OBSERVATIONS ON TWINNING IN NEW YORK STATE.

Abstract

The present investigation of the incidence of human twinning related to gestational and demo graphic characteristics is one of a series of birth-rate studies of all births recorded in New York State, excluding New York City. The twin births were selected from a population of approximately two million maternities registered during the 11-year period. A previous communication (Gittelsohn and Milham, 1964) described the information available for each pair of twins, and the linkage methodology for updating maternity records from the individual birth records of members of pairs of twins. The phenomenon of twinning in man has long aroused the curiosity and speculation accorded to the rare event. Recognition of the two distinct embryo logical processes whereby twins arise has stimulated research workers to explore various aspects of the condition. In effect, monozygotic and dizygotic twinnings are naturally occurring mechanisms which simulate controlled experimentation. Monozygotic twins have been assumed to be genetically identical, while dizygotic twins have been regarded as being no more alike, genetically, than siblings. By measuring differentials in the concordance of qualitative traits, and by correlating quantitative traits within the two types of twins, inferences have been drawn about the relative roles of heredity and environment in the development and expression of given phenotypes. From an alternative viewpoint, it has long been recognized that twins are at greater risk of perinatal death than single births. The various aspects of the lowered viability of twins during the pre-natal and post-natal period have been described in a number of commentaries based on vital statistics and hospital records (Barr and Stevenson, 1961; Lowe and Record, 1951; Record, Gibson, and McKeown, 1952; Karn, 1953; Yerushalmy and Sheerar, 1940). The excess mortality risk of twins over single births has been ascribed to increased occurrence of such compli cations as prematurity, toxaemia, placenta praevia, polyhydramnios, premature separation of the placenta, and malpresentation. In terms of the uniparous condition characteristic of human birth, binovular and uniovular twinning may be regarded as reproductive anomalies leading to lowered fitness. Several teratogenic agents and varied physical conditions have been observed to result in a higher incide ce of monozygotic twinning along with various types of d velopmental abnormalities in experimental animals (Witschi, 1952). Alternatively, the high loss rates of twins may be indicative of maladaptation of the human uterus to the simultaneous gestation of more than one foetus. Suggestions of underlying causal mechanisms may be obtained through study of differential loss rates by plurality and by zygosity groups.