Twenty–three cases of symmetrical conjoined twins were registered by the Latin-American Collaborative Study of Congenital Malformations (ECLAMC) in 1,714,952 births, which were observed during the 1967-1986 period in 95 maternity hospitals distributed in eleven Latin-American countries. This results in a birth prevalence rate of about 1/75,000 births. The secular and geographic distribution of this material do not depart from random in spite of one hospital with three cases, and two hospitals with two cases each, within a short time period. These 23 cases include one diprosopus, 3 dicephalus, one ischiopagus, 5 pygopagus, none dipygus, 3 syncephalus, none craniopagus, 9 thoracopagus, one omphalopagus, and none rachipagus. Sex distribution is even, with 12 male and 11 female cases.

The trends in births and in twinning in Australia are compared from 1854 to 1982. Until about 1930 the twinning rate increased as births fell. The two rates then became concordant. The relationship of fecundity to twinning is discussed. Comparison with other countries is made. The period embraces the demographic transition when changing contraceptive practices would have had a variable effect on twinning.

Twinning rates for the years 1975-79 in Catalonia (Spain) are presented. Crude rates are very low: 7.62 per 1,000 maternities, the DZTR and the MZTR being 3.74 and 3.88 respectively. Standardized rates remain very low. Sex ratio among twin couples is also very low (0.49 male vs 0.51 female births). A multiple linear stepwise regression on the twinning rates shows MZ rates to be influenced by birth order and father's age, and the DZ rates by mother's age and birth order.

Most population studies of twins estimate the number of monozygotic (MZ) and dizygotic (DZ) pairs by Weinberg's differential rule. This rule assumes that within the DZ twins the numbers of unlike-sexed (U) and like-sexed (L) twins are equal. The literature on the validity of Weinberg's rule is still controversial. In this prospective population-based study (EFPTS) of 2,589 twin pairs, of whom 2,577 were of known zygosity and placentation, the estimates of Weinberg's rule agree well with the results of direct zygosity determination.

A secular demographic study on the Iniö Island in SW Finland shows an even stronger decline of twinning rates than in the Åland Islands and the rest of the archipelago. Twinning rates in Iniö declined, in fact from about 30‰ to less than 10‰.

Mean MZ and DZ twinning rates in seven big Greek cities were 3.2 and 4.75 per 1,000 maternities, respectively, during the 1980-1985 period. The seasonal variations in twinning frequencies are not significant. The total twinning rate in Greece shows a decreasing trend from 1956 to 1985.

The components of a research program focussing on early mother-twin interaction is described. Preliminary data obtained from a questionnaire at two months post term, cross-sectional observations at the age of one year, a follow-up study involving home observation and parental interviews from birth to the age of 3, point to the specificity of this triadic situation. During the first months of life, the burden of material tasks and the increase in baby care leave little time for starting a relationship based on pleasure or play. The impossibility of responding simultaneously to the needs of two babies and the difficuty of forming relationships on an individual basis foster early concerns for egalitarianism. The degree of physical resemblance between the babies creates the problem of differentiating them. To tell twins apart, mothers rapidly tend to rely on behavioral characteristics to which they attribute a genetic basis. In contrast, differences in development between the babies that introduce the eventuality of the dominance of one of the twins are often denied. In this highly specific situation, mothers arrive at personal solutions of adjustment over the first 3 years, manifest in a certain number of psychological and educational attitudes. Analysis of these maternal attitudes may help to shed light on some of the features of later psychoemotional development in twins.

The results of the Bene Anthony Family Relations Test on 53 pairs of twins and 30 older siblings, as well as an analysis of personality inventories from their parents, are presented. It is shown that weight and birth order can affect not only personality and the way in which twins interact with each other, but can also affect family interaction. Particular attention has been given to the effect on and of the preceding sibling.

This three-year study was based on individual interviews of over 200 bereaved MZ and DZ adult twins. Its purpose was to record the Ione twins' response to the loss of their twin and to investigate factors that correlated with either the severity or amelioration of the loss.

The grief process is examined within the framework of multifetal pregnancy in a variety of demise situations. The case studies examined were obtained from approximately 120 lettere and interviews with families predominantly of higher order multiple births, who have experienced either partial or total loss in their pregnancy or afterwards. Objective and subjective factors were addressed. Results indicate the steps of the grief response are worked through in the same order as a singleton demise, but differ in intensity, duration, and frequency, depending on the individual circumstances, type of loss, gestational age, and the parents' backgrounds, expectations and beliefs. A unique feature discovered is that the entire grieving process, from shock through acceptance, appears to be completely experienced twice, with a large percentage of parents experiencing all of the stages three or more times. The general conclusion of the study is that the grieving pattern in multifetal death indeed follows a cycle and pattern of its own and requires special considerations.

In a retrospective survey of 336 mothers of recently born multiples, 71% reported suspicions of twin pregnancy prior to medical confirmation. Primiparous and multiparous reports of symptoms may be utilized to significantly increase the early diagnosis of the presence of multiples.

Parents of multiples suffer unique stresses which can severely impair family health and welfare. Access to information, counseling, and community resources increase parents' abilities to cope, and reduce the risk of child and spousal abuse. Twinline, a social service agency in Califormia, provides a variety of free and low-cost nonmedical perinatal services to meet the needs of a heterogeneous population of over 1,000 multiple birth families and parents expecting multiples in the urban and rural counties of the San Francisco Bay Area.