Traces of embryogenesis are the same in monozygotic and dizygotic twins: not compatible with double ovulation.

Common knowledge of over a century has it that monozygotic and dizygotic twinning events occur by unrelated mechanisms: monozygotic twinning 'splits' embryos, producing anomalously re-arranged embryogenic asymmetries; dizygotic twinning begins with independent ovulations yielding undisturbed parallel embryogeneses with no expectation of departures from singleton outcomes. The anomalies statistically associated with twin births are due to the re-arranged embryos of the monozygotics. Common knowledge further requires that dizygotic pairs are dichorionic; monochorionicity is exclusive to monozygotic pairs.

Embryogenesis of chimeras, twins and anterior midline asymmetries.

Human spontaneous chimerism, with one body built from cells of both twins of a dizygotic (DZ) pair, is supposed to be extremely rare, arising from the exchange of blood cells through placental anastomoses. Mosaicism is supposed to be far more common, arising from single zygotes by embryonic mutation. Because typical diagnosis of mosaicism can neither identify nor exclude chimerism, 'mosaicism' may often be chimerism undiscovered.

The epigenetic environment: secondary sex ratio depends on differential survival in embryogenesis.

Live human births are usually more than half male, in spite of excess losses of males throughout fetal development. These observations together demand an excess of males near the beginning of pregnancy greater than that seen at birth. Reductions of the usual excess of males among human live births have widely been considered to represent consequences of untoward circumstances surrounding conception.

[Intraoperative ventricular fibrillation in a patient with chronic cocaine abuse--a case report].

With increasing drug abuse of cocaine, the chances are growing that an anaesthetist comes into contact with an acutely intoxicated patient or chronic cocaine user while on call or during his daily routine. In South America chewing coca leaves is daily practise, while in the industrialised world the drug is sniffed, smoked or injected intravenously. Clinically, cocaine is used topically in ENT and ophthalmology due to its local analgesic and strong vasoconstrictive properties.

Effects of a behavioural rhythm on conception probability and pregnancy outcome.

This paper concerns a behaviour-driven weekday rhythm in conception rates in a large natural human population. From 1978 to 1991, North Carolina normal live single births arose from menstrual cycles which began on Monday in clear excess over other weekdays. Cycles beginning on Friday were also in excess.

Annual and sub-annual rhythms in human conception rates: time-series analyses show annual and weekday but no monthly rhythms in daily counts for last normal menses.

Methods of time-series analysis, which are widely used to good effect in physical sciences and econometrics, have found little use in much-needed analyses of cyclical biological phenomena. Here we apply those methods to analyses of rhythmic patterns in human conception rates. Our results confirm the annual periodicity of monthly counts of total viable conceptions, demonstrate a weekday rhythm reflecting interaction of conjugal coital rhythms with individual menstrual fertility cycles, and find no evidence of any other significant repeating pattern.

Annual and sub-annual rhythms in human conception rates. I. Effective correction and use of public record LMP dates.

Most studies of rhythms in human conception rates have used monthly total births back-dated 9 months. Such data are of little use for studying any pattern less than a few months in length. Even at that level, they are poor material for studying the possibility of altered conception rhythms in anomalous births.

Genotyping of macerated stillborn fetuses.

It is generally impossible to collect blood or to culture tissue from a macerated stillborn fetus. Accurate genotyping of such a fetus may, however, be critical for the diagnosis of genetic diseases and appropriate genetic counseling. In the East Flanders Prospective Twin Study, placental tissue of twin and triplet sets, in some of which one or both members were stillborn and macerated, has been stored at -20 degrees C.

Survival probability of human conceptions from fertilization to term.

Preterm death of the human conceptus is common. A consistent biphasic pattern in the rate of loss from biochemical pregnancy detection to term suggests that most wastage occurs prior to clinical recognition. After simple adjustments for varying methods, existing data show that at least 73% of natural single conceptions have no real chance of surviving 6 weeks of gestation.

Developmental differences between singletons and twins in distributions of dental diameter asymmetries.

Craniofacial development and behavioral development differs between human twins and singletons in several ways which are related to symmetry development and detectable in adults. In most of those ways, twin zygosity groups do not differ. Here we use distributions of dental diameters, as a model subsystem of craniofacial development, to show that twins, of both zygosities and both sexes, are substantially more symmetrical than singletons.

Twinning, nonrighthandedness, and fusion malformations: evidence for heritable causal elements held in common.

Certain relatively common congenital malformations that are reported to be excessive in frequency among twins are also found in excess among first-degree relatives of twins. They are familially associated with each other. Like twinning, they are familially associated with nonrighthandedness (NRH).

The organization of the oocyte and embryogenesis in twinning and fusion malformations.

Certain congenital malformations occur in excess among twins and also among first-degree relatives of twins. In the general population, these anomalies are familially associated with each other, and, like twinning, familially associated with unusual brain function asymmetry. They affect structures built by fusion of bilateral embryonic halves and remodeled under major influence of neural crest mesenchyme.

Race, zygosity, and mortality among twins: interaction of myth and method.

For epidemiological purposes, it is customary to assume that same-sex (SS) dizygotic (DZ) twin pairs are approximately equal in number to unlike-sex (OS)-DZs, the remainder of the SS pairs being monozygotic (MZ). It is also customary to consider OS-DZs to be epidemiologically representative of all DZs, which can only mean that difference in frequency of any trait between OS and SS twins is due to the MZ fraction of the SS twins. Since this is assumed as a premise, there is little value in its usual appearance as the result.

Interactions between opposite-sex dizygotic fetuses and the assumptions of Weinberg difference method epidemiology.

The Weinberg difference method in twin epidemiology may be subject to a major error related to a fundamental assumption that seems not to have been previously questioned. Results presented here indicate that the mortality of like-sex dizygotics (DZs) resembles that of monozygotics (MZs) much more than that of unlike-sex DZs, who are not representative of any other group. Unlike-sex twins enjoy a relative protection from fetal and neonatal mortality and probably from at least one effect of the transient perinatal hypoxia common for second twins.

Differences in protocols of craniofacial development related to twinship and zygosity.

Using 56 adult dental diameters as a subsystem model for craniofacial development, we show that monozygotic (MZ), dizygotic (DZ), and singleton groups differ significantly in developmental relationships assessed by multivariate statistical methods under commonly accepted assumptions. Given the differences observed, we suggest that any assumption of developmental equivalence between MZ and DZ twins, or between twins of either group and singletons, for variables of craniofacial or behavioral development, may be subject to serious doubt. Implications for twin study theory and methodology, and for study of early human development, are discussed.

On the distribution of nonrighthandedness among twins and their families.

In 773 three-generation families of twins, individual probability of nonrighthandedness (NRH) depends significantly on the handedness of that individual's parents. The parents of twins are much more often NRH than are their nontwinbearing siblings. The twins and their siblings apparently inherit the excess liability for NRH shown by their parents.

Schizophrenia, brain asymmetry development, and twinning: cellular relationship with etiological and possibly prognostic implications.

Discordance for schizophrenia in monozygotic twins has been interpreted as evidence of the role of environment in the etiology fo schizophrenia, with "environment" usually, in practical parlance, carrying Freudian and Sinnerian connotations. MZ twins are often discordant for certain brain development deviations (i) to which they are specially prone, (ii) which show other evidence of having genetic etiologies, and (iii) which involve embryonic symmetry development. MZ twinning is an embryonic symmetry anomaly; its association with others is not surprising.