There are various levels of screening tests available (but not required) like ultrasounds, but the more definitive diagnostic tests (like amniocentesis) are more invasive and carry some risk to the fetus.

If you aren't sure that you'd abort the fetus if a genetic abnormality were detected, you'd probably be less likely to take a screening test that poses even a minor risk to the fetus. (of course, there are other reasons to know, like if you are considering a home birth versus hospital birth, you'd want to know if the baby will have any special needs at birth)