Abstract： Objective To investigate the clinical features of the abnormal chromosome karyotypes in twin pregnancies complicated with fetal malformations. Methods Totally 181 twin pregnancies (362 fetuses) in which one or two fetuses had abnormalities diagnosed by ultrasound were referred to the First Affiliated Hospital of Sun Yat-Sen University from January, 2000 to September, 2010. They were divided into different groups according to ( 1 ) maternal age: the cases with maternal age ≥35 were divided into advanced pregnancy group ( 105 fetuses) , and those with maternal age ＜35 were divided into young pregnancy group (203 fetuses) ; ( 2 ) conceived method : those conceived by assisted reproductive technology were divided into assisted reproductive group (81 fetuses), and the natural conception pregnancies were divided into natural conception group (227 fetuses) ; ( 3 ) chorionicity: the monochorionic twin (MCT) pregnancies were divided into MCT group( 123 fetuses), and the dichorionic twin (DCT) pregnancies were divided into DCT group( 185 fetuses); (4) structural abnormalities: 205 fetuses with structural abnormalities were divided into the abnormal fetal group, and 103 fetuses without structural abnormalities were divided into the normal fetal group. All fetuses were examined by the ultrasound and chromosomes were examined in 308 fetuses. Results ( 1 ) The karyotype of fetuses: among 181 twin pregnancies, 23 cases had chromosomal abnormalities in 1 or 2 fetuses ( 12. 7％ ,23/181 ), and chromosomes were exarmined in both fetuses in 20 of 23 cases. Twenty-six of 308 fetuses were found with abnormal chromosomes ( 8.4％, 26/308 ) , and the aneuploid was the most common type of abnormal karyotypes ( 53.8％ , 14/26 ). Twenty-one of 205 fetuses with malformations were found with abnormal karyotypes (10. 2％, 21/205 ). (2) Seven of 123 fetuses in MCT group were with abnormal karyotypes (5.7％, 7/123), and 19 of 185 fetuses in DCT group were with abnormal karyotypes ( 10. 3％, 19/185 ). There was no statistical difference of abnormal chromosome incidence between the two groups. There were 14 fetuses with aneuploid in DCT group ( 7.6％, 14/185 ) ;but there was no fetus with aneuploid in MCT group. There was statistical difference between these two groups. In two cases of DCT group, only one fetus with malformation received chromosome examination because another fetus was dead, and the karyotypes were trisomy 21 and trisomy 18 respectively. Both fetuses of the rest 17 cases received chromosome examination, and the chromosomes of both fetuses in each pregnancy were different. Fifteen of 19 fetuses with abnormal chromosomes in DCT group were complicated with structural abnormalities, and 7 fetuses of 4 twin pregnancies in MCT group were with chromosomal abnormalities. (3) The comparison of the abnormal karyotype incidence between the advanced pregnancy group and young pregnancy group: the abnormal karyotype incidence of the advanced pregnancy group was 7. 6％ (8/105), and that was 8.9％ (18/203) in young pregnancy group. There was no statistical difference between the two groups ( P ＞ 0. 05 ). Six of 105 fetuses in advanced pregnancy group were aneuploids (5. 7％, 6/105), and 8 of 203 fetuses in young pregnancy group were aneuploids (3.9％, 8/203). The aneuploid incidence in advanced pregnancy group was significantly higher than that in young pregnancy group ( P ＜ 0. 05 ). (4) The comparison of the abnormal karyotype incidence between the assisted reproductive group and the natural conception group: 11 of 81 fetuses were with the abnormal karyotypes in assisted reproductive group ( 13. 6％, 11/81 ), and 15 of 227 fetuses were with the abnormal karyotypes in assisted reproductive group (6. 6％, 15/227). There was statistical difference between the two groups ( P ＜0. 05). There were 7 fetuses with the aneuploid in assisted reproductive group ( 8. 6％, 7/81 ) and 7 fetuses with the aneuploid in natural conception group ( 3. 1％, 7/227 ), which showed no statistical difference ( P ＞0. 05 ).(5) The comparison of the abnormal karyotype incidence between the abnormal fetal group and normal fetal group: 21 of 205 fetuses in abnormal fetal group were with abnormal karyotypes (10. 2％, 21/205), and 5 of 103 fetuses in normal fetal group were with abnormal karyotypes ( 4. 9％, 5/103 ) . There was no statistical difference (P ＞ 0. 05 ). 13 fetuses in abnormal fetal group were with the aneuploid (6. 3％, 1 3/205), and only one fetus in normal fetal group was aneuploid (1.0％, 1/103 ). There was statistical difference between the two groups ( P ＜ 0. 05 ). Conclusions Aneuploid is the most common abnormal karyotype in twin pregnancy complicated with fetal abnormalities, especially trisomy 21. Aneuploid mainly occurs in only one fetus of DCT, and chromosomal discordance is usually found in DCT. While in MCT, the twin fetuses with the same abnormal karyotype may have different pbenotypes. The results suggest that it is necessary to analyze both karyotypes of twins even if only one fetus is complicated with structural abnormalities.