Is zygosity or chorionicity the main determinant of fetal outcome in twin pregnancies?

OBJECTIVE: The purpose of this study was to examine whether fetal outcome in twin pregnancies is dependent on zygosity or chorionicity. STUDY DESIGN: This was a prospective observational study comprised of women with twin pregnancies who attended the fetal medicine unit at St Michael's Hospital, Bristol, Ireland, during the years 1998 to 2000 and who were delivered in hospitals in south west England. After delivery, zygosity was determined with umbilical cord blood with the use of microsatellite markers that were amplified by polymerase chain reaction. Placentae were examined histologically for chorionic type. The perinatal outcomes of 3 groups of monozygotic monochorionic, monozygotic dichorionic, and dizygotic pregnancies were compared with the use of the Mann-Whitney U test and the Fisher's exact test. RESULTS: All 92 dizygotic and 15 monozygotic dichorionic pregnancies resulted in live births. In 7 of the 39 cases in the monozygotic monochorionic group, either both twins were not live born or delivery occurred <24 weeks of gestation. The gestational age at delivery and birth weight were significantly lower, and there were a greater number of cases with birth weight discordancy of >25% in the monochorionic pregnancies compared with the other 2 groups (P < .05). There were no significant differences in any of the study parameters between the monozygotic dichorionic and dizygotic groups. CONCLUSION: Fetal outcome in twin pregnancies is related to chorionicity rather than zygosity.

Homonymous hemianopia following a triplet pregnancy: post-natal cerebral vasculitis or atypical eclampsia.

A 46-year-old woman with a triplet pregnancy developed seizures, an oculogyric crisis and a homonymous hemianopia post-natally. Abnormal neuroimaging studies and lumbar puncture suggested possible cerebral vasculitis. Treatment with aciclovir and prednisolone resulted in a slow resolution of symptoms. This case highlights the difficulty in distinguishing eclampsia from rarer neurological causes of peripartum seizures.

Prediction of chorionicity in twin pregnancies at 10-14 weeks of gestation.

OBJECTIVE: To examine the accuracy of sonographic determination of chorionicity in twin pregnancies at 10-14 weeks of gestation. DESIGN: Prospective study on the sonographic prediction of chorionicity at 10-14 weeks of gestation. PARTICIPANTS: During a 30 month period, from October 1997 to May 2000, 165 women attending the departments of fetal medicine or ultrasound. METHODS: Sonographic criteria used in the diagnosis of chorionicity were the number of placental sites, the lambda (lambda) and T signs and the thickness of the inter-twin membrane. The diagnosis of chorionicity was made at the time of the ultrasound examination using all these features and subsequently compared with the postnatal diagnosis, confirmed either by placental histology or discordancy in infant sex. RESULTS: In 150 cases with confirmation of chorionicity following delivery, 116 were postnatally classified as dichorionic and 34 monochorionic. Prenatal ultrasound examination correctly identified chorionicity in 149 (99.3%) cases. The most reliable indicator for dichorionicity was a combination using the lambda sign or two separate placentae with a sensitivity and specificity of 97.4% and 100%, respectively. The most useful test in predicting monochorionicity was the T sign with a sensitivity of 100% and specificity of 98.2%. Measurement of the inter-twin membrane thickness was a less reliable indicator where the sensitivity for dichorionicity and specificity for monochorionicity was only 92.6%. CONCLUSIONS: Ultrasound examination of twin pregnancies at 10-14 weeks of gestation predicts chorionicity with a high degree of accuracy using a combination of the number of placentae, lambda and T signs and inter-twin membrane thickness. All hospitals should encourage departments providing ultrasound services to undertake chorionicity determination when examining women with twin pregnancies at this gestation.