The justification for the legislation lies in a fact (or, rather, an assertion) that cannot be proven: a fetus can experience pain starting at twenty weeks. My source is an article that appeared in the Journal of the American Medical Association for August 24/31 2005 under the headline “Fetal Pain: A Systematic Multidisciplinary Review of the Evidence.”

The bill, which has been proposed in California, Kentucky, Minnesota, Montana, New York, Oregon, and Virginia, as well as the U.S. Congress, requires physicians to inform women seeking abortions that the fetus feels pain and to offer fetal anesthesia at 20 or more weeks.

The Journal focused on “human studies related to fetal pain, anesthesia, and analgesia.” The articles included in the study focused on “fetuses of less than 30 weeks gestational age” or specifically addressed fetal pain perception or nociception. According to the article:

[P]ain perception requires conscious recognition or awareness of a noxious stimulus. Neither withdrawal reflexes nor hormonal stress responses to invasive procedures prove the existence of fetal pain, because they can be elicited by nonpainful stimuli and occur without conscious cortical processing.

Fetal awareness of noxious stimuli requires functional thalocortical connections. Thalamocortical fibers  begin appearing between 23 and 30 weeks’ gestational age, while electroencephaolography suggests the capacity for functional pain perception in preterm neonates probably does not exist before 29 or 30 weeks.

This is the conclusion reached in the article:

Evidence regarding the capacity for fetal pain is limited but indicates that fetal perception of pain is unlikely before the third trimester. Little or no evidence addresses the effectiveness of direct anesthetic or analgesic techniques. Similarly, limited or no data exist on the safety of such techniques for pregnant women in the context of abortion. Anesthetic techniques currently used during fetal surgery are not directly applicable to abortion procedures.

Based upon the article that I quoted from, above, there is no scientific support for the proposition that a fetus can feel pain at 20 weeks.