Since military service in Italy is obligatory for all 20-year olds (“conscripts”) who do not choose to perform alternative civilian service, we thought of utilizing the data derived from the examinations all conscripts undergo, and selecting information relative to twins. The material we have gathered from the country's Draft Boards concerns the period from October 1987 to December 1988, and this information allows to determine: 1) the twin status of a conscript and the identification of his cotwin; 2) aptitude for military service; 3) occupation; 4) presence of twins in the father's or mother's lineage; 5) home address and telephone number.

The twin material collected concerns 2,739 male pairs and 1,433 pairs of both sexes, classified with respect to:

1) Aptitude for military service;

2) Choice of occupation;

3) Presence of twins in the family.

An attempt has been made to apply Payami's models to maternal age-specific twin birth prevalences in several countries. The models disclose the heterogeneity of a cohort and spell out the risks to susceptible members (who will actually get the disease) according to age (or time). Payami's method specifies that the typical cohort of susceptibles is ascribed to two exposures: a potent and generalised exposure and a very low or no risk secondary exposure. The models have been adjusted to international data from current as well as old populations, of Occidental and Japanese origin. Results show that cohorts of twin-prone mothers aged 25 to 45 are homogeneous. A single dominant etiology is suggested which applies to both MZ and DZ twins. Heterogeneity, from 10% to 25%, is present in all countries whenever the 20-24 age group is involved. A separate study of illegitimate twin births from Denmark reduces the heterogeneity and shows that MZ illegitimate twin births are due to a secondary exposure responsible for a distinct twinning etiology. The age-specific risks of a MZ illegitimate twin birth are much higher than those of any comparison group, and are constant until age 35 years. This suggests a single-hit exposure akin to a neuroendocrine stimulus which short-cuts the usual age-dependent etiologic pathway. MZ and DZ twins both experience the same maternal age specific risks, an observation which underscores the common etiology of both types of twins. Risks increase with age from age 20 to 45 years. The pattern according to age varies among countries and time periods. Occidental populations have a pattern varying from an exponential to a more linear increase in old and recent populations, respectively. Only present-day Japan displays a logarithmic-like growth curve. The concept of “reproductive maturity” is introduced, and related to the secular trend of the DZ twin birth risk and to its variation across countries. Two new conclusions are drawn: a) The higher the rate of reproductive maturity, the less the DZ twin birth risk and, b) Reproductive maturity determines the maternal age-specific gonadotropin levels.

We wished to establish the frequency, regularity and laterality of multiple ovulation in mothers of dizygotic (DZ) twins and controls. Subjects had regular menses and were not using oral contraceptives. Ovarian ultrasound scans were taken over a number of cycles in 21 mothers of DZ twins and 18 controls (including 13 mothers of monozygotic [MZ] twins). Multiple large follicles (≥ 12 mm diameter) were seen significantly more frequently in mothers of DZ twins (13/21 mothers, 24/77 cycles, average ± SE follicles/cycle 1.34 ± 0.11) than controls (2/18 mothers, 3/31 cycles, average 1.10±0.08). Both ipsilateral and contralateral multiple follicles were observed. In one case a mother of DZ twins showed multiple large follicles in 7 out of 10 cycles in which she was scanned, including both ipsi-and contralateral patterns of occurrence.

The objective of this study was to compute yearly neonatal mortality rates (NMRs) in twins and compare these to rates in singletons during the same time period. The focus was on time trends in birthweight-specific twin mortality in the birth population of New York City during the years 1968 to 1986. The study population was all twin livebirths ≥ 500 g birthweight (N = 45,605), with a comparison group of all singleton livebirths in the same birthweight range (N = 2,191,144). Data came from the New York City Department of Health's computerized vital records on livebirths and infant deaths. Between 1968 and 1986 the crude NMR declined 39% in twins and 47% in singletons. In twins there were birthweight-specific declines of 69% to 84% between 1000 g and 2499 g. However, there was only a 19% decline in the twin NMR over 2499 g. This contrasts with a 50% decline in the singleton NMR over 2499 g. In New York City, modern medical care has been remarkably successful in lowering the NMR in low birthweight twins. However, more effort must be made to understand the etiology of perinatal problems in twins with birth weights greater than 2500 g.

The patterns of sex ratio in Israeli twins by maternal age and parity are described in two ethnic subpopulation. Jews and Bedouins differ one from the other in genetic, environmental, lifestyle, and reproductive factors. In the Jewish subpopulation, the proportion of males was significantly lower in twin than in singleton births; parity increased the odds of male twin births while maternal age had the inverse effect. In the Bedouin subpopulation, the sex ratio did not differ significantly from that of singletons, and no consistent patterns were found by maternal age and parity. The data suggest that several factors may influence the sex ratio in twin births. In Jewish twin births, the findings are consistent with the literature and can be largely explained in terms of high level of maternal gonadotropins. In the Bedouin subpopulation, paternal influence which could be related to lifestyle may be present, and as yet undefined genetic factors may also be involved. It would be of great interest, therefore, to continue monitoring of the effects of changes in lifestyle on the Bedouin population in order to tease out the relative importance of the varying factors on the sex ratio of twin births.

Based on Dutch twin incidence figures since the beginning of the current century, evidence is provided in support of the idea that the DZ unlike/like sexed ratio has gradually shifted (since 1900) from unity to less than unity. Opposing conclusions with regard to the justification of the use of Weingberg's differential rule are very probably correct in themselves but could depend on country and period of birth of the twin sample used. Furthermore, the fast drop and subsequent rise in DZ twinning rate between about 1963 and 1990 can very likely for the greater part be ascribed to a parallel shift in maternal age.

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 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.

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.

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 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.