A Study of Multiple Deliveries in Portugal: Indications of an Iberian Peninsula Pattern

Aspects of the History of Twin Research: Statistical Congresses in the 19th Century and Hellin's Law

In the 19th century, a series of international statistical congresses began that were important for population studies, including twin research. The introduction of common rules for the national demographic registers enabled scientists to contribute to the genesis of statistical research. The congress in St. Petersburg in 1872, in particular, focused on the movements of the population, and how they should be registered. Among the facts to be recorded were in multiple births, the sex and number of children born alive or still-born, whether legitimate or illegitimate, and the age of the mother at the date of the births. During the history of twin research, Hellin's law has played a central role because it is an approximately correct association between the rates of multiple maternities. It has been mathematically proven that Hellin's law does not hold as a general rule. Analyses show divergences from the law that are difficult to explain and/or eliminate. Varying improvements of this law have been proposed. The majority of all studies of Hellin's law are based on empirical rates of multiple maternities, ignoring random errors. Such studies can never confirm the law, but only identify errors with respect to Hellin's law that are too large to be characterized as random. It is of particular interest to note and explain why the rates of higher multiple maternities are sometimes too high or too low when Hellin's law is used as a benchmark. Studies have shown that there were investigators before Hellin who have contributed substantially to Hellin's law. In this article, we re-examine some old data sets and contributions in which Hellin's law has been evaluated and also analyze recent data.

Biodemographic Analysis of Factors Related to Perinatal Mortality in Portugal (1988-2011)

Background. The purpose of this paper is to determine the relative mortality risks at delivery and during the first week of life with regard to maternal and foetal characteristics. Methods. Yearly individual digital records on live births and early neonatal mortality were used to infer the possible factors involved in perinatal deaths. Results. The results show that the number of births per year declined with time throughout the period studied. At the same time, rates decreased in 66.4% for stillbirths and in 70.2% for early neonatal mortality. Logistic regressions modelled the interaction of the two mortality indicators and covariables such as birth weight and the duration of gestation. Conclusions. This research provides a first biodemographic approach to the knowledge of factors influencing perinatal mortality in Portugal based on a set of foetal and maternal variables. Although the magnitude of the different perinatal mortality rates may be affected by the criteria used for selecting cases (multiple-singletons; minimum birth weight or minimum duration of gestation), one of the conclusions of the present analysis is that the relationship among the maternal and foetal variables that determine the relative risk remains unaltered. Certain resemblance with the factors determining negative birth outcomes in Spain is appreciated.