the 17th century. Until the 1920s, the rates of multiple maternities in Sweden were among the highest known among Europeans. Strong secular and regional fluctuations were noted.

Should the statistical analyses of twinning-rate data be improved?

Every statistical model is based on explicitly or implicitly formulated assumptions. In this study we address new techniques of calculation of variances and confidence intervals, analyse some statistical methods applied to modelling twinning rates, and investigate whether the improvements give more reliable results. For an observed relative frequency, the commonly used variance formula holds exactly with the assumptions that the repetitions are independent and that the probability of success is constant.

Association between the rates of multiple maternities.

We analyze the association between the rates of multiple maternities. Correlation analysis is suitable if there are at least monotonic relationships between the variables. A decreasing tendency can be observed in the rates of multiple maternities in Sweden and in its 25 counties for the period 1751-1960 and the effect of external variables can be assumed to be monotonic.

Demographic analysis of the variation in the rates of multiple maternities in Sweden since 1751.

To elucidate the causes and mechanisms of twinning and higher multifetal maternities, we have taken advantage of the statistical sources of Sweden, where continuous statistics for the whole population are the oldest available. We found strong secular and regional fluctuations. The rates of multiple maternities were the highest during the last three decades of the 18th century, when the twinning rate was more than 17 per 1,000, the triplet rate was more than 3 per 10,000, and the quadruplet rate was almost 7 per 1 million maternities.