Changes in the DZ unlike/like sex ratio in The Netherlands

Time trends in biological fertility in Western Europe

We investigated trends in biological fertility in a comprehensive analysis of 5 major European data sets with data on time to pregnancy (TTP) and proportion of contraceptive failures. In particular, we distinguished a period effect from a birth cohort effect (lifelong tendency) in both sexes. Attempts at conception not resulting in birth were excluded. We analyzed data on pregnancies occurring in 9,247 couples between 1953 and 1993 and performed sensitivity analyses to check the robustness of findings. Separate analyses of each time effect showed an increasing fertility trend. Mutually adjusted analyses demonstrated that this rise was visible as a male cohort effect for both TTP and contraceptive failure. On the other hand, the female birth cohort effect showed a slight fall in the first half of the study period for both TTP and contraceptive failure. As a period effect, fertility remained generally stable, the slight trends in TTP and contraceptive failure being in opposite directions, likely indicating an artifact. The rising trend accords with most previous evidence. The increasing trend in male fertility does not contradict the previously reported semen quality deterioration, the effects of which are calculated to be small. The declining female fertility accords with a falling dizygotic twinning rate during the same period.

Timing of Twinning, X-Inactivation and Sex Proportion at Birth

Twins are not a homogeneous group. According to zygosity and chorionicity essential differences exist. The lower sex proportion at birth is due to the monozygotic twins, especially the monoamnionic variety. X-inactivation patterns in monozygotic twin girls are totally symmetrical in monoamnionic pairs, almost symmetrical in monochorionic diamnionic pairs and can be very asymmetrical in the dichorionic variety.

Identification of Twin Pairs From Large Population-Based Samples

The basis of most twin studies is the ascertainment of twins, often through twin registries, and determination of zygosity. The current rate of twin births in many industrialized countries implies that in the near future around 3% or more of individuals will be a twin. Hence, there are and will be a lot of twins around and many of those will not participate in twin studies. However, if large population-based samples are available that include appropriate identifiers, then twins can be detected and twin studies performed, even in the absence of zygosity information. We quantified the number of twin pairs that could be detected from a longitudinal survey in the Netherlands, which aims to answer questions about educational strategies and performance in primary education in the Netherlands. We detected 2865 twin pairs if we used a coded name identifier, date of birth, school, grade and year of survey, which is 2.01% of 284,945 pupils in five cohorts. Relaxing our selection criteria increased the number of apparent twin pairs identified, most of which are false positives due to chance matching of identification criteria. We show that the intraclass correlation on measured phenotypes can be used as a quality control measure for twin identification, and quantify the proportion of false negatives (true twin pairs not identified) due to missing data and data coding errors. We compared our estimated rate of twins in the sample to census data and estimate that with our most stringent selection criteria we detect more than 80% of all twin pairs in the sample. We conclude that the identification of twin pairs from large population-based samples is feasible, rapid and accurate if the appropriate identifiers are available, and that twin pairs from such sources are a valuable resource for studies to answer scientific question about twins versus nontwins and about genetic and environmental factors of twin resemblance.

The East Flanders Prospective Twin Survey (Belgium): a population-based registe

The East Flanders Prospective Twin Survey (EFPTS), started in 1964, is unique among the 17 major European twin registers because it is population based, the twins (and higher order births) are ascertained at birth, basic perinatal data are collected, chorion type is established and, when appropriate, genetic markers including DNA fingerprints, are determined. The total number of sets is 5089 twin, 158 triplet and 14 of higher order. Zygosity has been diagnosed on the basis of sex, placental structure and genetic markers in more than 95% of pairs. The EFPTS is the only large register that includes placental data and allows differentiation of three subtypes of monozygotic twins based on the time of the initial zygotic division: the dichorionic—diamnionic pairs (early), the monochorionic—diamnionic pairs (intermediate), and the monochorionic—monoamnionic pairs (late). Methodology and basic results in twins are considered in this article; detailed studies will be reported later. The sex proportion in dizygotic (DZ) twins is the same as in singletons, whereas monozygotic (MZ) twins number more girls than boys. The difference in perinatal mortality between DZ and MZ twins is limited to the monochorionic MZ subgroup. Birth weight is highest in DZ twins and diminishes stepwise in MZ dichorionic and MZ monochorionic twins. Duration of pregnancy follows the same trend but is limited to a few days. Iatrogenic pregnancies are increasing to the point of representing almost 50% of the twin births in 1997.

The Estimated Probability of Dizygotic Twins: A Comparison of Two Methods

This study presents a general model of two binary variables and applies it to twin sex pairing data from 21 twin data sources to estimate the frequency of dizygotic twins. The purpose of this study is to clarify the relationship between maximum likelihood and Weinberg's differential rule zygosity estimation methods. We explore the accuracy of these zygosity estimation measures in relation to twin ascertainment methods and the probability of a male. Twin sex pairing data from 21 twin data sources representing 15 countries was collected for use in this study. Maximum likelihood estimation of the probability of dizygotic twins is applied to describe the variation in the frequency of dizygotic twin births. The differences between maximum likelihood and Weinberg's differential rule zygosity estimation methods are presented as a function of twin data ascertainment method and the probability of a male. Maximum likelihood estimation of the probability of dizygotic twins ranges from 0.083 (95% approximate CI: 0.082, 0.085) to 0.750 (95% approximate CI: 0.749, 0.752) for voluntary ascertainment data sources and from 0.374 (95% approximate CI: 0.373, 0.375) to 0.987 (95% approximate CI: 0.959, 1.016) for active ascertainment data sources. In 17 of the 21 twin data sources differences of 0.01 or less occur between maximum likelihood and Weinberg zygosity estimation methods. The Weinberg and maximum likelihood estimates are negligibly different in most applications. Using the above general maximum likelihood estimate, the probability of a dizygotic twin is subject to substantial variation that is largely a function of twin data ascertainment method.

Use of dizygotic to monozygotic twinning ratio as a measure of fertility

BACKGROUND

Monozygotic (Mz, identical) twinning occurs at a rate of around three per 1000 maternities in all populations, whereas dizygotic (Dz, fraternal) twinning is highly heritable, and varies with age and race. The Dz/Mz twinning ratio reflects the frequency of twin ovulations, and can provide a useful measure of human fertility.

METHODS

1625 pairs of twins of known sex were born in Isan Yuk Obstetric Hospital in Hong Kong during the period 1960-95. The yearly Dz/Mz ratio was calculated, and trends were analysed by chi 2 test with the Bonferroni correction. Ages of mothers of all opposite-sexed twins were recorded and trends analysed by ANOVA and linear regression.

FINDINGS

The Dz/Mz ratio declined significantly from 1.12 in 1960 to 0.05 in 1978 (p < 0.001), and then rose significantly to an average of 0.86 in 1994-95 (p < 0.003). There was a significant declining trend in age of mothers of opposite-sexed twins from 1960 to 1978 (p < 0.001), but there were no significant changes in maternal age after 1978 (p = 0.38).

INTERPRETATION

If we are correct in assuming that the frequency of Mz twinning remained constant during the study period, the declining Dz/Mz ratio from 1960 to 1978, which also occurred in many developed countries, could reflect some adverse environmental effect on human fertility. The increasing proportion of Dz twins in the past two decades is probably due to increasing use of ovulation-inducing drugs such as clomiphene citrate, which could mask a serious and continuing decline in human fertility. It is therefore important to continue to monitor the Dz/Mz ratio in the future in this and other subpopulations, after exclusion of any women who have taken drugs that stimulate fertility.