If you have 2 healthy children, does that lower your chances of having a child with Down syndrome or other birth defect?

Rolling the dice. A woman can give birth to a ds infant at any age. The frequency varies with age @ 1/1500 from 15-29, 1/100 @40-45 and 1/50 over 45.There are some non specific screening tests available early in pregnancy to see if it is more likely, but examination of fetal cells for a chromosome count is the most accurate.
No. The most significant risk factor for down's syndrome is the mother's age. Please speak to your doctor about the pros and cons and risks to you and the baby of conceiving at the age of 43.