A 25-year-old woman is G5, P0, Ab4. All of her previous pregnancies ended in spontaneous abortion in the first or second trimester. She is now in the 16th week of her fifth pregnancy and has had no prenatal problems. Laboratory findings include maternal blood type of A positive, negative serologic test for syphilis, and immunity to rubella. Which of the following laboratory studies would be most useful for determining a potential cause of recurrent fetal loss in this patient?