Final answer:
The greatest risk for a 44-year-old client with a positive pregnancy test and a history of lost pregnancies is chromosomal abnormalities, specifically due to the increased risk associated with advanced maternal age.
Step-by-step explanation:
Considering the client's prenatal history and age, the nurse would recognize chromosomal abnormalities such as trisomy 21, which is associated with Down syndrome, as the greatest risk for the client at this time. The incidence of chromosomal abnormalities increases significantly with maternal age, particularly for women aged 35 and above. This risk is outlined clearly in studies that show a correlation between increased maternal age and the likelihood of conceiving a child with trisomy 21 or other chromosomal disorders.