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.

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.

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.

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.