they can screen for downs early in the pregnancy so you will know. they do a test called amniocentisis. i am not sure but i think down's is genetic in origin. as in something is wrong with gene sequencing while the baby was developing. not hereditary wherein it is passed from family to family.
down syndrome is also known as trisomy 21, meaning that instead of the normal two copies of each and every 24 chromosomes, there is an extra copy of number 21. it can be tested for within the first 10-12 weeks using Chorionic villi sampling, which samples the placenta, which contains DNA (which is neatly packaged into chromosomes) This test will only be performed if there are other serious risk factors for genetic abnormality (age of mother]35, family history, etc)
Good luck! and don't worry about it, stress is not good for you or the baby!