The mothers of a consecutive series of 125 same sexed pairs of twins aged 6 months to 6 1/2 years completed a mailed questionnaire concerning the physical similarity of the twin partners. Zygosity diagnoses were first determined blindly on the basis of the answers given by the mothers, who did not know the result of the blood group test when fulfilling the questionnaire. Next, the results were compared with the zygosity determination based on examination of erythrocyte-, serum- and enzyme-groups. It turned out that a few simple questions distinguished well enough between MZ and DZ pairs, with a frequency of misclassification of 4%, leaving 5% as unclassifiable. It is concluded that twin zygosity can be estimated by a simple questionnaire with sufficient accuracy even in very young twin pairs. This has relevant implications for more extensive twin studies where the use of a wide spectrum of serological characters would imply too high expenses.

A case-control study was conducted to evaluate the association between maternal age and parity and dizygotic (DZ) and monozygotic (MZ) twinning, respectively. Mothers of all twins born alive in Denmark in 1984 or 1985 were included into the study as cases, and a random sample of mothers of singletons were controls. Data on maternal age at delivery and number of previous births were collected from the Danish Medical Birth Register. By means of logistic regression analysis, a significant and positive association was found betwen maternal age and DZ twinning, and a significant and negative association between parity and MZ twinning. No association was found for parity and DZ twinning, nor for maternal age and MZ twinning. The study suggests that human MZ twinning has predictors too, as has DZ twinning. The finding has implications for zygosity classification in future twin research.

SUMMARY A study was made of seasonality in twin birth rate in Denmark between 1977 and1984. We studied all twin births (N = 45 550) in all deliveries (N= 3 679 932) TBRt= e)T T + (1-0)) T' for t=2 ...., 588 B'/1000 T' and B' denote monthly twin and total birth frequencies, respectively, adjusted for length of month (ie, observed frequencies were multiplied by 30*4/number of days in month). A weight factor, 0), was used to adjust for differences in mean gestational length between singleton and twin pregnancies. A mean difference of 20 days has been reported in the literature,'5 which corresponds to 0) = 0-65. It

Abstract. In a case‐control study an association was found between mothers' smoking habits and the frequency of dizygotic twinning. As cases were included all twins born in Denmark in 1984 and 1985. A random sample of 1.5% of mothers to singletons, born in the same period were selected as controls. Only live‐borns, conceived after no hormonal treatment, were included in the study. The finding may be due to the well‐known anti‐estrogen effect of smoking.

In a comparative study to examine the effect of social factors on human twinning, data on sociodemographic and other factors were collected from parents of all twins born alive in Denmark in 1984 or 1985 and from a random sample of parents of singleton infants born in the same years. A postal questionnaire was used. The twins were classified as monozygotic (MZ) or dizygotic (DZ) twins by the similarity method. A trend was found in DZ-twinning, with significantly fewer DZ-twins born in the lower social classes, but not in MZ-twinning. All results were controlled for maternal age and parity.