Hellin's law states that if the twinning rate is w, then the triplet rate is w2, the quadruplet rate is w3, and so forth. The opinion of today is that Hellin's law holds only approximately. In this study the inaccuracy of Hellin's law is studied and the discrepancies are explained mathematically. In our earlier studies we built linear models for the twinning rate. Because most of the mothers are younger than 40 years of age and because in this age interval the twinning rate depends linearly on age, linear regression methods have been applied. Hellin's law suggests using the square-root transformation of the triplet rate r. Statistical arguments speak in favor of using the arcsin square root of r transformation. We discuss both transformations. Despite the fact that Hellin's law is only approximate, the arcsin transformation proves valuable. The transformed triplet rate can be modeled in a way similar to the twinning rate. We consider secular data from Finland for 1881-1990 and from Sweden since 1751. Using Hellin's law, we compare the triplet rates and the twinning rates and study the time trends of the observed twinning and triplet rates. The data are standardized. Our theoretical results are applied to multiple maternity data for Finland. Using maternal age as the regressor, we build a linear model for the twinning rate and for the arcsin-transformed triplet rate. This analysis shows a decreasing linear time trend in the triplet series for the period 1881-1950 but not in the twinning series. The triplet rate has an increasing trend after 1960, which seems to be mainly caused by artificial induction of ovulation.
OBJECTIVE: To assess secular trends for birthweight by gestational age in twins in Norway and to develop current national birthweight standards by gestational age for twin and triplet births using population-based data. MATERIAL AND METHODS: The analysis of secular trends for birthweight and gestational age in twins was based on 32,379 twin livebirths (1967-95). Taking into account the observed secular trends in birthweight for 35-40 weeks of gestation, data on twins born during 1987-95 only were included in the calculation of birthweight percentiles for 35-40 weeks, while for lower and upper weeks, data on twins born during 1967-95 were used. The construction of birthweight-for-gestation curves for triplets was based on the data on 690 triplets. RESULTS: Whereas the overall mean birthweight and gestational age decreased in 1987-95 compared with the previous years, the mean birthweights by gestational age for the 35-40 weeks of gestation was significantly higher in 1987-95. Male twins weighed more than female twins throughout the gestation with consistent and significant differences from 27 to 42 weeks of gestation. Smoothed curves for birthweight-by-gestational-age percentiles of male and female twins are plotted. The birthweight-by-gestational-age curves of triplets were almost identical with twin curves before 30 weeks of gestation, starting to diverge from them progressively thereafter. The intrauterine growth of twin births also starts to differ markedly from singletons at approximately 30 weeks of gestation. CONCLUSION: This study shows that plurality-specific birthweight-by-gestation standards should be used for assessment of fetal growth in multiple births rather than singleton standards.
Secular changes in twinning and triplet rates were analyzed using vital statistics in Austria, the Czech Republic, the Slovak Republic, England and Wales, Germany, the Netherlands, Switzerland, Denmark, Finland, Norway, Sweden, Canada, Australia, Hong Kong, Israel, Japan, and Singapore during the period from 1972 to 1996. Among those 17 countries, the twinning and triplet rates in the Czech Republic and in the Slovak Republic remained constant from 1972 to 1994, whereas these rates increased significantly year by year in the other 15 countries during the examined period in each country. Twinning rates increased from 1.2-fold in Austria to 2-fold in Denmark from 1972 to 1996. As for triplets, the rate increased from 3-fold in Denmark to 9-fold in Norway during that period. With one exception, that being the Slovak Republic, the triplet rate was highest in the Scandinavian countries, followed by the other European and Asian countries. The rising twinning and triplet rates have been attributed to the higher proportion of mothers treated with ovulation-inducing hormones and partially attributed to IVF.
Annual changes in twinning and triplet rates by zygosity were investigated in eight countries during the period 1972-1999 using vital statistics. The monozygotic (MZ) twinning rates in Denmark, Switzerland and the Slovak Republic remained more or less constant throughout this period, whereas those in England and Wales, the Federal Republic of Germany (Germany), the Netherlands, the Czech Republic and Japan increased significantly year by year. With the exception of the Slovak Republic, the dizygotic (DZ) twinning rate increased significantly year by year in each country. It was 2.9 times higher in Denmark and 1.5 times higher in Germany in 1999 than in 1972, and within the same range in the other countries. With two exceptions, the MZ triplet rates remained more or less constant in each country. On the other hand, the DZ and trizygotic (TZ) triplet rates increased significantly year by year in each country. The TZ rate increased 30-fold in Germany, 16.6-fold in Japan, 11.7-fold in Switzerland, 9.7-fold in the Czech Republic, 8.7-fold in the Netherlands, 6.4-fold in Denmark, 5.6-fold in England and Wales and 3.5-fold in the Slovak Republic. The higher DZ twinning rate and higher DZ and TZ triplet rates since 1983 have been attributed to the higher proportion of mothers being treated with ovulation-inducing hormones and in vitro fertilization (IVF) in Denmark, England and Wales, Germany, the Netherlands, Switzerland and Japan. After the introduction of fertility drugs and IVF, variations in the DZ twinning and triplet rates and the TZ triplet rates were not only due to biological factors, but also depended on the popularity of fertility drugs and IVF in each country. In the Slovak Republic, where human fertility might not be affected by some adverse environmental factors, the DZ:MZ ratio remained constant during the period 1972-1999.
The purpose of this study was to investigate whether perinatal health outcomes changed during the 1990s with the increasing use of IVF.
Data were from the Finnish Medical Birth Register for periods 1991-1993 and 1998-1999. Outcomes of IVF infants and other infants were compared, both overall and separately for singleton and multiple births, by adjusting for mothers' background variables by logistic regression.
The IVF multiple birth rate, especially the number of triplets, declined from the first (1991-1993) to the second (1998-1999) time-period. The outcomes for IVF newborns improved, especially for multiple births. After adjusting for mothers' background variables, the odds ratios for preterm birth and low birthweight decreased among singletons from 2.2 [95% confidence interval (CI) 1.8-2.8] to 1.8 (CI 1.5-2.1) and from 2.4 (CI 1.9-3.1) to 1.7 (CI 1.4-2.1) respectively and more among multiples from 2.4 (CI 2.0-2.9) to 1.5 (CI 1.2-1.7) and from 1.9 (CI 1.6-2.3) to 1.1 (CI 1.0-1.3) respectively. Still, overall the outcomes for IVF infants remained poorer than those for other infants. A correlation was found between increased use of antenatal services and improved outcomes, but causality cannot be assumed.
A trend of improved perinatal health of multiple IVF children was found, mainly due to a decrease in higher order multiple births.
We studied the effects of twins and triplets on perinatal health indicators in the overall population in the 1980s and 1990s in Canada, England and Wales, France, and the United States.
Data were derived mostly from live birth registration. We used rates, relative risks, and population attributable risks for twins and triplets separately.
In each country, the increase in multiple births, and the increase in preterm delivery among multiple births, contributed almost equally to the rise in or stabilization of the overall rates of preterm delivery. Twins contributed a much larger proportion of the preterm deliveries and low-birthweight newborns than did triplets.
Twins have a major population-based impact on the trends of perinatal health indicators.
Cites: Lancet. 1999 Nov 6;354(9190):1579-8510560671
Cites: Dev Med Child Neurol. 2000 Jan;42(1):14-2010665970
This paper examines Canadian trends and patterns in multiple births in relation to total confinements, singleton births, maternal age, parity, gestational age and birth weight using vital statistics from 1974 to 1990. Multiple-birth rates in Canada increased from 912.8 to 1,058.9 per 100,000 confinements between 1974 and 1990. The increase is especially noticeable for women over 30. The rate of triplet and higher-order births increased from 8.3 to 21.7 per 100,000 confinements between 1974 and 1990. The proportion of multiple-birth babies that were pre-term (
Recent increases in the frequency of multiple births and simultaneous increases in preterm birth among multiple births have focused attention on such births. However, most previous studies have examined twins rather than higher-order multiples. We carried out a study to examine rates and trends in preterm birth and in gestational age-specific fetal and infant mortality among triplet births in Canada. We used data from the stillbirth, live birth and mortality files of Statistics Canada for the years 1985-97. All births in Canada (excluding those occurring in Ontario and Newfoundland) were included in the study, with two periods (1985-90 vs. 1991-96) being contrasted for assessing temporal change. Changes were estimated using relative risks, 95 confidence intervals [CI] and two-tailed P-values. The rate of preterm birth among triplet live births increased by 6 (95 CI 3, 9) from 90.4 in 1985-90 to 96.0 in 1991-96. Stillbirth rates among triplets did not change significantly and were 30.3 per 1000 total births in 1985-90 and 33.8 per 1000 total births in 1991-96. Infant mortality among triplets declined from 112.7 per 1000 live births in 1985-90 to 73.8 per 1000 live births in 1991-96. In spite of temporal reductions in infant mortality, triplet births continue to be associated with very high rates of preterm birth and fetal and infant mortality. Fetal mortality among triplets has not changed over the last ten years.
After a substantial decrease in the middle of the 20th century, multiple pregnancy rates have increased in many Western countries. Between the mid-1970s and 1998, the rate of twin pregnancies increased by 50% to 60% in England and Wales, France, and the United States. The rates of triplet or higher-order multiple pregnancies increased by 310% in France, 430% in England and Wales, and 696% in the United States. One fourth to one third of the increase in twin or triplet pregnancies are attributable to a contemporaneous increase in maternal age. Furthermore, in countries with high occurrence of multiple births, 30% to 50% of twin pregnancies and at least 75% of triplet pregnancies occur after infertility treatment. The impact of the increase in multiple births on preterm delivery rates in the overall population is mainly attributable to twin pregnancies. In Canada, France, and the United States, an increase in preterm births among multiples contributed almost as much as the increase in occurrence of multiple births to the increase or stabilization of the overall rates of preterm delivery observed in these countries.