Humans have two sets of teeth because a child’s jaw is too small to fit the adult teeth. Plus, the second set of teeth lets accidents and poor dental hygiene have fewer long term consequences.
Week 9 to 10
Growth spurt, nails, teeth and fingerprints
Human Prenatal Age
- PCW 10iPost-Conception Week
- Days 63-69
- Gestational Week 12
Highlights
This week, the unborn child’s fingernails start to grow. Just like an adult’s, the fetus’s nails grow out slowly from the nail bed. The fingernails won’t reach the fingertips until approximately 30 weeks after conception.4 The toenails start growing at 14 weeks after conception and finish growing by 34 weeks after conception.5
This image shows a real living fetus in the womb, recorded using fetoscopy. The nerve receptors on the face, hands, and feet of this fetus respond to light touch. If something touches the sole of her feet, she will curl her toes. Also notice how much her arms and legs have grown from the previous week. (Image Credit: The Center for Bio-Ethical Reform)
By nine weeks after conception, the fetus responds to light touch on the face, palms of the hand and soles of the feet.6 A light touch on the sole of the foot may cause the fetus to bend his knee and curl his toes.7
The first recorded eye movements come from the 10th week after conception. When something touches the upper eyelid, the eyes roll downward and the muscles around the eye ‘squint’.8 Furthermore, if researchers touch the palm of the fetus’s hand at this age, she curls her fingers in towards the object, but not her thumb. The fetus does not include her thumb in grasping an object until about 13 weeks after conception.9
Why do people get two sets of teeth - baby teeth and adult teeth?
How do teeth form?
Teeth start as buds in the seventh week after conception. They form from the interaction of cells near the early neural tube, called neural crest cells, and the external tissue layer, called ectoderm. Basically, the enamel coating on teeth comes from the ectoderm while the inner structure of a tooth, including the roots and dental pulp, comes from the neural crest cells.10 About 9 weeks after conception, 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.11 Next, the jaw bone forms around the baby tooth and permanent tooth, holding them in place. Then the tooth grows by adding layers of dentin inside the tooth and enamel outside the tooth. Finally, the baby teeth start to poke out of the gum, starting roughly six months after birth.12 Most children have their full set of 20 baby teeth by age 2. Permanent teeth form inside the jaw bone through early childhood. When children shed their baby teeth, only the crown and the uppermost part of the root fall out. The permanent teeth use the same artery and bone socket that the baby teeth used.13 Adult teeth replace the baby teeth throughout childhood.
A frontal view of the jawbone and skull shows how the permanent teeth are developing underneath the baby teeth. This skull comes from a 6 1/2 year old. (Image Credit: Henry Gray, 1918, Public Domain)
