This report examines the impact of two major kinds of unequal ascertainment on the estimation of true probandwise concordance (Cpbt ) in twin studies: 1) concordance-dependent – where the ascertainment rate differs in affected members of concordant vs discordant pairs, and 2) non-independent – where ascertainment rates differ in affected members of concordant pairs where the cotwin has vs has not been ascertained. Concordance-dependent ascertainment is easily modeled algebraically; non-independent ascertainment is more complex and we here propose a model based on survival analysis. Overall, concordance-dependent ascertainment produces greater bias in estimates of probandwise concordance than does non-independent ascertainment. The bias introduced by concordance-dependent ascertainment is greatest when Cpbt is low and/or when the ascertainment rate for twins in concordant pairs is low. The bias introduced by non-independent ascertainment is greatest when Cpbt is high and/or when the ascertainment probability for an affected twin in a concordant pair where the cotwin has already been ascertained approaches unity. The impact of concordance-dependent and non-independent ascertainment on estimates of heritability and common environment is examined. Correction terms to estimate Cpbt in the presence of concordance-dependent and/or non-independent ascertainment are presented.

During the 10-year period, 1975-1984, 105 triplet pregnancies were delivered at Harare Maternity Hospital, Zimbabwe, among 286,338 pregnancies in the Greater Harare Unit, giving an incidence of triplets of 1:2,727. The mean gestational age at delivery was 32.5 wk with 81 women (77.1%) delivering before 37 wk. Primigravidas delivered at a significantly earlier mean gestational age (P < 0.05) and had a higher perinatal mortality (P < 0.001) compared with grand multigravidas. Of the 315 babies, 277 (87.9%) weighed < 2500 g. The overall perinatal mortality rate was 327‰, with a pernital mortality rate of 146‰ for infants weighing ≥ 1000 g. Women hospitalised for bed rest during the antenatal period had fewer perinatal deaths compared with those diagnosed during the antenatal period, but not hospitalised for bed rest (P < 0.02). No difference was found in the mean gestational age at delivery or the mean birth weights between these two groups. Among infants ≥ 28 wk gestation there were fewer perinatal deaths in triplets delivered by cesarean section compared with triplets delivered vaginally (P < 0.0004). This suggests that cesarean section may offer the optimal mode of delivery in triplet pregnancy.

The perinatal outcome of seven recurrent twin deliveries after 28 weeks gestation, was compared to the first twin delivery and to randomly selected controls matched for parity, maternal age and gestational age. The outcome of the recurrent twin gestation was associated with significantly increased gestational age (P < 0.04) and mean twin birth weight (P < 0.05) compared to the first twin pregnancy. When compared to controls matched for maternal age and gestational age they were of significantly higher parity (P < 0.005) but had otherwise similar obstetric characteristics as when compared to controls matched for parity. These data suggest a better outcome for the recurrent compared to the first twin gestation. The frequencies of repeat twinning (0.0165% of all deliveries) and the calculated probability of having by chance a recurrent twin delivery were significantly different (P < 10−6) suggesting that both deliveries may be stochastically dependent events.

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.

Doppler arterial velocimetry waveforms was used as a method of antenatal surveillance for triplets. Six sets of triplet pregnancies were studied. Each fetus was identified by real-time-ultrasound scanning and the peak systolic/end diastolic (S/D) ratio was calculated. In one set of triplets two fetuses showed absent end diastolic flow. Both subsequently suffered from necrotising enterocolitis and had major bowel resection. The five other sets of triplets maintained a normal S/D ratio. Three had a normal outcome and two delivered prematurely.

The mothers of a consecutive series of 125 same sexed pairs of twins aged 6 months to 6 1/2 years completed a mailed questionnaire concerning the physical similarity of the twin partners. Zygosity diagnoses were first determined blindly on the basis of the answers given by the mothers, who did not know the result of the blood group test when fullfilling the questionnaire. Next, the results were compared with the zygosity determination based on examination of erythrocyteserum- and enzyme-groups. It turned out that a few simple questions distinguished well enough between MZ and DZ pairs, with a frequency of misclassification of 4%, leaving 5% as unclassifiable. It is concluded that twin zygosity can be estimated by a simple questionnaire with sufficient accuracy even in very young twin pairs. This has relevant implications for more extensive twin studies where the use of a wide spectrum of serological characters would imply too high expenses.

The incidence and evolution of patent ductus arteriosus (PDA) was evaluated in twins and preterm singletons with birth weight ≤ 1750 g admitted to our Department in 1987 for respiratory distress syndrome (RDS). Screening by echocardiography and Doppler-flow studies (AT MK 600) was performed on the third day of life. Out of 91 neonates who needed intubation and ventilation during this 12-month period (23.8% of admissions), 40 weighed less than 1750 g and of these 40, 14 were twins (35%). Hemodynamically significant PDA was documented in 13 patients; of these, only 5 were preterm singletons and 8 were twins. Two twins weighing < 1000 g received no therapy for ductus closure; one ductus closed spontaneously, the other had an early demise. Three twins and 2 preterm singletons received indomethacin; one of the twins needed a second cycle for definitive ductus closure. Three twins and three preterm singletons underwent surgery, while one twin died on the 10th postoperative day. Screening and early therapy of PDA during RDS could be of great clinical importance. Twinning seems to play a role in the incidence and evolution of PDA and this needs to be evaluated in further studies.