Week 12

More complex behaviors can be observed at 12 weeks, such as bicycle kicking, punching, and clenching his fists.¹⁴ Just as more adults are right-handed than left-handed, these preferences can be observed long before birth. About 90% of unborn babies prefer their right arm and will move their head to the right more often than the left, too.¹⁵ The baby continues to touch his face, stretch, and yawn.^{16 17 18} This not only showcases the baby’s evolving capabilities but also indicates healthy brain development.¹⁹

As early as 12 weeks, connections form between the thalamus and the rest of the brain that researchers contend are capable of processing sensations including unreflective pain. Unreflective pain is an immediate sensory experience of pain without necessarily knowing one is in pain.^{20 21 22}

Fingerprints start forming around week 12 in a beautiful combination of genetics and environment. Genes provide the basic plan, but small details—like the baby’s movements, the flow of amniotic fluid, and where ridges start forming—shape the final pattern.^{25 26} Even identical twins, who share nearly the same DNA, develop different fingerprints because no two babies move in exactly the same way.²⁷ By 26 weeks, the familiar patterns of loops, whorls, and arches are permanently set.²⁸ Because each fingerprint is truly one-of-a-kind and remains unchanged for life, it serves as a reliable and enduring marker of personal identity.

By 8 weeks of pregnancy, the first signs of a tooth appear. A tiny tooth germ begins to grow inside the baby’s jaw. Teeth develop as a carefully ordered part of human development. Cells from the lining of the mouth and nearby neural crest cells, which come from the area next to the developing brain and spinal cord, work together to build a tooth.³²

By 10 weeks gestation, the bud stage begins. Small enamel organs form. These are the early structures from which each tooth will develop.^{33 34}

By about 12 weeks, the cap stage is underway. The tooth germ grows and folds inward. This helps shape the future crown, the part of the tooth seen above the gum. Important cells also begin to specialize. Some will later make enamel. Other nearby cells form the dental papilla, which will become the tooth’s pulp. The bud of a second tooth starts to form behind the first tooth. The second tooth will slowly develop into the permanent adult tooth. The first tooth to form becomes the baby tooth.^{35 36}

By 16 weeks, the tooth enters the bell stage. More layers appear, and the tooth becomes even more organized. These layers protect the developing tooth, carry nutrients, and help determine its final form, including the root.³⁷ The jaw bone forms around the baby tooth and permanent tooth, holding them in place.³⁸