Data are presented from a large-scale population-based study in Northern Ireland, in which multiple sources of ascertainment were used. As found in other studies, the overall prevalence at birth of congenital anomalies amogst twins (285.4/10,000) was somewhat higher than the rate amongst singletons (241.8/10,000). Unlike in other studies, however, the rate amongst twins of like sex (287.8/10,000) was not markedly higher than that amongst twins of unlike sex (252.3/10,000). Problems of comparison between series are discussed.

In a large population-based study in Northern Ireland during the period 1974-1979, the rate of anencephalus in twins (9.1/10,000) was found to be less than that in singletons (24.3/10,000). This finding is in contrast with most other studies and the possibility of underascertainment of twin cases is considered, but it is concluded that chance is the likeliest explanation. The rate of spina bifida in twins (36.4/10,000) was similar to that in singletons (31.9/10,000). All of the twins with anencephalus were female and from pairs of like sex. Rates of spina bifida in twins from pairs of the two sex types were similar but, unusually, there was a male preponderance. As in previous studies, the great majority of twins with NTDs had unaffected cotwins.

Rates of congenital anomalies of the cardiovascular system were compared between twins and singletons in a population-based study in Northern Ireland during the period 1974-1978. Multiple sources of ascertainment were used. As in previous studies, the rate of anomalies of the cardiovascular system in twins (91.0/10,000) was higher than the rate in singletons (66.4/10,000). The excess was confined to twins from pairs of like sex and, in the main, anomalies of the circulatory system other than of the heart itself were involved. Problems in the interpretation of this excess are discussed. No twins were concordant for congenital cardiovascular anomalies of any type.

Annual serial records in the form of facial contour maps were examined for 18 like-sexed twin pairs of near equal zygosity distribution. Zygosity diagnosis was based primarily on hematological reports for 26 of the 36 children and the remainder were diagnosed on a basis of the concordance or discordance of various physical characteristics: standing height, finger print ridge count, tooth size, and hair and eye colour. Thirteen facial parameters were measured on 274 maps. After age correcting and theree-point smoothing, more than 1,150 intrapair differences of individual facial parameters were measured. In general, the dizygotic twin pairs had the larger mean intrapair differences in facial parameters and the monozygotic twin pairs had the smaller intrapair mean differences. The more important facial parameters for distinguishing the two groups were identified and used to calculate a “facial similarity index”.

A pair of preterm, opposite-sex twins were examined during the lying-in period for behavioral and morphological effects of maternal alcohol and pentazocine abuse during pregnancy. A few morphological features typical of fetal alcohol syndrome were observed in each infant, and the male infant only was above the standardized mean in minor physical anomalies. The male's behavioral scores were more likely to be deviant from the standardized mean than the female's. Specifically, the male was more irritable, both spontaneously and in response to specific stimuli; highly active while awake and handled for the presentation of stimuli; more active than average during sleep; and low on ratings that reflect the attitude of the examiner toward the infant. These findings imply individual and gender differences in behavioral susceptibility to teratogens.

The twinning rate during a twenty-year period (1965-1985) was investigated at five-year intervals in four Brazilian hospitals. During this period the twinning rate has decreased significantly (from 10.68‰ to 8.11‰), being highly negatively correlated with the five-year intervals (r = −0.97). This change was due mainly to the decline in dizygotic twinning, since the incidence of DZ twins has fallen from around 7‰ in 1965 to around 3.6‰ in 1985. The detected decline in DZ twinning seems to be due to the remotion of the fertility advantage of the more fecundable DZ twin-prone women by the introduction of effective birth control at the present, while some other mechanisms are causing the decline irrespective of the type of twinning.

Nation-wide data in Japan on births and prenatal deaths of 16 sets of quintuplets during 1974-1985 were analysed. Among the 16 sets, 3 sets were liveborn, 8 were stillborn, and 5 were mixed, with a stillbirth rate of 0.64 (51/80). Effects of sex, maternal age and birth order on the stillbirth rate were not considered because of the small sample size. Effects of gestational age and birthweight on stillbirth rate were also examined. The mean weight of the 40 quintuplet individuals was 1,048 g.

From the 474 twins born in 1950 in 100 Flemish cities and towns in Belgium, 85% reached the age of 20 yr, against 37% in Austria a hundred years ago. More female than male twins are married (P = 0.03), in particular nonidentical female twins (P = 0.02). Fertility seems to be lower in male, especially male-male twins (P < 0.03), and most strikingly so in identical male twins (P = 0.05). There seems to be no influence of premature birth or low birthweight.