You see a 42-year-old patient in your office who is now 5 weeks pregnant with her fifth baby. She is very concerned regarding the risk of Down syndrome because of her advanced maternal age. After extensive genetic counseling, she has decided to undergo a second-trimester amniocentesis to determine the karyotype of her fetus. Prior to performing the procedure, you inform the patient that all of the following are possible complications of the amniocentesis except