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From http://www.tnr.com/013100/easterbrook013100.html

"The zygotes that do implant soon transform into embryos. During its early growth, an embryo is sufficiently undifferentiated that it is impossible to distinguish which tissue will end up as part of the new life and which will be discarded as placenta. By about the sixth week the embryo gives way to the fetus, which has a recognizable human shape. (It was during the embryo-fetus transition, Augustine believed, that the soul is acquired, and this was Catholic doctrine for most of the period from the fifth century until 1869.) Also around the sixth week, faint electrical activity can be detected from the fetal nervous system. Some pro-life commentators say this means that brain activity begins during the sixth week, but, according to Dr. Martha Herbert, a neurologist at Massachusetts General Hospital, there is little research to support that claim. Most neurologists assume that electrical activity in the first trimester represents random neuron firings as nerves connect--basically, tiny spasms.

The fetus's heart begins to beat, and by about the twentieth week the fetus can kick. Kicking is probably a spasm, too, at least initially, because the fetal cerebral cortex, the center of voluntary brain function, is not yet "wired," its neurons still nonfunctional. (Readings from 20- to 22-week-old premature babies who died at birth show only very feeble EEG signals.) From the twenty-second week to the twenty-fourth week, connections start to be established between the cortex and the thalamus, the part of the brain that translates thoughts into nervous-system commands. Fetal consciousness seems physically "impossible" before these connections form, says Fisk, of the Imperial College School of Medicine.

At about the twenty-third week the lungs become able to function, and, as a result, 23 weeks is the earliest date at which premature babies have survived. At 24 weeks the third trimester begins, and at about this time, as the cerebral cortex becomes "wired," fetal EEG readings begin to look more and more like those of a newborn. It may be a logical consequence, either of natural selection or of divine creation, that fetal higher brain activity begins at about the time when life outside the mother becomes possible. After all, without brain function, prematurely born fetuses would lack elementary survival skills, such as the ability to root for nourishment.

At about 26 weeks the cell structure of the fetal brain begins to resemble a newborn's, though many changes remain in store. By the twenty-seventh week, according to Dr. Phillip Pearl, a pediatric neurologist at Children's Hospital in Washington, D.C., the fetal EEG reading shows well-organized activity that partly overlaps with the brain activity of adults, although the patterns are far from mature and will continue to change for many weeks. By the thirty-second week, the fetal brain pattern is close to identical to that of a full-term baby...."

So personhood as defined by the brain starts at about 22 weeks of gestation...

Until then, abortion should be on demand... After only if the health or life of the woman host is at risk... Which actually would be perfectly OLK with Roe v. Wade...