Even though both parents have normal chromosomes, the fetus may still have chromosomal abnormalities due to anomalies in the sperm or egg. These abnormalities can occur randomly with a low recurrence rate, but some patients may have chromosomal structural abnormalities like balanced translocation or Robertsonian translocation, leading to abnormal gametes and embryos. It is recommended that both partners undergo chromosomal karyotype analysis to determine the cause of the abnormality and develop a corresponding reproductive plan.