Neonatal Perception - Medical Studies and Reviews

Medical Studies and Reviews

The hypothesis that human fetuses are capable of perceiving pain in the early stages of a pregnancy has not received sufficient evidence to be proven or disproven; the developmental stage of research and instrumentation is so far insufficient to this task. The issue is considerably complicated by the usual difficulties in perceptual research of unresponsive subjects: "Though techniques such as positron electron tomography scanning might reveal those parts of the brain that respond to a painful stimulus, this does not tell us what the individual is experiencing."

The accepted hypothesis of the means by which pain is perceived states that it requires certain physical structures and operations. These are not formed in fetuses until 30 weeks or more. The consensus of the scientific community at this time is that only fetuses of this age or older are capable of perceiving pain.

Researchers from the University of California, San Francisco in the Journal of the American Medical Association concluded in a meta-analysis of data from dozens of medical reports and studies that fetuses are unlikely to feel pain until the third trimester of pregnancy. There is an emerging consensus among developmental neurobiologists that the establishment of thalamocortical connections (at about 26 weeks) is a critical event with regard to fetal perception of pain. Because pain can involve sensory, emotional and cognitive factors, it may be "impossible to know" when painful experiences are perceived, even if it is known when thalamocortical connections are established.

Electroencephalography suggests the capacity for functional pain perception in premature infants probably does not exist before 29 or 30 weeks; this study asserted that withdrawal reflexes and changes in heart rates and hormone levels in response to invasive procedures are reflexes that do not indicate fetal pain.

Also in 2005, Mellor and colleagues reviewed several lines of evidence that suggested a fetus does not awaken during its time in the womb. Mellor notes that much of the literature on fetal pain simply extrapolates from findings and research on premature babies. He questions the value of such data:

Systematic studies of fetal neurological function suggest, however, that there are major differences in the in utero environment and fetal neural state that make it likely that this assumption is substantially incorrect.

He and his team detected the presence of such chemicals as adenosine, pregnanolone, and prostaglandin-D₂ in both human and animal fetuses, indicating that the fetus is both sedated and anesthetized in the womb. These chemicals are oxidized with the newborn's first few breaths and washed out of the tissues, allowing consciousness to occur. If the fetus is asleep throughout gestation then the possibility of fetal pain is greatly minimized. “A fetus,” Mellor told the NYTimes, “is not a baby who just hasn’t been born yet.” Nevertheless, several studies show that during pregnancy fetuses have phases of wake; and it is clear that even during sleep pain can be felt.

There is also discussion among researchers about how pain is perceived over-all. Some researchers believe that because pain can involve sensory, emotional and cognitive factors, pain may not be sensed until after birth. Other researchers argue that pain is felt during the second trimester of pregnancy.

In 2001, a working group of the Medical Research Council (UK) in the United Kingdom called for more research regarding fetal pain. According to the Daily Telegraph, Eve Johnstone, the chair of that working group "makes a strong case for additional research." Ms. Johnstone told the newspaper, "We ought to study this carefully."

Though many researchers in the area of fetal development agree a fetus is unlikely to feel pain until after the seventh month of pregnancy, developmental neurobiologists suspect that the establishment of thalamocortical connections (at about 26 weeks) may be critical to fetal perception of pain.