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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.

A retrospective study was carried out on a sample consisting of 89,491 deliveries occurred from 1979 through 1998, in the city of São Paulo, SP, Brazil. These deliveries included 935 twin and 26 triplet births. The mean incidence per 1,000 deliveries was estimated at 10.45 for twins (5.34 for dizygotic and 5.11 for monozygotic twins) and 0.29 for triplets. The average maternal age increased significantly during this period for both twins and singletons. The temporal variation in years significantly influenced the increase of monozygotic twins and of triplets. Temporal variation and maternal age significantly influenced the dizygotic twinning rate increase. An increased proportion of monozygotic twins born to mothers aged 31 -35 years was also detected.

We have evaluated the relation between height and rate of clinical progression in boys with Duchenne muscular dystrophy (DMD). In all, 111 DMD patients with age ranging from 2 to 23 years (mean 8.2 +/- 3.4 years) were assessed; of these patients, 92 had their height measured. Clinical course was determined through Vignos scale of functional disability, motor ability, and timed functional tests. All patients had grossly elevated serum creatine-kinase (CK) and pyruvate-kinase (PK) levels. When height was adjusted for patients' age, a statistically significant correlation was found between height and clinical course (positive with Vignos scale and negative with motor ability), suggesting that smaller boys have a better clinical course than taller patients of comparable age. These results support our previous hypothesis and suggest that growth inhibition seems to be effective in diminishing the progression of DMD.

Systolic and diastolic blood pressures were measured on 254 monozygotic (MZ) and 260 dizygotic (DZ) male twin pairs, during middle age (average age 48 years) and at two later age points. Genetic and environmental components of covariation were modeled by time series. For both measures, shared environmental influences were absent and specific environmental influences were largely time-specific. Although heritability was about 0.5 at each time point, genetic variation present at middle age contributed only about 60% to that present 9 years later, the remaining 40% being new. Fifteen years later, at the third time point, no new genetic variation was evident, variation in individual differences being entirely attributable to genetic differences laid down at the two earlier ages.

This population based study compares the rates of multiple births in the 1990s in four hospitals of different socioeconomic levels. It is well known that women from higher socioeconomic groups have easiest access to infertility therapies because of greater financial resources. The hospital of lower socioeconomic level presented multiple birth rates of approximately 8 per thousand during the decade, which may be considered as the natural one. The other three hospitals presented increased rates that were positively correlated to socioeconomic level. This increase occurred mainly due to dizygotic twins and to triplets and was as high as 4.8 per thousand in 1999. Maternal age was also positively correlated to socioeconomic level for singletons as well as for twins. However, during the decade the mean maternal age increased only in the two hospitals with better socioeconomic levels. Gestational order decreased as socioeconomic levels increased, mainly for twins and triplets. The percentage of singletons with low birthweight and very low birthweight decreased as socioeconomic level increased. However, twins presented with an equal distribution in the four hospitals, indicating that better socioeconomic level did not affect the incidence of low birthweight and very low birthweight among twins. Fetal death rate decreased as socioeconomic level increased but twin/singleton fetal death ratio is three times greater in the hospital of higher socioeconomic level suggesting that even in ideal conditions of medical and hospital facilities, the mortality of twins continues to be much higher than that of singletons.

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