How far into a pregnancy is it before visible signs of it is apparent and risk of physical trauma increases
Around the second trimester.  That's an article from WedMD, which says quite a few things.  To summarize, by the end of the first trimester:


  You may be noticing some of the first physical signs of pregnancy: breasts that are sore or tingle at the slightest touch, skin that's drier or oilier than usual, and "morning sickness" -- which may or may not restrict itself to the morning hours. As many as 70% to 80% of pregnant women have it, but not feeling morning sickness doesn't mean there's anything wrong with the baby.


In the second trimester:


  Most pregnant women begin to "show" during the second trimester. You likely gained less than 5 pounds during your first trimester, but now the number on the scale is edging relentlessly upward. As your second trimester proceeds, you'll gain an average of 1 to 2 pounds per week.


The exact time and rate of "showing" can vary, including whether or not this is your first pregnancy.  By the third trimester, the infant is rapidly becoming more and more capable of surviving on its own.