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The brain structures and nerve-cell connections that characterize the thinking and feeling parts of the brain are not completed until between the 7th and 8th months of gestation. Only after 30 weeks to the brain waves show patterns of waking consciousness when pain can be perceived. The reflex actions that are present before this stage do not indicate ability to feel pain. Abortions virtually never occur after 24 weeks.
Elements of the pain system first appear much earlier, beginning with pain receptors under the skin of the face at eight weeks and early connections within the pain pathways of the spinal cord at seven weeks. The spinal cord is the "highway" that carries pain information from the limbs and trunk upward to the brain. By 20 weeks, the human fetal brain contains the full complement of one billion neurons in the gray matter, waiting to receive ascending pain impulses. The final connections between pain fibers and the gray matter neurons are completed in the period between 20 to 24 weeks. This is how we as adults "feel" pain; there is no reason to believe this is any different for humans inside the womb.
Observations of human fetal response to painful stimulation support this. A 1994 study published in England by Professor Nicholas Fisk showed that two types of hormones released during pain and stress rose to high levels when blood was drawn (for necessary testing) from the living fetus by puncturing the abdomen. As a "control" experiment, blood was drawn from the painless umbilical vein source in other cases; these unborn babies did not release the pain and stress hormones.
These observations were made in fetuses as early as 19 weeks of age. An earlier study by Dr. Joachim Partch of Kiel, Germany, detected similar hormones from the amniotic fluid as early as 16 weeks into the pregnancy.