Objectives: To determine the prevalence of discordant twins in Siriraj Hospital and to evaluate associated factors.
Design: Cross-sectional study.
Setting: Department of Obstetrics and Gynecology, Faculty of Medicine Siriraj Hospital, Mahidol University.
Subjects: A total of 150 women with twin pregnancy, >28 weeks of gestation, who had deliveries of both live twins at Siriraj Hospital from 2003 to 2004.
Method: A review of medical records was conducted. Discordance was defined as > 20% difference in birth weight. Prevalence of discordance was calculated and associated risk factors were evaluated.
Results: Discordance was found in 35 cases; therefore, the prevalence was 23.3%. No significant association was found between discordance and various factors, including maternal age, maternal complications, parity, pregnant by assisted reproduction, gestational age at first diagnosis, chorionicity. However, discordant twins delivered at earlier gestational age compared to concordant twins (34.9 + 3.2 and 36.2 + 2.4 weeks, p = 0.037). Infants of discordant pairs were more likely to be admitted to the NICU than those of concordant pairs (17.1% and 3.9%, p < 0.001, both larger and smaller infants). Other neonatal morbidities were not significantly different.
Conclusion: Discordance twin was found in 23.3% of the cases. No significant associated risk factor was found. Infants of discordant pairs were more likely to be admitted to the NICU than those of concordant pairs.

Keywords: Twins, Birth weight discordance, Pregnancy outcome