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?

Multiple fetal losses earlier in gestation suggest the likelihood of a chromosomal abnormality - the mother or father may be the carrier of a balanced translocation. Cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations may lead to cystic fibrosis, and like most inborn errors of metabolism, with appearance of findings postnatally. The maternal serum level of a-fetoprotein can help identify fetal neural tube defects, but these defects are not a cause of early fetal loss. Maternal HIV infection is not a cause of significant fetal loss. Because the mother is blood type A positive, fetal loss with erythroblastosis fetalis is unlikely, although other blood group incompatibilities potentially may result in erythroblastosis fetalis.