Studies on twins. IV. Twinning in Madhya Pradesh

Twinning and inbreeding in India: the fraternal component

It is suggested that the rise in a population depends not only on the nu number of females per thousand males, their fecundability, and the survival of offspring (mean live births), but also on the genetic background or inbreeding status of the fetuses. Abortions and stillbirths add to the interference. A hypothetical formula has been devised (Formula: see text), where Mb = mean birth rate, G = growth percentage of population sample from preceding Census, F = inbreeding coefficient, Sa = Stillbirths + abortions, and Sr = Sex ratio. It is intriguing that the product, being termed Fraternal Component (FC), is found to be almost equal to the DZ twinning rate of the population sample. Should that be confirmed, one could perhaps estimate the DZ twinning rate by an altogether new approach.

Analysis of multiple birth rates in Japan. 1. Secular trend, maternal age effect, and geographical variation in twinning rates

Mean twin, triplet, and quadruplet birth rates in Japan from 1951 to 1968 were 6.41, 0.056, and 0.00094 per 1,000, respectively. In 1974 the corresponding figures were 5.83, 0.059, and 0.00329. No quintuplets were born in the former period, but a set was born in the latter year, the rate being 0.47 per million. From 1955 to 1966 the MZ twinning rate increased slightly, but decreased thereafter. This increase was limited to live-born MZ twins, particularly in the higher maternal age groups. The DZ twinning rate declined in the entire period, particularly in higher maternal age groups. This decline appeared to be essentially limited to fetal deaths. Among live births the MZ twinning rate underwent a nearly linear increase with maternal age, whereas the DZ twinning rate attained a mode in the maternal age group 35--39 years. The MZ and the DZ twinning rates among fetal deaths by maternal age had unimodal distributions with modes in maternal age groups 25--29 and 30--34 years, respectively. As to the DZ twinning rate, a geographical cline was noted, with a high rate in the northeast of Japan; the rate was positively correlated with latitude, which also positively correlated with the presence of multiple births among relatives. A negative but nonsignificant correlation was seen between the DZ twinning rate and the proportion of mothers treated with ovulation-inducing hormone. The proportion was higher in mothers of unlike-sexed twins than in those of like-sexed twins and in mothers of triplets than in those of twins. An association between DZ twinning rate and age-specific fertility per married woman is suggested to exist among higher maternal age groups in the northeast part of Japan in earlier years.

Twinning in India

The incidence of twinning in India, as estimated on the basis of data from the various types of maternity centers, so as to include both urban and rural population, appears to be 15.97 per thousand (12.6 DZ, 3.3 MZ). The incidence is significantly higher in rural than urban populations (18.1 vs. 11.8 per thousand). It is suggested that this may be accompanied with a higher production of DZ, possibly opposite-sexed twins.