Mostly, I am basing this on what I know about the science of fetal development in mammals. A little has to do with personal observation and my religious beliefs. I am putting this in a diary because there are so many websites written by people who oppose abortion that contain wildly inaccurate information about the science of fetal development. Even if you do not try to explain the science to someone else, it can be helpful to know when what someone else says is bull shit.
First, my limited personal experience and my religious beliefs.
I believe that life of any mammal begins with the first breath outside the female's body. And death occurs right after the last breath. I have seen puppies and kittens and foals and calves born. I have seen dogs and cats and people die. In both birth and death there is a very definite change at the time of first and last breath. It looks to me like the soul entering and leaving the body. If in fact that is when the soul enters and leaves the body, then the fetus is not yet the person, and the corpse is no longer the person. The fetus is merely a collection of cells that will someday be a person, and the corpse is merely a collection of cells that was once a person.
On the other hand, I have never had a baby myself. I know that some people who have had a baby have already experienced the fetus as a person long before birth. So for them, my position is nonsense. I am inclined to think and feel that the soul is attracted to the body that it will one day inhabit, and is therefore hanging around the fetus waiting to move in with the first breath. I know that babies can recognize and respond to sounds that were in the fetuses' environment during gestation, and so there must be something, either the brain or the soul that heard and remembered the sounds in some fashion. But I also know that there is a very visible change in the appearance of the body when breathing starts and when breathing ends.
There are also some religions that believe that life begins and ends at first and last breath. Others set the start at conception, at implantation, at quickening.
The rest is mostly about the science.
In the "third trimester" is roughly when the fetus is capable of breathing outside the female body, even if it is only capable of breathing with supplemental oxygen. For a normally developing human fetus, the lungs are developed enough that the fetus can breath oxygen rich air at about five months post conception, or five and a half months post last menstrual period. Until then, the oxygen must enter the blood through the mother-to-be's lungs and the fetus will die if the placenta detaches, the cord is crushed or cut, or something else prevents the mother-to-be's lungs from providing oxygen to the baby.
If the development of the fetus is significantly abnormal, the fetus may be capable of breathing air at a later point, or the fetus may never be capable of breathing air, if the brain or the lungs do not develop normally, or the spinal cord and peripheral nerves do not develop, or if the glands do not provide the proper amounts of the proper hormones, or several other developmental defects occur. In these cases, the fetus will not survive after birth, and we are not even close to being able to keep the fetus alive after birth.
At this point if the fetus is normal, inducing labor will result in a live birth, as will performing a Caesarean section. At this point, if giving birth will not kill the mother, it may be possible to save both the fetus and the mother. Before this point, the only way to save the fetus is by continuing the pregnancy.
Some birth defects cannot be discovered until this point. One of the main "causes" of third trimester abortions is birth defects that are serious that could not be discovered earlier. Some of these defects do not occur until it is time for that part of the fetus to develop. For example, you cannot have a defect in the lungs until the baby has lungs. These are defects that we will probably never be able to predict at a younger age. Sometimes the fetus is so abnormal that it would die after being born. This is one reason that third trimester abortions should be legal. No diagnosis of fetal abnormalities is 100% reliable, because no diagnosis is 100% reliable, but we have gotten pretty good at finding serious abnormalities in fetuses in the third trimester that we could not have found earlier. I would rather trust the woman to make the final decision about whether to have an abortion in the third trimester, without having to get court approval or spousal or parent approval, or even a second opinion if she is convinced that the results are obvious enough that she has all the information she needs to make her decision. But I think that the woman should have the opportunity to know whether the doctor believes that the fetus might survive birth, and so either inducing labor or a Caesarean section are possible options in those cases. In many cases at this point an abortion is also more risky for the mother than a live birth. But each case is different. That is the scientific and medical information.
Unfortunately, many antiabortion web sites give wildly inaccurate scientific and medical information, some even claiming that the fetus is a fully formed human that just needs to get bigger by twelve weeks after the last menstrual period or 10 weeks post conception, which is two and a half months earlier than this is true. At 10 weeks post conception, not only is the fetus only about the size of a finger, the heart and brain are not fully functional and the lungs are virtually nonexistent. The arms and legs are about like the legs of a salamander or newt compared to the body, thin and short and with only partially formed fingers and toes. The "heart" does not yet have four chambers. The fetus does not have a separate blood supply from the mother, nor does it have a different blood type from the mother, even if it will someday have a different blood type from the mother. Why would a site that was opposed to abortion feel that it was necessary to tell this many lies? Probably because the people that created the site do not feel that people would be against abortion at this stage if they knew the truth. If the people who want to make abortion illegal do not believe the truth about fetal development, or if they think the truth about fetal development would lead people to support allowing abortion at this point, then it sounds like allowing abortions at this point is the rational thing to do.