I have 2, both relating to my pregnancy.
During my pregnancy, and before we traded in my car for our van, my DH was driving my car during the summer because the air conditioning didn't work, and during a Washington, DC summer, AC is very important! As we moved into fall, and cooler temperatures, we switched back. I was about 5 months pregnant with twins and driving to work on one of the major highways in the area. Traffic was traveling at ~55 mph, in a construction zone, so no shoulder to pull off on. Suddenly, my vision started to narrow, I was seeing black spots, and I just knew I was going to pass out. All I could think was that I was going to crash, because there was no place to pull over, and cars all around me, and that all 3 of us were going to die. Just as my vision tunneled down to nothing, a break in the construction opened up and I was able to pull over safely. I'm not actually sure if I fully passed out at that point or not, but I did sit there for a solid 10 minutes or so before finishing up my drive to work. Where I was promptly driven home by my boss because my face was just white. It turned out that the angle of the drivers seat in my car meant the babies were putting pressure at just the right spot to cause slowed blood flow and therefore, fainting. It still terrifies me to think about it!
The other story isn't as dramatic, and I was never really in danger, but if it had been in the not too distant past, I would have been. I went into preterm labor at 31 weeks, 1 day. After contractions were slowed, I developed HELLP syndrome and I started going into renal failure. They delivered the twins at 31 weeks, 2 days, and while they had to spend 6 weeks in the NICU, they were amazingly healthy for preemies, and are now fully caught up nearly 5 year olds. But, if modern medicine wasn't what it is, again, all 3 of us could have died.