Objective To observe the chromosome karyotype and the polymorphism of umbilical cord blood in patients with high risk in middle and late pregnancy. Methods Cordocentesis was performed in 593 pregnant women [501 cases in the elder group(age > 35 years) and 92 cases in the non-elder group(age < 35 years) ] during the 17 to 38 gestational weeks to acquire the umbilical cord blood, which was used to examine the fetal chromosomal karyotypes and to observe the polymorphism. Results Forty abnormalities in 593 samples(6. 75%) were checked out including 16 in the elder group group(17. 39%) and 24 in the non-elder group(4. 79%). There was a statistically significant difference in the detection rate of chromosomal karyotype abnormalities between these two groups(P < 0. 05). Eleven and 6 cases of trisomy 21 were detected in elder and non-elder groups, respectively, with statistically significant difference between these two groups(P < 0. 05). Twenty-five cases of chromosomal polymorphism in 593 samples(4. 21%) were identified including 5 cases of inv(9)(0. 84%), 11 cases of Y chromosomal polymorphism(1. 85%), 3 cases of secondary constriction variation of No. 9(0. 51%) and No. 16, and 6 cases of chromosome polymorphism in the group D/G(1. 18%). Conclusion In high-risk patients of middle and late stages of pregnancy, the detection rate of chromosomal karyotype abnormality and chromosome polymorphism in umbilical cord blood is 6. 75% and 4. 21%, respectively. Trisomy 21 is the main and common abnormal karyotype, especially in elderly high-risk pregnancy patients. The most common chromosome polymorphism in umbilical cord blood is Y chromosome polymorphism. Observation of the chromosome karyotype and the polymorphism of umbilical cord blood in patients with high risk in middle and late pregnancy is helpful to prenatal diagnosis of fetal chromosomal disease. [ABSTRACT FROM AUTHOR]