This study examined sleep patterns in twins, paying special attention to the mirroring phenomenon. Concordance and discordance of sleep related patterns (parasomnias) in a group of 27 monozygotic (MZ) mirror image twins were compared to sleep patterns in a group of 53 MZ non-mirror image twins and a group of 24 dizygotic (DZ) twin pairs. Sleep patterns had the lowest concordance among MZ mirror twins. “Mirroring” was observed not only in facial features but also in some physiological patterns such as sleep and sleep deviations. These facts suggest that mirroring in MZ twins is not merely a superficial epiphenomenon, but a reflection of a biological polarization. Biological polarization in this context is a descriptive term emphasizing the role of biological (physiological, biochemical or even genetic) versus psychological or environmental factors causing not only mirror body image but opposite tendencies in the development of personality, professional and sex orientation and, most importantly, the opposite presentation of pathology.

In a sample of 105 concordant sex MZ and DZ twin pairs, the following characteristics were measured: red cell count, haemoglobin concentration, package cell volume, mean cell volume, mean cell haemoglobin, mean cell haemoglobin concentration, reticulocytes, platelets, white cell count and the six types of leucocytes, lymphocytes, monocytes, band and segmented neutrophils, eosinophils and basophils. The statistical model employed in the univariate twin analysis allows for three sources of variation: genetic (h2), shared environmental (c2) and specific environmental influences (e2). A genetic component was significant for red cell count, haemoglobin and mean cell haemoglobin (0.64, 0.60 and 0.46 respectively), with heritable variation suggested for package cell volume, mean cell volume, mean cell haemoglobin, lymphocytes and monocytes. Shared environmental variation was only present for neutrophils.

During a screening protocol of early echocardiographic diagnosis (ATL MK 600) and treatment of “silent” PDA in RDS preterms with BW ≤ 1.750 kg, clinical data on premature twins were collected, including diagnosis of both PDA and BPD, to investigate whether twin birth influences PDA incidence and BPD development. Out of the 290 RDS preterms evaluated, 96 (33%) showed evidence of PDA, and a total of 79 (27%) developed BPD, 47 (16%) with associated PDA and 32 (11%) without PDA. Out of 238 singletons, 74 (31%) presented “silent” PDA and a total of 75 (31%) developed BPD, 44 (18%) with associated PDA, and 31 (13%) without PDA. In 52 other twins (18% of the total number of babies studied), 22 (42% of this subgroup) presented evidence of “silent” PDA, and 4 (8% of the subgroup), developed BPD, 3 with associated PDA (6% of the subgroup), and 1 without PDA (2% of the subgroup). From these data, it is inferred that that low-birthweight twins are at high risk for PDA hemodynamic complications during RDS, and may benefit from early induced ductal closure. Instead, in RDS twins, BPD was statistically less frequent (at the 99% C.L.) probably because twinning enhances fetal lung maturity, influencing enzymatic and nonenzymatic protective systems of lung defence.

It is estimated that at least one dizygotic (DZ) twin maternity in twelve is preceded by superfecundation (the fertilization of two ova by sperm from different coitions). Presumably this parameter varies from population to population eg. with coital rates and rates of double ovulation. Sometimes superfecundation occurs by two different men. The frequency with which this occurs must depend on rates of infidelity (promiscuity). It is suggested that among DZ twins born to married white women in the U.S., about one pair in 400 is bipaternal. The incidence may be substantially higher in small selected groups of dizygotic twin maternities, eg. those of women engaged in prostitution.

White male twins from the NAS-NRC twin sample, who were born in the U.S. between 1917 and 1927 and served in the military, are used to estimate variability in hazard functions for those twins who died during the period 1974-1990. Roughly the same number of MZ and DZ twins died during this period, but their death rates are similar. DZ twins exhibit greater within-pair variation. Using hazard and other analyses, the only statistically significant variables are found to be being a DZ twin (in level equations), date of birth, and, sometimes, wife's religious preference. Variables not significant for level or within-pair equations, include own religion, parental education, working overtime frequently, and number of children. The greater variation in life expectancy of DZ twins is hardly surprising and may say something about the lack of comparability in phenotype variance of DZ twins, which in turn may be worrying.

The present study aimed at clarifying the characteristics of twins' physical growth. First, 557 pairs of normal Japanese twins were analyzed according to the following three life stages: (1) intrauterine growth, (2) body weight and height from birth to 6 years, and (3) body weight and height at school age (from 6 to 11 years). The following results were obtained. 1. Intrauterine growth of twins was very different from that of singletons, especially as regards weight, so twins should be estimated by twin standards. 2. Size deficit at birth was appropriately recovered over the first 6 years. 3. No size deficit was observed by school age. Second, the similarity of bodyweight and height according to zygosity was analyzed using 605 pairs of normal Japanese twins, 427 monozygotic (MZ), 113 same-sexed dizygotic (DZ) and 65 opposite-sexed DZ pairs. The similarity between MZ and DZ pairs was almost the same at birth. However, MZ pairs became increasingly more similar with age, whereas DZ pairs became more dissimilar. This tendency was very clearly seen through early infancy, thus suggesting that genetic factors became more apparent during this life stage.

Birth statistics (on 13,887.943 births) from 11 Indian States indicate a fall in the dizygotic twinning rate and a rise in the monozygotic twinning rate over the decade between 1982 and 1991. These figures seem to reverse the trend of the period 1960-1982. One plausible explanation is offered by the age-data on mothers, which indicate that twins of different sex are more frequently born to mothers above the age of 35. Since extensive family planning measures have reduced the upper age-limit of mothers, the birth-rate of twins of different sex has also fallen. Due to the increased proportion of twins of the same sex, Weinberg's differential method will therefore report a decline in the dizygotic twinning rate, wherever the mean age of mothers has fallen (i.e. most mothers do not reproduce after the age of 35).

The distribution of menarcheal interval-length between cotwins in 67 pairs of female identical twins in Tokyo was found to cluster at fairly constant intervals. When delayed in one twin, the menarche occurred in the second twin at intervals of approximately 28 days after that of the earlier twin. This finding suggests the existence of a latent lunar or menstrual cycle in pre-menarcheal girls, evidence for which was detected by recording the menarcheal intervals between MZ cotwins in days.