This article explores some rare sides of twin research. The focus of this article is the sad plight of the Dionne quintuplets, born in Canada in 1934. However, several other studies belong in this category, such as Dr Josef Mengele’s horrifying twin research conducted at the Auschwitz concentration camp, Dr John Money’s misguided attempt to turn an accidentally castrated male twin into a female, Russian scientists’ cruel medical study of conjoined female twins and Dr Peter Neubauer’s secret project that tracked the development of separated twins. Reviews of current twin research span twins’ representation of self-image, twins with Kleine–Levin Syndrome, heteropaternal twinning in lemurs and factors affecting risk of dental caries. Media coverage includes a pair of high-society models, a book about the ‘Winkelevii’ twins, Super Bowl twin teammates, a family with three sets of fraternal twins, a twin sister surrogate and a near presidential twin.

Given the uniqueness of the Hungarian conjoined twin sisters Helen and Judith (1701–1723) and their lasting influence as an anatomical showcase if not model for mental or social deviant states, we present here a closer scrutiny of their introduction into the scientific literature of the later 18th century by analyzing depictions of the twins from 1707 onwards.

Craniofacial duplication or diprosopus is a very rare malformation that is present in approximately 0.4% of conjoined twins. Here is presented a case of craniofacial duplication in association with bilateral cleft lip/palate in both heads found in a ceramic representation from the early Chimú culture from Peru. A comparative analysis is made with a current case of a 28-week-old fetus with similar characteristics. After reviewing the medical literature on conjoined twins, very few reports of facial cleft in both twins were found, with no reports at all of bilateral cleft lip/palate. This ceramic crock is considered one of the first representations suggestive of craniofacial duplication, and probably the first reporting it in association with facial cleft.

This paper discusses an unusual etching of unlike-sex conjoined twins of the syncephalus type dated 1547. The authenticity of the case is discussed in terms of the biological plausibility of such twins and in the light of mediaeval understanding of the twinning process. Unlike-sex syncephalic twins may occur as a result of dispermic fertilisation, or be an erroneous depiction of an asymmetrical anomaly of the external genitalia of a like-sex pair, mimicking the genitals of the other sex. On the other hand, the Löwen etching conforms with mediaeval beliefs that conjoined twins are divine punishment and therefore were used for ecclesiastical and perhaps commercial purposes. The historical context of the drawing, in which the relation between religion, media, and medicine satisfied the eagerness of the public for curios of Nature, is no longer valid. Thus the Löwen etching opens a window to the past from which modern medicine can be appreciated. Twin Research (2000) 3, 185–188.

Case reports of thoracopagus/xiphopagus conjoined twins by Godofredus Christell (1751) and Francesco Luigi Fanzago (1803) have been recovered and can be added to the history of conjoined twins.

One of conjoined thoracopagus twins, with separate hearts and a common pericardial sack, presented with respiratory distress because of a persistent arterial duct causing congestive heart failure in the neonatal period. Surgical ligation of the duct was performed prior to subsequent separation, with an excellent outcome.

In order to examine perinatal mortality and prevalence of major congenital malformations in twins, deliveries in four teaching hospitals in Taipei City were studied. Among a total of 73,264 deliveries from October 1985 to June 1989, there were 844 pairs of twins. The zygosity of the twin pairs was determined by sex, placentation and 12 red blood cell antigens. There were 482 MZ and 252 DZ twin pairs identified, but the zygosity of a further 110 twin pairs was indeterminable due to lack of information on plancentation and/or blood types. A total of 4,573 singletons delivered in one study hospital from July 1986 to June 1987 were also studied as controls. The perinatal mortality rate was 7.5% for MZ twins, 1.4% for DZ twins, and 0.7% for singletons. The concordance rate of perinatal death was significantly higher in MZ (60%) than in DZ (0%) twins. The prevalence of major congenital malformations was 2.7% for MZ twins, 1.0% for DZ twins, and 0.6% for singletons. The concordance rate of major congenital malformations was 18% for MZ twins, but no DZ pair was concordant in any major congenital malformation. The concordance rate of facial clefts was 29% for MZ twins. There were 2 sets of conjoined twins giving a prevalence rate of 2.7 per 100,000 deliveries. These findings showing the prevalence of perinatal mortality and major congenital malformation to be highest in MZ twins, intermediate in DZ twins and lowest in singletons, suggest the importance of intrauterine environments in the determination of perinatal mortality and congenital malformations.

Data on conjoined twins have been collected from 14 different malformation monitoring programs around the world. Among over 28 million births, 312 cases were identified. After considering underascertainment in one large program, the best estimate of the incidence based on the sum of induced abortions and births is 1.3 per 100,000 births. The distribution according to type of twinning, the sex distribution (39% males) and the stillbirth rate (47%) are presented. The presence of malformations not directly related to the area of fusion is discussed. In three women, thyroid disease was present and five women had been treated for infertility before conception.

The total prevalence of conjoined twins (birth + prenatally diagnosed) was 1:68,000 in the study of 1970-1986. Symmetrical cases (the so-called Siamese twins) have an obvious predominance (92.3%). Associated major malformations occurred in 80% of conjoined twins and more than 1/5 were discordant. The surviving time of liveborn conjoined twin sets was not more than two days except in two surgically successfully separated pairs. The family study did not indicate a higher recurrence risk. The case group was compared to two control groups and it appeared that the periconceptional use of oral contraception and ovulation induction were mentioned more frequently in pregnancies resulting in conjoined twins.

Nation-wide data in Japan on the 112 sets of conjoined twins from fetal deaths and from postnatal deaths during 1979-1985 were analysed. Female conjoined twins accounted for 60% of cases. The incidence rate of conjoined twins remained Constant except in 1985. Overall incidence rate was 10 per million births. Maternal age effect was found in mothers over the age of 40, where the highest incidence rate was obtained. The incidence rate of conjoined twins increased with birth order. There was no seasonal variation in the time of conception.

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.

During a 10-year period (1971-1980) 16 sets of conjoined twins were delivered in the author's hospital. The incidence of those twins was 1 in 4242 deliveries, 1 in 69 multiple births, and 1 in 139 congenitally malformed babies. All diagnoses were made during labour or at operation and all babies were lost. There were no maternal deaths.

A study of cigarette smoking was undertaken in a pair of craniopagus twins to determine how a transfer of products of smoking is occurring between the twins. Alternately and independently, one twin smoked a nicotine-free cigarette, then the second twin smoked a nicotine-containing cigarette. The procedure enabled the investigators to study the migration of nicotine and carbon monoxide from one twin to the other. Salivary determination provided a noninvasive method of measuring cross circulation in conjoined twins. Measurements of salivary nicotine, however, indicated that, although the nicotine levels rose following smoking, there was relatively little transfer from one twin to the other through the circulation.