The twinning incidence from 1984 to 1993 was investigated in Campinas, State of São Paulo, Brazil, and the analyzed data were combined with previous series from the same population. This study has shown that the mean annual incidences and the standard deviations for DZ, MZ and DZ plus MZ twins for the period of 1984 to 1993 are estimated respectively as 4.7 ± 0.92, 4.1 ± 1.11, and 8.8 ± 0.87 per 1,000. In the same period the mean annual incidence of triplets was 0.15 ± 0.16. It was also shown that the incidence of DZ twins is steadily decreasing since 1925, while the incidence of MZ pairs, after a period of decline is increasing since the sixties, due perhaps to the widespread use of oral contraceptives. Concerning the twinning rate as a whole, it has been shown that its declining trend has disappeared over the last few years.

Analysis of 116,699 deliveries (1062 twin pairs and 115,637 singletons) which occurred from 1984 to 1996 at the largest obstetric center in Campinas, SP, Brazil, provided the following conclusions: 1) despite the low percentage of twin births (0.9%), 10.7% of all early neonatal deaths and 3.5% of all stillbirths were twins. Compared to singletons, the likelihood of stillbirths and early neonatal deaths among twins was 1.9 and 6.5 times greater, respectively. 2) The proportion of twins among early neonatal deaths tended to decrease, probably because of improvements in prenatal and perinatal care.3) The critical period for early neonatal deaths in singletons and twins lasted for the three first days after birth. 4) The incidence of perinatal deaths among twins was highest among MM pairs, followed by FF pairs, and lowest among MF pairs. 5) There was a trend towards a decrease in the annual rate of early neonatal deaths among both singletons and twins, and of stillbirths among singletons, the latter being more conspicuous among females. 6) The sex ratio of singletons was practically stable among living newborns, but showed strong cyclic variation among early neonatal deaths, and a tendency to increase among stillbirths. Twins who died in the first week, as well as living twins showed a strong increase in their sex ratio from 1984 to 1996. 7) The mean gestational age of singletons was highest among living newborns followed by stillbirths, and was lowest among early neonatal deaths. The mean gestational age of living twins was higher than that of early neonatal deaths, but not different from that of stillbirths. 8) The mean birth weight of singletons and twins was highest among living newborns, followed by stillbirths, and lowest among early neonatal deaths. 9) Apgar scores ≥ 7 obtained five minutes after birth are inadequate as a reliable indicator of the clinical condition of the newborn, since almost half of the early neonatal deaths had 5-min Apgar scores varying from 7 to 10.

Intraclass correlation coefficients for one- and five-min Apgar scores of 604 twin pairs born at a southeastern Brazilian hospital were calculated, after adjusting these scores for gestational age and sex. The data support a genetic hypothesis only for 1-min Apgar score, probably because it is less affected by the environment than 4 min later, after the newborns have been under the care of a neonatology team. First-born twins exhibited, on average, better clinical conditions than second-born twins. The former showed a significantly lower proportion of Apgar scores under seven than second-born twins, both at 1 min (17.5% vs. 29.8%) and at 5 min (7.2% vs. 11.9%). The proportion of children born with "good" Apgar scores was significantly smaller among twins than among 1,522 singletons born at the same hospital. Among the latter, 1- and 5-min Apgar scores under seven were exhibited by 9.2% and 3.4% newborns, respectively.
Os coeficientes de correlação intraclasse foram calculados para os índices de Apgar 1 e 5 minutos após o nascimento de 604 pares de gêmeos em uma maternidade do sudeste brasileiro, depois que esses índices foram ajustados para idade gestacional e sexo. Os dados obtidos apoiaram a hipótese genética apenas em relação ao primeiro índice de Apgar, provavelmente porque ele é menos influenciado pelo ambiente do que 4 minutos depois, quando os recém-nascidos já estiveram sob os cuidados de uma equipe de neonatologistas. Os gêmeos nascidos em primeiro lugar apresentaram, em média, melhor estado clínico que os nascidos em segundo lugar, visto que os primeiros mostraram uma proporção de índices de Apgar inferiores a 7 significativamente menor do que os nascidos em segundo lugar, tanto um minuto (17,5% contra 29,8%) quanto cinco minutos após o nascimento (7,2% contra 11,9%). A proporção de recém-nascidos com índices de Apgar que indicam bom prognóstico foi significativamente menor nos gêmeos do que em 1.522 conceptos únicos nascidos na mesma maternidade. Índices de Apgar menores do que 7, obtidos um e cinco minutos após o nascimento, foram encontrados respectivamente em 9,2% e em 3,4% dos recém-nascidos de parto único

The annual variation of the sex ratio (SR) of 1385 twin births was analyzed and compared to that of 85909 singletons. These births referred not only to live births but also to stillbirths in two southeastern Brazilian maternity hospitals from 1984 to 1993. While the annual variation of the SR of singletons was very small, that of twin births was extremely high, due to the significant heterogeneity of the annual data. It is suggested that the large SR annual variation of the twin birth might be a consequence of the variation of male or female monozygotic twins. The hypothesis that twin births show a lower sex ratio than singletons could be supported by the present data.

Intraclass correlation coefficients were calculated for the birth weights of twins born at three southeastern Brazilian hospitals, after adjustment of the natural logarithms of these weights for gestational age, its quadratic and cubic terms, sex, and their interactions. The data indicate that fetal genetic effect on birth weight might have the opportunity to be demonstrated by children born to undernourished women. Undernourishment, acting as a selective force, might enhance the existence of genotypes that determine less need of food for normal development.
Os coeficientes de correlação intraclasse foram calculados para o peso de gêmeos nascidos em três maternidades do sudeste brasileiro, depois de ajustar os logaritmos naturais desses pesos para a idade gestacional, seus termos quadrático e cúbico, sexo e interações dessas variáveis. Os dados obtidos indicaram que o efeito genético fetal sobre o peso ao nascer teria a oportunidade de ser demonstrado por recém-nascidos de mães subnutridas. A subnutrição, atuando como força seletiva, realçaria a existência de genótipos que determinam menor necessidade nutricional para o desenvolvimento normal