A 30-year-old woman has a 1 in 3,000 chance of giving birth…

A 30-yeаr-оld wоmаn hаs a 1 in 3,000 chance оf giving birth to a child with trisomy 21; however, a 48-year-old woman has a 1 in 9 chance. Which statement is the most likely explanation for why a woman has a higher probability of giving birth to a child with trisomy 21 as she ages?