To analyze a fetus with abnormal cardiac ultrasound by using various techniques and explore its genotype-phenotype correlation. Lymphocytes derived from umbilical cord blood sample were subjected to G-banding analysis. Short tandem repeats quantitative fluorescence PCR (STR-QF-PCR) was used for analysis of fetal DNA as an auxiliary test. Low-coverage whole genome sequencing (WGS) was used to detect chromosomal deletion/duplication which exceeded 100 kb in size. The karyotype of the fetus was 47,XN,+mar. As detected by STR-QF-PCR, the copy number of GATA178F11 locus on chromosome 18 was 4, and the duplicated fragment was derived from the mother. WGS suggested that the fetus to be 46,XN,dup(18p11.21p11.32).seq [GRCh37/hg19](10 001-15 378 887)× 4, with the duplicated fragment spanning approximately 15.38 Mb. The cardiac malformation of the fetus may be attributed to the partial duplication of chromosome 18p. Combined cytogenetic and molecular methods can facilitate prenatal detection of genetic abnormalities.