Usually "viability" is used to refer to the point at which a baby who is born at that gestation has about a 50% chance of survival. That line now is 24 weeks. 23 weeks, I believe, is the earliest which some NICUs will take a baby and try to provide life-saving treatment, though chances the baby will live at 23 weeks are substantially less than 50%.
The smallest chance now is at 20 weeks with it being about 4-5% that bay will live. 24 weeks is when they start giving the best chances though. Also from everything I've seen girls are more likely to survive before 24 weeks then boys, they "fight" more.
I can't find any information about there ever having been a 20 week baby surviving. (20 weeks gestation you mean, right?) Do you have a link for this? The earliest I have ever seen even claimed by a news source is something like very late 21 weeks or very early 22 weeks, but the pregnancy dating in those cases if often contested.
I'd LOVE it to be the case that there was a 4-5% chance at 20 weeks as we are counting down the days to viability given we lost a baby in my last pregnancy at 18.5 weeks, but I have just never seen anything indicating that is the case.
It simply means they baby has a small chance
of surviving outside the womb if it were to be born now. Yes the baby
needs to keep baking until at least 36 weeks to be healthy. Even babies
born after 23 weeks may not survive but a miscarriage will not happen.
It will be born but whether or not is survives varies. Some miracle
babies have made it with all our technology. I posted about it today.
I've had several miscarriages and this is a big milestone for me :) I
know there is some chance that if something goes wrong they might be
able to save him. The longer the baby bakes the more likely survival
Maybe I'm just misunderstanding what you mean by the bolded, but whether a baby is "born" or "miscarried" really has nothing to do with the 23 week or 24 week line. I'm not sure really at what point in pregnancy a "miscarriage" becomes a "birth". At 20 weeks the term miscarriage is no longer technically used; a loss will usually then be referred to as a stillbirth. Technically, though, when preterm labor is the cause of the loss a baby may be born alive even a few weeks before 20 weeks. (At least, when I went into labor at 18.5 weeks and was waiting to deliver we were told the baby might be born alive and they would tell us immediately if there was any sign of a heartbeat, though of course there was no chance she would live more than a few minutes.)