Fetal Pain: It’s Not Pseudoscience

Recently, decisions by several state legislatures, including Texas and North Carolina, that limit abortions after 20 weeks have stoked political and moral debate. Even the U.S. House of Representatives passed legislation in June banning abortions after 20 weeks of fetal gestation.

Various motivations underlie the 20-week abortion ban, but a scientific one has moved to the forefront, with the premise that first and second trimester fetuses feel pain. This claim has been derided by the pro-choice community that touts scientific evidence against fetal pain.

Last week, a Salon article completely dismissed fetal pain as “phony science,” “a nebulous concept,” and “pseudoscience.” A Columbia OB/GYN (and abortion provider) was quoted as saying that “the fetus at 20 weeks can’t actually feel anything.” Her argument was that brain connections required to process pain “are not connected until after 26 weeks.” Other articles have also pronounced that science — based on a review article published in JAMA — has completely debunked the myth of fetal pain.

Fetal pain is a tricky area of study. The ambiguity stems primarily from the complex definition of pain. Interestingly – and as an aside – the scientific literature on this subject is not focused on the issue of abortion, but on evaluating approaches for fetal surgery. The dilemma is: should the mother only or mother and baby be given anesthesia when surgery is performed?

However, a logical extension of this question is that if there is a possibility that a second trimester fetus feels pain during surgery, then it is just as likely to feel pain during an abortion.

The best understanding of pain is from verbal responses. But since a fetus obviously can’t talk, scientists and medical professionals use symptoms (conscious expression) and signs (unconscious or involuntary reactions) that include visual, physiological, and biological markers. Facial expressions are useful measures of pain or distress, especially as the fetus grows, but as with all scientific data they can be variably interpreted.

The human nervous system consists of sequential, orchestrated steps to establish neurons and synaptic connections. Increasingly sophisticated neuroscience research is clarifying the links between these developmental stages and pain perception. Many neurobiology studies are performed in animals, such as mice or rats. But comparisons to neural development in animal models are not absolutely equivalent, since human brain development — starting with the production and growth of neurons — initiates earlier (first trimester) in humans.

Fetal pain studies often center on the development and function of nociceptors -– the sensory neurons that translate external stimuli into electrical pulses that are interpreted and remembered as pain. The nervous system forms early in the fetus. By 6 weeks of gestation, the spinal cord has already formed synapses with developing sensory neurons. Think of the nervous system as a huge electrical system in which various wires (neurons) pass electrical or chemical signals to each other. Synapses are the conduits that pass those signals between neurons.

During early pregnancy, neurons multiply at a rate of about 250,000 per minute. Biological markers of mature neurons are present as early as 8 weeks. By 11 weeks, the sensory neurons have reached the limbs, and at 15 weeks they are distributed throughout the trunk of the fetus. At 20 weeks, neurons have been established throughout the skin and mucosal (eyes, nose, lungs, stomach) surfaces. Therefore, the structures that carry impulses in response to stimuli are present before 24 weeks.

A major argument by scientists who deny fetal pain prior to the third trimester (before 24 weeks) is based on the composition of the nerves.

Most adult nerves are myelinated. Myelin is like insulation, allowing electrical impulses to travel along the nerves at high speeds. Myelin is built as a “sheath” that surrounds the nerve. More insulation equals faster conductivity of an impulse.

At 12 weeks gestation, myelin production begins, and in 20 week old fetuses, myelinated neurons are readily observed. Thus, in the second trimester, the fetus has both myelinated and unmyelinated nerves. Some claim that unmyelinated neurons equate with lack of function or a simple reflex response rather than a meaningful interpretation of pain.

But wait, not so fast…

Unmyelinated nerves still carry impulses, but the rate of transmission is just slower than myelinated ones. Impulses travel continuously on unmyelinated neurons (think: a person walking steadily). Upon myelination, the impulses travel rapidly in bursts (think: a person hopping or jumping).

The point is, myelinated or not, at 20 weeks the nerves are still transmitting impulses.

These neurons transmit information to the subplate of the fetal brain, an area of the cerebral cortex (the outer layer of the brain) that is required for consciousness, memory, and perception. The subplate organizes incoming nerve fibers, funneling information from the body to the thalamus. The thalamus, located in the midline of the brain, is like a switchboard operator that receives signals from the sensory neurons, and then determines which ones to forward to the cerebral cortex where they are interpreted as sensation. Thalamus-cortical connections first appear between 12 and 18 weeks. Between 23 and 30 weeks, direct thalamus-cortex connections become more visible. A reasonable conclusion, then, is that in the second trimester, nerves and receptors – important parts of the pain pathway — are connected to the thalamus.

Even more importantly, at 20 weeks, the fetal brain has the complete complement of neurons (~100 billion) present in adulthood.

The second trimester fetal brain appears smooth, not ridged or folded like we normally think of a human brain. Some argue that this “immature brain” translates to lack of awareness. Couple that with the continuous sleep-state of the fetus (caused by hormones in the amniotic fluid), and a common interpretation is that the fetus is not likely to register pain.

However, even in the absence of a mature cortex, stimuli still can be processed. And increasing evidence suggests that even minimal consciousness, such as a vegetative state, does not preclude awareness and sensory perception.

An even better argument for perception in the early brain is that at 13 weeks gestation an important fold appears on each side of the brain. By 15 weeks, this fold becomes part of the insular cortex, a key area of the brain for experiencing external stimuli — like pain.

In addition to these anatomical developments, between 8 and 20 weeks, the fetus also exhibits chemical and physiological responses  — production of stress hormones, increased blood pressure and heart rates — known to be associated with pain. Premature infants (age equivalent of a second trimester fetus) undergoing surgery require higher concentrations of anesthesia and also exhibit a stress hormone response that is 3-5 times greater than in adults. It becomes hard to argue that these second trimester fetal responses are merely reflexive.

Overall, scientific evidence does exist to suggest that there is pain perception in the fetus by 20 weeks gestation. But the indirect nature of measuring fetal pain will continue to draw controversy, similar to the dilemma in interpreting pain in non-verbal adults.

It comes down to how one chooses to interpret scientific data. I personally find it difficult to follow the logic of scientists who are convinced that lobsters — an invertebrate with a simple nervous system of less than 100,000 neurons — can feel excruciating pain when boiled, but deny that a 20-week old fetus that has 100,000,000,000 neurons is aware of pain in any significant way.

While researching this topic, I read the conclusion of one paper that aptly summed up the state of the fetal pain field:

 “When the development of the fetus is equal to that of a premature baby in whom we commonly measure and treat pain, we should suppose that the fetus can feel pain.…”

I’m more than content to suppose as well.