The decrease trend of multiple births temporarily broke off in Hungary in 1970, with an increase from 18.18‰ (1968) to 20.94‰ (1970), i.e., about 400 cases of excess.

The male preponderance in twin births is lower than in single births, and triplets actually show a female excess.

There are more boys than girls among the first-born of multiple births.

According to the 1959 and 1968 figures, both DZ and MZ twin births were significantly more frequent in mothers aged 16 or less than in mothers aged 17-19 years.

The monthly distribution of DZ twins is characteristic, while the monochorial MZ twins did not show seasonal variations.

Since 1 January 1970, multiple births have been recorded by the Budapest Twin Register. The maternity wards report multiple births by telephone, retain the placenta, mark the umbilical cord of the first born and fill in the Registration Form of Multiple Births indicating the most important personal and obstetrical data. Placentae are examined by a pathologist. In like-sexed twins with dichorial placenta zygosity is determined 6-24 months after birth by applying blood and serum protein group determinations and dermatoglyphics. A total of 85.9 and 84.2% of all multiple births (356 and 316) were reported in 1970 and 1971, respectively. So far, the zygosity of twins was determined in 53.8 and 50.5% of all twins in 1970 and 1971.

With the introduction of obligatory registration of twins in Budapest on 1 January 1970, our team started the genetic and somatopsychic examination of twins. The program is being carried out in collaboration with the institutions of the Child Health Care Organisation of Budapest and the National Institute of Public Health.

Since 1 January 1970, the placentae of twins born in Budapest are submitted to pathologic and histologic examination, and the twins living in Budapest are examined systematically by our clinical group at the age of six months, and one, two, and three years.

In addition to studying the social environment, examinations are carried out with regard to pediatrics, anthropometric measurements, photographic documentation, dental examination, dermatoglyphics, cardiology, and neuropsychiatry. In order to define the genotype of the twins we used haptoglobins and blood groups.

A total of 400 DZ and 76 MZ twin pairs were studied for: maternal age, in which no significant differences were found; birth order, which was significantly higher in DZ twins; weight at birth; frequency of ABO blood-group phenotypes, where no significant differences were found; and, lastly, frequency of twinning in relatives, where, there was significantly higher occurrence, namely in siblings of probands.

Macroscopic examination of the placenta and histological examination of the membranous partition between amniotic sacs was peformed in approximately 1400 twin maternities in Ibadan, Nigeria.

Monochorionic placentation is mainly characterised by two layers (as opposed to four in the dichorionic one) in the dividing septum between the two amniotic sacs, as well as by the absence of a membranous ridge of tissues at the base of the septum, that is instead present in the dichorionic one.

Monochorial twins appear to be invariably MZ, whereas dichorial twins may be either MZ or DZ. In the present study the chance of being DZ was 95% in the case of single and of 99% in the case of double dichorionic placentation.

When the level of sex chromatin in a newborn female remains as low as 2-8% after the first few days of life, some genetic disorder may be involved.

A clinical case is presented where only one of MZ twins appears to be affected.

A total of 70 karyotypes of infants from triplet pregnancies and their parents from 17 families were examined. The karyograms of 16 families showed a normal number of chromosomes and no abnormal morphology. Only in the 17th family a girl showed an additional chromosome in group G and deletion of the long arms in another from group C. A high concentration of developmental malformations was however present in this family.