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 risk for specific defects among twins compared to singletons was studied using data collected by the Spanish Collaborative Study of Congenital Malformations (ECEMC). A total of 136 twins had a major and/or minor congenital defect. The overall rate of congenital defects in twins (2.37%) did not deviate significantly from the rate in singletons (2.21%). Like-sex (LS) and male-male (MM) twin pairs had a slightly higher rate of birth defects than unlike-sex (US) and female-female (FF) pairs, respectively. Defects of the central nervous system, cardiovascular system and genitourinary system were significantly more frequent in LS twins than in singletons, with relative risks of 2.8, 2.5 and 1.6, respectively. No significantly increased risk was found among US twins. Among defects of the central nervous system, the rates of anencephaly, encephalocele and hydrocephaly were significantly higher in total and LS twins; however, no significantly increased risk for spina bifida was observed when compared to singletons. MM twins were also 1.9 times more likely to have hypospadias, but the risk among males of male-female (FM) pairs was decreased.

In order to more adequately characterize patterns of intrauterine growth retardation in twins, the mean birthweights of all nonanomalous white or black twins born between 24 and 41 weeks of gestation and surviving until discharge over an 11-year period (547 infants) and all similar singletons (19,072 infants) were compared by completed weeks of gestation. Between 24 and 35 weeks of gestation, the mean birthweights for the 547 twins and the 19,072 singletons were comparable and did not consistently differ statistically. From 36 to 41 weeks gestation, however, the difference became large, consistent, and statistically significant for each week at P < 0.0001. This difference was present among all subgroups of twins, ie, in all males, females, blacks, and whites; it was still evident when the sample was further stratified by both race and sex (black males, white males, black females, white females). These data suggest a pattern of growth retardation in twins compared to singletons which is large, consistent, and statistically significant beginning at 36 weeks gestation. Clinically, these data also suggest the need for ultrasonic examination early in gestation (24-32 weeks) to document normal growth and to provide baseline data, and show the importance of such monitoring later in gestation, specifically after 36 weeks.

We examined the neonatal outcome of 644 twins weighing 500 g or more and 656 singletons, born in the years 1984-1986 in the Soroka Medical Center, Beer-Sheva, Israel. There was nearly a four-fold risk of antepartum death in twins vs singletons, which disappeared when birth weight was controlled for. The risks for intrapartum and early neonatal mortality were not raised in this population. A statistically significant relative risk for congenital heart malformations in twins vs singletons remained (RR = 5.0, 95% CI = 1.5-16.3), after controlling for maternal age. Significantly higher rates of hyalin membrane disease, hypoglycemia, hyperbilirubinemia, anemia and septicemia were found in twins. Controlling for the confounding of the association between twinning and mortality or morbidity caused by differences in distributions of mode of delivery or gestational age between twins and singletons, was not as efficient as the controlling for birth weight. Thus, adjustment for birth weight removed all the excess risks detected except in hypoglycemia. Our findings suggest that the-lower birth weight of twins, which is so intimately associated with multiple gestations, is probably the single most important factor associated with neonatal problems found in twin births.

In order to establish a protocol considering the definition, diagnosis, and management of growth-discordant twin gestations, a questionnaire was sent to 96 authors of twin-related obstetric articles. The views of the 61 responders comprise this international census survey. The data suggest that a clear cut-off value for discordancy is still needed; however, the data indirectly supported a two-grade definition, namely, mild (> 15% and < 25% birth-weight disparity) and severe (> 25%) growth discordants. Expectant management was advocated by the majority of participants with out-patient follow-up for mild discordants, while severe discordants may preferably be hospitalized. Follow-up should be done by non-stress testing (daily - 2/wk), biophysical profile (1-2/wk), Doppler velocimetry (1/wk - bi-weekly) and sonographic biometry (bi-weekly). The opinions considering termination of pregnancy because of intertwin growth discordancy were divided; however, discordancy per se, was not considered an indication for cesarean delivery. An adapted management flowchart that summarizes the survey's data is presented and may be used as a standard for future investigations.

Environmental (b2) and genetic heritability (h2) of serum isocitrate dehydrogenase (ICDH) were estimated in 96 pairs of healthy twins according to a path analysis model. The results showed significant heritability (0.42) and a very low environmental component (based on obesity and level of habitual physical activity), suggesting that these factors do not influence ICDH serum activity.

Dizygotic (DZ) World War II veteran twins who participated in the National Heart Lung and Blood Institute (NHLBI) Twin Study have been reported to have greater variance than monozygotic (MZ) twins for plasma high-density lipoprotein cholesterol (HDL-C), cholesterol in the low-density fraction of HDL (HDL2-C) and apolipoprotein A-I, a major protein component of HDL. It was hypothesized that a possible source of this difference in zygosity variance could be prenatal environmental influences related to placental type. Dermatoglyphics were used to provide a retrospective index of placental type in a subset of the NHLBI MZ twins aged 59-70. The MZ twins classified as dichorionic were found to have significantly greater within-pair variability than the monochorionic MZ twins for HDL-C, HDL2-C and Apo A-I. These findings indicate that intrauterine environmental influences on HDL are manifest later in life.

To evaluate the relationship between genetic factors and back pain, 5029 MZ and 7876 DZ twin pairs were investigated by a questionnaire. The data were analyzed with regard to physical symptoms, eg, back pain, and certain work conditions. Pain was reported by about 17% of the male and 13% of the female twins in the material. The occurrence of back pain was strongly associated with work load, especially among the male twins. Twin concordance regarding back pain was considerably higher in MZ than in DZ twins. Of four groups who were studied, statistically significant differences between the two twin categories were observed in three cases. This result supports the assumption that a relationship exists between genetic factors and the occurrence of back pain. Official registrations for sick-leave pay in a sample of 247 twins confirmed the interview information on back pain. Just a few cases were wrongly classified, ie, they were sick-listed because of back pain but had stated no back pain in the questionnaire.

A prospective study was undertaken which examined 179 sets of twins, 68 premature (less than 36 weeks of gestation) and 111 term. The purpose of this study was to assess differences in the acid-base status between twins related to gestational age, birth order and the time interval between twin births. Although the twin blood-gas data is within the range considered normal, statistically significant differences favoring the first-born were noted for both preterm and term twins. These differences do not depend on gestational age, route of delivery or presentation, and become evident when the interval between twin births exceeds one minute. We postulate that after delivery of the first twin, the reduced uterine size causes a decrease in the intervillous blood flow and consequently a reduction in the respiratory exchange between the second fetus, still in utero, and its placenta.

A histomorphological study of the amniotic membranes in full-term twins with double and single placenta was carried out by means of the silver impregnation staining technique suitably modified. Specimens of interface of amniotic membranes were prepared by means of sections. The constant presence of a tubelike structure was observed. Proceeding from the amniotic cavity, the following histological layers were noted: 1) single layer of amniotic cells; 2) amorphic substance with fibrocytes; 3) single layer of endothelial cells. The same order of single layer is present in the amniotic membrane of the second fetus. This tubelike structure is present only in cases of twins with double placenta. If the placenta is single with two umbilical cords, the tubelike structure is not present and only a central amorphic substance surrounded by two single layers of amniotic cells is observed, to confirm the single embryogenetic derivation (monovular). Therefore, through this histological method, we can recognize the true single placenta of twin pregnancy from the pseudosingle placenta so said for the presence of adherences of adjoining surfaces that make it appear single. On the contrary, by manual dissection it is possible to identify a twin pregnancy with two placentae. From the physiological point of view, the walls of the tubelike structure have probably the function to realize exchanges of amniotic liquids between the two fetuses, so as to obtain a balance of electrolytic ions and of intercavity pressure. Growth factors (vascular endothelial factor) are probably involved in the genesis of the endothelial tubelike structure.

A prospective study was undertaken to determine the effect of twin birth-weight discordancy on Apgar scores and umbilical blood acid-base parameters. Using the paired t-test, small but statistically significant differences were seen in these parameters favoring the heavier twin over its lighter sibling. These differences were also affected by birth order, with the first-born being favored.

Maternal plasma IGF-I and hPL concentrations were examined in 10 singleton pregnancies and in 11 twin gestations near term. hPL concentrations were higher in the twin pregnancies (14.4 ± 2.4 μg/1 vs 6.9 ± 0.9 μg/1, P < 0.02). In contrast, plasma IGF-I concentrations were similar in the singleton and twin pregnancies (533 ± 45 μg/1 vs 572 ± 60 μg/1, respectively). IGF-I concentrations failed to correlate with hPL concentrations in either group separately or when all subjects were considered together. These data do not support the hypothesis that maternal IGF-I secretion is an hPL-dependent process.

The perinatal characteristics of 16 vaginal-abdominal deliveries of twins were evaluated. The primary indication for the cesarean delivery was compound vertex presentation (n = 4), prolapsed umbilical cord (n = 4), transverse lie (n = 7), and mentoposterior face presentation (n= 1). Ther outcome of Twin A was not different from that of Twin B. About 90% of the twins were eventually discharged on time. A significant correlation (R = 0.9722, p < 0.0003) was found between the reported rates of combined deliveries and cesareans in twins. The data suggest that a higher rate of combined deliveries is expected in practices where abdominal deliveries are performed more often in twin gestations, while in obstetric services with low cesarean rates in twins, combined deliveries seem to be unfortunate occurrences dictated by unexpected intrapartum events.

Two cases of antepartum demise of one fetus in a twin pregnancy are reported. Obstetrical management and fetal outcome are discussed with particular respect to changes in maternal coagulation parameters.