Twin births are known to vary across seasons in several countries. It has been hypothesized that this variation may be due to seasonal changes in luminosity leading to pineal gland-mediated multiovulation among susceptible mothers. To describe seasonal variation of twin births in Washington State, all mothers residing in Washington State who gave birth to both a pair of twins and a singleton baby between 1984-1990 (n = 1168) were identified through linkage of computerized State birth certificates. Using a “matched-on-mother” case-control design, the estimated month of conception of twin gestations (the “case” events) were compared to that for their singleton siblings (the “control” events) to determine their relative occurrence during periods of high vs low sunlight in accordance with local climatological data. For the study population as a whole, there was only a slight tendency for twins to have been conceived during the period of high sunlight compared to their singleton siblings (OR = 1.3, 95% C.I. = 1.0-1.7). When stratified by concordant-sex vs discordant-sex, however, more discordant-sex twin pairs were conceived during the light period than corresponding singletons (OR = 1.7, 95% C.I. = 1.0-2.8), whereas no association was found for concordant-sex twins (OR =1.1, 95% C.I. =0.8-1.6). The presence of an association only among discordant-sex twins, all of whom are dizygotic, is consistent with the hypothesis that exposure to sunlight may stimulate multiple ovulation, and thus increase the incidence of twin gestations among twin-prone mothers.

This study examined sleep patterns in twins, paying special attention to the mirroring phenomenon. Concordance and discordance of sleep related patterns (parasomnias) in a group of 27 monozygotic (MZ) mirror image twins were compared to sleep patterns in a group of 53 MZ non-mirror image twins and a group of 24 dizygotic (DZ) twin pairs. Sleep patterns had the lowest concordance among MZ mirror twins. “Mirroring” was observed not only in facial features but also in some physiological patterns such as sleep and sleep deviations. These facts suggest that mirroring in MZ twins is not merely a superficial epiphenomenon, but a reflection of a biological polarization. Biological polarization in this context is a descriptive term emphasizing the role of biological (physiological, biochemical or even genetic) versus psychological or environmental factors causing not only mirror body image but opposite tendencies in the development of personality, professional and sex orientation and, most importantly, the opposite presentation of pathology.

To evaluate the influence of rate of gestational weight gain on newborn birthweight and birthlength in twin gestations complicated by preeclampsia, 74 cases of preeclampsia in twin pregnancy were matched to 148 non-preeclamptic twin controls for maternal race, height, pregravid weight, age and length of gestation. Total weight gain was significantly higher for all cases versus controls and for mild cases versus controls. However, the rate of early weight gain was lower for severe cases and severe cases with thrombocytopenia compared to their controls. Mean birthweight and birthlength did not differ between cases and controls, although the proportion with birthlength below the 10th percentile was significantly higher among cases than controls. In addition, the proportion of birthweights and birthlengths < 10th percentile was significantly higher among cases than controls. These findings suggest that inadequate early weight gain in twin gestations complicated by preeclampsia results in retarded birthlength and birthweight. The implication of these findings are discussed.

In order to evaluate the influence of birth order and fetal presentation on antenatal growth of twins we conducted a comparison of prospective measurements of five fetal biometric indices in 50 vertex-vertex and 47 vertex-breech twins. We compared (a) twin A to twin B in both groups; (b) the second and (c) the first twins of both groups. Both groups had similar maternal and neonatal characteristics. The growth curves of the twins were also very similar except for three significant (p<0.05) deviations: (a) Twin A of the vertex-vertex group, had larger femur length (FL) at 18-19 weeks, abdominal circumference (AC) and estimated fetal weight (EFW) at 29 weeks, and EFW measurements at 36 weeks, (b) Second breech twins, compared to their second vertex cohorts, had significantly smaller biparietal diameter (BPD), head circumference (HC) and FL at 18-19 weeks, BPD and HC at 29 weeks, and EFW at 37 weeks, (c) First twins of the vertex-breech group, as compared to first twins of the vertex-vertex group, had significantly smaller BPD and AC at 18-19 weeks, FL and AC at 21-22 and 29 weeks, FL at 31 weeks, and EFW at 27-28 and 36 weeks' gestation. We concluded that significantly different sonographic fetal indices may be measured at about 20 and 30 weeks' gestation, but not later. An adaptive mechanism attributed to fetal presentation is suggested to explain similar birthweights in spite of these antepartum differences.

In a sample of 105 concordant sex MZ and DZ twin pairs, the following characteristics were measured: red cell count, haemoglobin concentration, package cell volume, mean cell volume, mean cell haemoglobin, mean cell haemoglobin concentration, reticulocytes, platelets, white cell count and the six types of leucocytes, lymphocytes, monocytes, band and segmented neutrophils, eosinophils and basophils. The statistical model employed in the univariate twin analysis allows for three sources of variation: genetic (h2), shared environmental (c2) and specific environmental influences (e2). A genetic component was significant for red cell count, haemoglobin and mean cell haemoglobin (0.64, 0.60 and 0.46 respectively), with heritable variation suggested for package cell volume, mean cell volume, mean cell haemoglobin, lymphocytes and monocytes. Shared environmental variation was only present for neutrophils.

Washington State birth certificate data for the years 1984-1988 were analyzed for 2,804 mothers of twins in order to determine whether black mothers of twins have a higher risk of delivering a low birthweight twin infant than white mothers. Seventy four percent of black mothers of twins gave birth to an infant weighing < 2500 grams (white 52.9%), while slightly less than 20% gave birth to an infant weighing <1500 grams (white 9.4%). After adjustment for maternal age, parity and marital status, the risk of black mothers giving birth to at least one twin infant <2500 grams relative to white mothers was 1.3 (95% confidence interval, 1.2, 1.5). When this analysis was restricted to very low birthweight babies (<1500 grams), the relative risk for infants of black mothers compared to white mothers adjusted for the same factors was 2.1 (95% confidence interval, 1.5, 3.0). The frequency of neonatal mortality in the study population was also assessed. The overall frequency of mortality in black twin infants was double that in white twins infants. When stratified by birthweight category, the frequency was higher in white infants (30.2%) than in black infants (24.1%) weighing less than 1500 grams at birth. However, within higher birthweight categories, (≥1500 grams to <2500 grams and >2500 grams), relative frequencies of neonatal mortality were higher in black infants. This analysis reveals that black mothers of twins are more likely to deliver low birthweight twin infants than white mothers. The increased risk is even more pronounced in very low birthweight twin pairs. However, the additional risks for low birthweight black twin pairs and very low birthweight twin pairs found in this study are lower than the risk increase commonly reported for the respective birthweight categories in black singleton infants relative to white singleton infants. This implies that while black twin pregnancies require many of the same special antepartum considerations given to black singleton pregnancies, the increased baseline risks associated with low birthweight and very low birthweight twin deliveries offset racial disparities traditionally seen in singleton infants.

Subjects were 74 twin pairs, 61 MZs and 13 same-sexed DZs who entered the High School affiliated with Tokyo University, Japan. Their mothers also participated. The twins’ zygosity was previously identified by many genetic markers. This study aimed at clarifying the effectiveness of zygosity diagnosis by questionnaires distributed to twins’ mothers. The questionnaire consisted of three questions concerning the degree of similarity of twins at one year of age; whether they were confused, and if so, by whom. It was slightly modified from that reported earlier by Ooki et al [11] for twins themselves. According to the degree of similarity of the twins, points were allotted thus: from 1 to 3 points for answers to questions (1) and (2) and from 1 to 4 points for answers to questions (3). The sum of the points was calculated, then ranged from 3 to 10. Zygosity was determined by the sum of these points. If the sum was 3-6, the twin pairs were considered MZ and if the sum was 7-10 DZ. It was revealed that more than 90% of twins were identified correctly as MZ or DZ by applying this cutting point and this result was in accordance with that obtained by use of discriminant function analysis. Moreover, the accuracy of the mothers’ responses was nearly the same as that obtained by the questionnaire for twins themselves. It was concluded, therefore, that the information from twins’ mothers is as correct as that from twins themselves. This questionnaire is simple, practical and especially useful when twins are still too young to participate personally.

During a screening protocol of early echocardiographic diagnosis (ATL MK 600) and treatment of “silent” PDA in RDS preterms with BW ≤ 1.750 kg, clinical data on premature twins were collected, including diagnosis of both PDA and BPD, to investigate whether twin birth influences PDA incidence and BPD development. Out of the 290 RDS preterms evaluated, 96 (33%) showed evidence of PDA, and a total of 79 (27%) developed BPD, 47 (16%) with associated PDA and 32 (11%) without PDA. Out of 238 singletons, 74 (31%) presented “silent” PDA and a total of 75 (31%) developed BPD, 44 (18%) with associated PDA, and 31 (13%) without PDA. In 52 other twins (18% of the total number of babies studied), 22 (42% of this subgroup) presented evidence of “silent” PDA, and 4 (8% of the subgroup), developed BPD, 3 with associated PDA (6% of the subgroup), and 1 without PDA (2% of the subgroup). From these data, it is inferred that that low-birthweight twins are at high risk for PDA hemodynamic complications during RDS, and may benefit from early induced ductal closure. Instead, in RDS twins, BPD was statistically less frequent (at the 99% C.L.) probably because twinning enhances fetal lung maturity, influencing enzymatic and nonenzymatic protective systems of lung defence.

To better understand behavioural and genetic influences upon breathing, the breathing patterns of 8 pairs of monozygous (MZ) twins were measured under 4 behavioural conditions; relaxed without standardisation; eyes closed; eyes open; and reading. Breathing was quantified by inspiratory and expiratory durations (TI, TE), tidal volume (VT) and derived variables. Airflow shape was normalised and quantified using 8 dimensions. Reading caused breathing to increase by > 500 ml/min compared to the other four conditions. Differences in breathing between combinations of two conditions were compared by testing whether the differences within an individual were smaller than the differences between random pairs of individuals from the same 16 subjects. For almost all respiratory variables, and whatever the behavioural condition, there were highly significant similarities within an individual (p < 0.00025 on 32/80 comparisons). Under each condition, the differences within MZ twin-pairs were compared to the differences within random-pairs from the same subject population. There were highly significant similarities within twin-pairs for the airflow shape across all conditions. However, TI, TE and VT failed to consistently show significant similarities within twin pairs. Hence, an individual's airflow shape appears to be a fundamental characteristic which is conserved when behavioural condition and level of ventilation changes. Further, MZ twins have similar airflow shapes – whatever the behavioural situation. Hence, behavioural influences upon airflow shape act upon monozygous twin pairs in similar ways, or such influences were negligible under the conditions of the present study.

Neonatal sleep behaviors and behavioral state cycling were observed for 20 pairs of same-sex, fullterm twins in which one twin of the pair was appropriate-for-gestational-age (AGA) and the other twin was small-for-gestational-age (SGA). Time-sampling recordings were made in active sleep of number and vigor of limb movements, body and head movements, and mouth movements. No group differences were observed for time spent in first active sleep, first quiet sleep, or length of first sleep cycle. Examination of specific behaviors indicated a significantly higher incidence of vigorous limb movements and right hand-to-mouth movements, with a trend for more small limb movements and left hand-to-face movements, for AGA twins when compared with SGA twins. SGA twins had significantly more spontaneous smiles and a trend for more spontaneous startles than AGA twins. A stepwise discriminant analysis indicated that a composite of the variables smile, large limb movement, startle, and left hand-to-face significantly discriminated between the two groups, with 90% correct classification of the AGA twins and 75% correct classification of the SGA twins. The results demonstrated the utility of evaluating specific sleep behaviors, rather than state cycling only, to describe differences in neonatal sleep characteristics between AGA and SGA twins.

It is estimated that at least one dizygotic (DZ) twin maternity in twelve is preceded by superfecundation (the fertilization of two ova by sperm from different coitions). Presumably this parameter varies from population to population eg. with coital rates and rates of double ovulation. Sometimes superfecundation occurs by two different men. The frequency with which this occurs must depend on rates of infidelity (promiscuity). It is suggested that among DZ twins born to married white women in the U.S., about one pair in 400 is bipaternal. The incidence may be substantially higher in small selected groups of dizygotic twin maternities, eg. those of women engaged in prostitution.

Total birth records for the Bengalee Muslim population (BMP) and the Bengalee Hindu caste population (BHCP) for the period 1980-1988 were 17,720 and 119,107 respectively. Of these, the number of twin pairs were 363 BMP and 1,229 BHCP. These data were obtained from the registers of the following hospitals: Islamia Hospital, NRS Medical College and Hospitals and RG Kar Medical College and Hospitals, Calcutta and Medinipore Sadar Hospital, West Bengal, India. The twinning rates found were 20.48 and 10.57 per thousand deliveries in the BMP and BHCP respectively. The proportion of twins, 0.02048, in the BMP was significantly higher (Z = 12.38, p ≤ 0.01) than that in the BHCP, 0.01057. This finding of a higher twinning rate in the BMP is corroborated by the fact that available data on the Muslim population of Srinagar in Kammu and Kashmir, and Lucknow and Kanpur in Uttar Pradesh show higher twinning rates than the other populations of India. This increased twinning rate may be due to the greater amount of inbreeding in the BMP.

White male twins from the NAS-NRC twin sample, who were born in the U.S. between 1917 and 1927 and served in the military, are used to estimate variability in hazard functions for those twins who died during the period 1974-1990. Roughly the same number of MZ and DZ twins died during this period, but their death rates are similar. DZ twins exhibit greater within-pair variation. Using hazard and other analyses, the only statistically significant variables are found to be being a DZ twin (in level equations), date of birth, and, sometimes, wife's religious preference. Variables not significant for level or within-pair equations, include own religion, parental education, working overtime frequently, and number of children. The greater variation in life expectancy of DZ twins is hardly surprising and may say something about the lack of comparability in phenotype variance of DZ twins, which in turn may be worrying.

The present study aimed at clarifying the characteristics of twins' physical growth. First, 557 pairs of normal Japanese twins were analyzed according to the following three life stages: (1) intrauterine growth, (2) body weight and height from birth to 6 years, and (3) body weight and height at school age (from 6 to 11 years). The following results were obtained. 1. Intrauterine growth of twins was very different from that of singletons, especially as regards weight, so twins should be estimated by twin standards. 2. Size deficit at birth was appropriately recovered over the first 6 years. 3. No size deficit was observed by school age. Second, the similarity of bodyweight and height according to zygosity was analyzed using 605 pairs of normal Japanese twins, 427 monozygotic (MZ), 113 same-sexed dizygotic (DZ) and 65 opposite-sexed DZ pairs. The similarity between MZ and DZ pairs was almost the same at birth. However, MZ pairs became increasingly more similar with age, whereas DZ pairs became more dissimilar. This tendency was very clearly seen through early infancy, thus suggesting that genetic factors became more apparent during this life stage.

We observed persistent first trimester growth disparity in a twin pregnancy following IVF-ET. The crown-rump length of the two fetuses was substantially different at 7 and 11 weeks and from the 20th week discordant growth was observed by intertwin differences in abdominal circumferences and estimated fetal weights. Birth weight discordance was 26.6% (1600/2180). This is apparently the first documentation of first trimester growth discordance persisting throughout pregnancy.

Although numerous genetic and anthropological markers are available for determining zygosity of twins, there is still a need for a more practical and informative method in zygosity diagnosis. Dinucleotide repeats or other short repeats (microsatellites) are highly variable between individuals and offer a simple, fast, cheap, and exact approach for zygosity determination. The feasibility of a set of microsatellites to be used for this purpose is demonstrated.

Birth statistics (on 13,887.943 births) from 11 Indian States indicate a fall in the dizygotic twinning rate and a rise in the monozygotic twinning rate over the decade between 1982 and 1991. These figures seem to reverse the trend of the period 1960-1982. One plausible explanation is offered by the age-data on mothers, which indicate that twins of different sex are more frequently born to mothers above the age of 35. Since extensive family planning measures have reduced the upper age-limit of mothers, the birth-rate of twins of different sex has also fallen. Due to the increased proportion of twins of the same sex, Weinberg's differential method will therefore report a decline in the dizygotic twinning rate, wherever the mean age of mothers has fallen (i.e. most mothers do not reproduce after the age of 35).