Weeks 19 & 20

Even more of the brain connections for processing pain have developed by 20 weeks. Specifically, neural connections between the brain and the rest of the body are functionally complete after 18 weeks.¹⁶ Furthermore, two important brain structures for processing pain include the diencephalon and the thalamus. The diencephalon is sufficiently developed at 15 weeks to create a pain experience,¹⁷ and the thalamus is sufficiently developed at 17 weeks to transmit pain information.^{18 19}   

Information from all the senses, except smell, makes a neural connection in the thalamus before moving to the cortex for further processing. Decision making, perception, and language are all controlled by the cortex. None of these skills are fully developed at birth, and neither is their underlying brain structure. Before the cortex starts forming, a transitional structure called the subplate forms. This temporary and functional structure can be found in the developing brain between 12 and 35 weeks and has the capacity to process some pain information.^{20 21} Taken together, the current science shows that the fetus can feel pain well before 20 weeks.

Although the eyelids are fused shut between 10 and 22 weeks,²⁷ the eyes keep developing. The back of the eye, called the retina, which converts light into neural signals, now has discrete layers, each with a different function. The center of the eye matures before the outer edges.²⁸

In animals, waves of spontaneous electrical activity cross the retinas. This allows the retinal cells to properly connect to the visual cortex in the brain, and to organize inputs from the right and left eyes.²⁹ Although these spontaneous retinal waves have never been directly measured from the human fetus, they likely begin around this age.

By the midpoint of pregnancy, several key brain structures are beginning to take on recognizable form. Around 20 weeks, the hippocampus—a region central to memory—starts to resemble its adult structure.³⁰ At the same time, subtle asymmetries emerge between the brain’s hemispheres, with extra growth in regions that will later process language.³¹ The cerebral cortex is also organizing rapidly. By this stage it has developed the six-layered structure characteristic of the newborn and adult brain, a crucial scaffold for the complex neural circuits that will support perception, thought, and learning.³²

Around 19 weeks of pregnancy, the fetus begins creating body fat. By 7 months, fat deposits make the baby’s cheeks chubby.^{48 49} At 20 weeks of pregnancy, the skin has developed hair follicles and early sweat glands.^{50 51}

By 19 weeks, the unborn baby’s heart has beat over 20 million times.⁵² The fetal heart pumps about 55 quarts of blood each day.⁵³ For comparison, that is about a tank full of gas in a small sedan.