Fetal pain

Science asks awkward questions, doesn’t it? I got a link to a recent paper in the BMJ (thanks, SEF!) that asks one of those questions—can fetuses feel pain?—and then takes it apart clinically, coming up with an answer that will make some adults feel pain: that answer is no.

The first step is to work out when the machinery of the nervous system is first present, and when it is simply possible for the fetus to detect unpleasant stimuli. The nervous system has its beginnings early in development, with neurulation at around 3 weeks after fertilization, but it is initially little more than a thread of dedicated tissue that is more focused on raw proliferation than on putting together connections. The first pain sensors grow into the periphery around 7 weeks, and the first projections from the spinal cord to the thalamus of the brain also occur at 7 weeks. Prior to that time, there’s just no way for signals to travel to the brain, so sensory information doesn’t exist.

The thalamus, now, is a relatively low-level structure, the posterior part of the forebrain. To put it simply, it’s a kind of relay station that integrates information from multiple sensory modalities and forwards that on to the higher brain centers of the cortex, that stuff that we think of as more significant in generating conscious thought. It’s just not where the action is at. Also, at this early stage, the thalamus has not yet sorted out its structure and doesn’t have the capacity for much processing. Neither does the cortex; the next important step is for these tissues to organize themselves into layered structures, and for the thalamus to send projections to the cortex. This doesn’t happen until the embryo is 23-25 weeks old. That date represents a minimal, rock bottom bare essential level for the presence of any connections that would confer even a remote possibility of sensory function.

There is evidence that the 25 week old fetus can generate cortical responses to noxious stimuli. There are noninvasive tools to measure blood flow to regions of the brain, and poking needles into preemies and infants (not for fun, but as a necessary part of their medical treatment) elicits elevated blood flow into the somatosensory cortex, a sign that something is going on in their brain. The basic wiring is there.

The author, Derbyshire, is a psychologist, and goes on a little further. There’s more to conscious awareness of pain than raw, reflexive nociception. It also requires awareness, a level of experience that enables the mind to associate a pattern of electrical signals with elements of the environment. You can’t have pain without an ability to relate it to memory or emotion, and the fetus hasn’t yet developed any of those linkages.

So when can we first feel pain? Later than 25 weeks of gestation, that’s for sure. As you can see from the summary diagram below, Derbyshire argues that it occurs post-natally—substantially post-natally.

Key developmental stages before and after birth. Colours illustrate gradual emergence of
indicated feature. Solid colour indicates that feature is clearly apparent although not necessarily
fully developed (frontal cortex synaptogenesis, for example, continues into adolescence). Colour
becoming dim again indicates that feature is transient (hyperexcitability to noxious stimulation,
for example, appears at about four months’ gestation but is no longer a feature of behaviour
after three months of age).

You might argue with the specific age at which infants are first capable of experiencing pain, but I think his reasoning is sound.

Another obstacle to equating fetal pain experience
with that of adults or older children is the developmen-
tal process that begins after birth. Theories of
development assume that the early human mind begins
with minimal content and gradually evolves into the rich
experience of older children and adults. Although
the view of a neonate as a blank slate, or tabula rasa, is
generally rejected, it is broadly accepted that psychologi-
cal processes have content concerning people, objects,
and symbols, which lay in the first instance outside the
brain. If pain also depends on content derived
from outside the brain, then fetal pain cannot be possi-
ble, regardless of neural development.

Why does this matter? There is currently federal legislation pending that would require fetal anesthesia in all surgical procedures, including abortion, on fetuses older than 22 weeks of gestational age, with large penalties for doctors who ignore the ruling. We’re also seeing more and more efforts to carry out corrective surgery on fetuses. Anesthesia does have costs and risks to both the mother and fetus (in particular, I would think, to delicate developing tissues—why risk exposing them to strange chemicals unless absolutely necessary?). I don’t think the article is advocating indifference to the possibility of fetal pain, but is saying that an assessment of risks and benefits should recognize that fetal pain is not a serious concern.

Derbyshire SWG (2006) Can fetuses feel pain? BMJ 332:909-912.

Slater R, Cantarella A, Gallella S, Worley A, Boyd S, Meek J, Fitzgerald M. (2006) Cortical pain responses in human infants. J Neurosci 26(14):3662-6.