Charlotte Lozier Institute

Phone: 202-223-8073
Fax: 571-312-0544

2776 S. Arlington Mill Dr.
Arlington, VA 22206

Get Notifications

Sign up to receive email updates from Charlotte Lozier Institute.

This field is for validation purposes and should be left unchanged.

Week 12

Growth spurt, nails, and fingerprints

Human Prenatal Age
  • Post-conception week 10
  • Days 63-69
  • Gestational Week 12
  • The fetus’s heart has beat over 10 million times already!1

  • All of the intestines, which grew so fast that they extended outside the embryo, just under the umbilical cord, are back in the abdomen.2 

  • The fetus’s body weight increases by over 75%.3

In gestational week 12, the preborn 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 32 weeks.4 The toenails start growing at 16 weeks and finish growing by 36 weeks.5

This image shows the embryo alive in the uterus 10 weeks following fertilization (or 12 weeks gestation), recorded using fetoscopy. The nerve receptors on the face, hands, and feet of this fetus respond to light touch. Notice how long her arms and legs have grown from the previous week. (Image credit: The Center for Bioethical Reform)
What can a 12 week old fetus do?

By 12 weeks, 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 fetus continues with complex behaviors, such as thumb-sucking, swallowing, and stretching.8

The first recorded eye movements come from week 12. When something touches the upper eyelid, the eyes roll downward and the muscles around the eye ‘squint’.9  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 15 weeks.10

Why do people get two sets of teeth - baby teeth and adult teeth?

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.

How do teeth form?

Teeth start as buds in week 9. 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.11 Around 11 weeks, 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.12 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.13 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.14 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)
Dive Deeper
Fingerprints are unique because of the fetus's size, location, and movement patterns...
We describe 12 amazing facts about the developing human being at 12 weeks of gestation. Enjoy the journey!
Sperm-egg fusion