Personhood is attained at about 22 weeks gestation:
This argument is based on the definition of death:
"Death is not just another disease that can be specified, analyzed, and catalogued as viral or bacterial, infectious or auto-immune. Death is the final cessation of life. Thus defining death requires more than medical and technical expertise: It requires also some agreed understanding of what is constitutive of human life, and what it is that must be absent before the person can be said to be dead."
"Sometimes it will be obvious to any reasonable observer that someone is dead, or alternatively, that someone is still alive. Someone who is breathing [without a respirator] and talking and walking around is obviously alive. Someone whose body is rotting away and hanging off the bones is obviously dead. However there are some cases, perhaps many cases, where it will not be obvious to an unqualified layman whether someone is alive or dead. In these cases it is the decision of competent physicians that decides the issue." 1
Prior to about 1960, a person would be declared dead if both their heartbeat and breathing had ceased and could not be re-started. But newer technological developments made this definition invalid. Heart pacemakers can keep the heart beating indefinitely long after all other internal systems have wound down. Respirators can keep the person apparently breathing forever.
Death is generally defined in most U.S. states as a situation in which the brain "flat-lines." That is, there is no major central nervous system activity and there is no detectable electrical activity in the brain's cerebral cortex. At this point, the person may be declared dead in many jurisdictions. The patient may appear to be breathing, as a result of the action of a respirator. Her/his heart may still be beating, either on its own or as a result of a heart pacemaker. But he/she is judged to be dead. Unplugging the patient from life support systems at this point will not actually kill the patient; she/he is already considered to be dead.
The great rise of transplant medicine has, then, been wholly dependent upon organ harvesting from so called 'beating-heart cadavers', that is, patients who are determined to be dead on the basis of brain death criteria. 1 But their hearts continue to beat (sometimes with external help), to keep the body's organs fresh for transplanting.
If the point of death is defined as a lack of electrical activity in the brain's cerebral cortex one might use the same criteria to define the start of human life. One might argue that fetal life becomes human person when electrical activity commences in the cerebral cortex. Human personhood, would then start when consciousness begins and ends when consciousness irrevocably ends. One could then argue that a fully-informed woman should have access to abortion at any point before the point that human personhood begins.
According to author Richard Carrier: "...the fetus does not become truly neurologically active until the fifth month (an event we call 'quickening.' This activity might only be a generative one, i.e. the spontaneous nerve pulses could merely be autonomous or spontaneous reflexes aimed at stimulating and developing muscle and organ tissue. Nevertheless, it is in this month that a complex cerebral cortex, the one unique feature of human -- in contrast with animal -- brains, begins to develop, and is typically complete, though still growing, by the sixth month. What is actually going on mentally at that point is unknown, but the hardware is in place for a human mind to exist in at least a primitive state."
When medical ethicist Bonnie Steinbock was interviewed by Newsweek and asked the question "So when does life begin?," she answered: "If weâre talking about life in the biological sense, eggs are alive, sperm are alive. Cancer tumors are alive. For me, what matters is this: When does it have the moral status of a human being? When does it have some kind of awareness of its surroundings? When it can feel pain, for example, because thatâs one of the most brute kinds of awareness there could be. And that happens, interestingly enough, just around the time of viability. It certainly doesnât happen with an embryo." 8
Under this argument, some primitive neurological activity in the cerebral cortex begins during the fifth month, perhaps as early as the 22nd week of pregnancy. If we allow a two week safety factor, then we could set the gestation time limit at which abortions should not be freely available at 20 weeks. Abortions could then be requested up to the start of the 20th week for normal pregnancies, or at a later time if unusual conditions existed. Many state and provincial medical associations in North America have actually adopted this limit, probably using a different rationale.
http://www.religioustolerance.org/abo_argu.htm