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Week 12

Fingerprints and fingernails

Human Prenatal Age

Post-conception week 10
Days of life 70-76
Gestational Week 12

Highlights

This week, the unborn baby’s fingernails start to grow,¹ and fingerprints start to form.²
The unborn baby’s brain starts to form connections capable of processing sensations including unreflective pain.³
The earliest successful surgery in the womb was performed at 12 weeks and 5 days gestation.⁴

In gestational week 12, the unborn child’s fingernails start to grow. The baby’s nails grow out slowly from the nail bed, just like an adult’s. The fingernails will not reach the fingertips until approximately 34 weeks.⁵ The toenails start growing around 16 weeks and finish growing by 38 weeks.⁶

This composite image rendered from light-guided endoscopy recordings shows the fetus alive in the uterus at 10 weeks following (or 12 weeks gestation). The nerve receptors on the face, hands, and feet of this baby respond to light touch. Notice how long his arms and legs have grown from the previous week. (Image Credit: Education Resource Fund)

What can the baby do at 12 weeks of pregnancy?

By 12 weeks gestation, the unborn baby responds to light touch anywhere on his limbs or face.⁷  In week 12, the baby’s response to a light touch on the sole of the foot changes. While in previous weeks, the baby may have curled his toes if something touched the bottom of his foot, now he lifts his big toe up and fans his other toes out like newborn babies do.⁸ When something touches his palm, he curls his fingers toward the object but leaves the thumb out; the thumb does not join the grasp until about 15 weeks.⁹ This responsiveness can also be seen in twin pregnancies, and the baby will observably react when his twin brother or sister touches him.¹⁰

The first recorded eye movements come from week 12. When something touches the upper eyelid, the eyes roll down and the muscles around the eye ‘squint’.¹¹ The unborn baby starts swallowing amniotic fluid around 12 weeks gestation.¹² ¹³

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

The human brain at 12 weeks gestation. (Image Credit: Science Source)

12 weeks gestation: the first fetal surgery

If the unborn baby cannot release urine in the womb, the pressure can damage the developing kidneys and may kill the baby. This condition can be discovered as early as 12 weeks of pregnancy. Early treatment is critical. In fact, the first successful fetal surgery to relieve a blocked bladder was performed at 12 weeks and 5 days. Outcomes are best when this procedure is done before 16 weeks, giving the baby a better chance to survive and grow.²³

The earliest successful surgery in the womb was performed at 12 weeks and 5 days gestation.²⁴

How do fingerprints form?

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.

The nails on the fingers and toes start to grow, as does the baby's individualized fingerprints. The fingerprint patterns established by 26 weeks last for a lifetime.²⁹ (Image Credit: Priests for Life)

How does the inner ear grow?

Sounds are detected in the cochlea, a snail shaped tube in the inner ear. The young cochlea grows in a spiral fashion so that by week 12 it has completed 2 ½ turns and reached its adult shape. Internally, the structures needed to turn sounds into brain signals keep growing.³⁰ ³¹

Why do people get two sets of teeth?

Humans have two sets of teeth because a child’s jaw is too small to fit all 32 adult teeth. Plus, the second set of teeth serves as an important back-up should accidents or poor dental hygiene damage the first set of teeth, so the child will have fewer long-term consequences.

How do teeth form?

By 8 weeks of pregnancy, the first signs of a tooth appear as tiny tooth buds begin growing inside the baby’s jaw. These buds form through a remarkable partnership between cells lining the mouth and neural crest cells that migrate from the developing nervous system.³² Over the next several weeks, each tooth bud passes through the bud, cap, and bell stages, gradually taking on the shape of a future tooth. During this time, specialized cells begin forming the tooth’s crown, pulp, enamel, and dentin. Around 12 weeks, a second tooth bud appears behind each baby tooth. These hidden buds will eventually become the permanent adult teeth.³³ ³⁴ As development continues, the jawbone forms around both sets of teeth, while roots, cementum, and the periodontal ligament anchor each tooth securely in place. ³⁵

This animation illustrates how teeth form and change shape through the bud, cap, and bell stages. The purple cells represent the dental lamina while the blue cells represent the jaw tissue. (Image Credit: Wikimedia Commons)

Finally, the baby teeth start to poke out of the gum, starting roughly 6 months after birth.³⁶ Most children have their full set of 20 baby teeth by age 2. Permanent teeth form inside the jaw bone throughout 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.³⁷ Adult teeth replace baby teeth throughout childhood.

Continued development at 12 weeks of pregnancy

Most guidelines today recommend an 11 to 14 week dating ultrasound as the best practice for calculating a due date.³⁸ Over the next two weeks, the baby will triple in body weight.³⁹ This rapid growth allows the medical professionals to measure different body parts to make an accurate estimate of the baby’s age.

At 12 weeks gestation, approximately 10% of the total body weight is the liver. Large patches of blood-producing cells lie between the walls of the blood vessels and the liver cells,⁴⁰ At this time, the liver is the primary source of the fetal red and white blood cells.⁴¹ Between weeks 8 and 12, the intestines grow so fast that they extend outside the fetus, spilling into a cavity in the umbilical cord. By week 12, the abdomen has enlarged enough that the intestines can fit in the abdomen again.⁴² This week, the bone marrow also starts producing blood cells.⁴³ However, it will not become the primary source of red blood cells for the baby until about 7 months gestation.⁴⁴

The unborn baby’s heart has beat over 10 million times already!⁴⁵

At 12 weeks of pregnancy, the unborn baby’s heart has already beat over 10 million times!⁴⁶

Dive Deeper

When and How Fingerprints Form

Fingerprints are unique because of the fetus's size, location, and movement patterns...

12 Facts at 12 Weeks

We describe 12 amazing facts about the developing human being at 12 weeks of gestation. Enjoy the journey!

Teeth

By 12 weeks, the permanent teeth are already forming beneath the baby teeth, at least 6 years before they will emerge ...

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Week 12