I understand that you are feeling that everything we learn will somehow happen to you. That is because all you see around you in the hospital is usually on the sick end of the spectrum- i mean your teachers pick the patients you see- and most of the time they don't give you run of the mill cases, because that wouldn't be challenging enough. However when there are multiple pathologies- there are multiple learning opportunities.
That being said, I understand that you have relatives that went through a health scare with kids. I think almost every person has some part of their family with a health problem or other (including neonates), so you're not alone in that concern. But you need to look at the odds also: It may be that certain cromosomal illnessess are attributed to late in life procreation. but again, that is not 100%... I don't remember the figure-- i doubt it is even in the double digits. again, it's about the odds.
Imagine if you would that the general public knew about Achondroplasia (a form of dwarfisim). almost all cases are because of a new gene mutation and not the fact that it's a recessive disorder (i think... genetics was a few years ago). And how often does this happen? Not as often as we (the hypochondriac nursing students) may think. again- a numbers game. And the general public doesn't worry about it- because it hardly happens.
From personal experience, both my parents married when they were both over 30 - and together they had 5 healthy children, two years apart between each kid, (all top university grads) in good health. In the 80's the only genetic test that was available and of relevance was Tay-sachs, and that was a negative result. Nowadays who knows how many mutated genes and genetic issues might show up if my mom would have done the same tests. But having that information wouldn't have changed the physical outcome, only the mental burdon of decision.
I wouldn't call your thinking "irrational", because this is what we learn! pathology! But I believe that some things in life are never certain, and children fall into that catagory and until you have that pink squishy bundle of joy in your arms, you can never be sure that everything will be 100% smooth and OK.
I hope this was somehow comforting.... as the above person said, you should speak to someone professional about this - might I advise that you speak to a OB doc and ask them how THEY handled having their own kids--- I mean, they are the ultimate person to worry about everything surrounding a birth- so maybe they could put your mind at ease.