This is my second pregnancy. My first born is now 6. At first I thought I was done having children, since my first came along at 16/17. I ended that relationship with her bio father, as it was very toxic and we moved across the country.

I met someone else and he really is a wonderful man. We married and decided to have a child.

I'm due in December and the closer I get to giving birth the more terrified I am of this baby.

The doctors tell me he/she (keeping the gender a surprise) is growing well and is healthy.

I fear that once outside of the womb the baby will be disfigured or have some sort of serious health issues, like Down syndrome, severe autism, cerebral palsy... The list goes on.

I'm just so scared that this baby isn't going to be healthy.

Is this normal to feel this way? Have any of you felt this way during your pregnancy?

Don't get me wrong, I don't regret having this child, I'm simply scared of him/her.

First let's clarify if you mean scared OF or scared FOR. Scared of implies an alien with fangs bursting out of your womb and attacking you (or something). Scared for sounds like what you mean, in terms of health.
Assuming the latter, I don't think it's so uncommon. Did you take any recreational drugs or a lot of alcohol or eat a crappy diet in the last few years? Get a STD? Do anything a young parent shouldn't do? See a lot of pictures of birth defects, or ads on TV from law firms who want to sue doctors for them? Know someone personally who had a child with defects?

It's also a variation on 'what is this THING filling up my belly?' Women giving birth often scream out horrible expletives against the baby, and say they don't want it anymore, just get it out of there. Nurses are used to that and just ignore it.

Regardless of your life and what may be behind this, the present is what matters, and that means trust of your doctors. For some reason you don't trust them when they say your baby is healthy. At your next appt, ask for an extra 5 minutes to look over the ultrasounds and to talk about your fears. Tell them everything you said here.
Also do a lot of research online about which birth defects are detected before birth, how, when, and how often in the population. Go to family far and wide and ask if any occur in your family. Well informed is the best weapon against fear.