The simplest reason for my time frames is hearbeat and organ function. At around 12 to 14 weeks the fetus is a fully functioning human in the biological sense that all of its organs are functioning. At around this time the fetus even urinates. One of the reasons that the baby can survive outside the womb is basically just size and an inability to breathe outside of the womb. Those inabilities to survive the environment doesn't negate that they are for all purposes a full being at around 12 to 14 weeks.

The second question as to why 24 weeks is outrageous to me is the fact that the earliest natural premature birth so 21 weeks. That child is living today. My wife worked for a teaching hospital for awhile in the NICU and it got to the point where if a baby was born at 22 or 23 weeks they were surprised if the baby didn't live and survive. If the baby is strong enough to survive on their own then I feel the choice is no longer one someone else can make.

Lastly, the awareness issue, if that's truly your measure for abortions and justification for legal time frame, then an abortion should be legal up until 5 months after the baby is born. That is when babies start gaining the ability for memory and consciousness of their surroundings. Before that they are effectively just in a dry version of the womb and have only base instincts controlling actions the same as they would have in the womb. Here's an article about that last part.

http://www.wired.com/2013/04/baby-consciousness/

My 12 week mark comes in spite of my religious beliefs and comes from listening and reading pro choice articles and talks. After 12 weeks surgical abortion does have an increased potential for harm to the mother to experience complications and infections after the procedure.

If there is a true medical need meaning death of the mother after 12 weeks, then abortion is an option but that is an entirely different discussion since the choice is no longer do I live with the burden of pregnancy or a child but instead it's do I live or die.