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Fetal Development

Fetal Development

Fetus at 12 weeks (source:

National Institutes of Health

When sperm is deposited in the vagina, it travels through the cervix and into the Fallopian tubes. Conception usually takes place in the Fallopian tube. A single sperm penetrates the mother’s egg cell, and the resulting cell is called a zygote.

The zygote contains all of the genetic information (DNA) necessary to become a child. Half of the genetic information comes from the mother’s egg, and half from the father’s sperm.

The zygote spends the next few days traveling down the Fallopian tube and divides to form a ball of cells. Further cell division creates an inner group of cells with an outer shell. This stage is called a “blastocyst”. The inner group of cells will become the embryo, while the outer group of cells will become the membranes that nourish and protect it.

The blastocyst reaches the uterus at roughly the fifth day, and implants into the uterine wall on about day six. At this point in the mother’s menstrual cycle, the endometrium (lining of the uterus) has grown and is ready to support a fetus. The blastocyst adheres tightly to the endometrium, where it receives nourishment via the mother’s bloodstream.

The cells of the embryo now multiply and begin to take on specific functions. This process is called differentiation, which produces the varied cell types that make up a human being (such as blood cells, kidney cells, and nerve cells).

There is rapid growth, and the baby’s main external features begin to take form. It is during this critical period of differentiation (most of the first trimester) that the growing baby is most susceptible to damage from:

  1. Alcohol, certain prescription and recreational drugs, and other substances that cause birth defects
  2. Infection (such as rubella or cytomegalovirus)
  3. Radiation from x-rays or radiation therapy
  4. Nutritional deficiencies

Specific Changes by Week

Week 3

Beginning development of the brain, spinal cord, and heart

Beginning development of the gastrointestinal tract

Weeks 4 to 5

Formation of tissue that develops into the vertebra and some other bones

Further development of the heart which now beats at a regular rhythm

Movement of rudimentary blood through the main vessels

Beginning of the structures of the eye and ears

The brain develops into five areas and some cranial nerves are visible

Arm and leg buds are visible

Week 6

Beginning of formation of the lungs

Further development of the brain

Arms and legs have lengthened with foot and hand areas distinguishable

Hands and feet have digits, but may still be webbed

Week 7

Nipples and hair follicles form

Elbows and toes visible

All essential organs have at least begun to form

Week 8

Rotation of intestines

Facial features continue to develop

The eyelids are more developed

The external features of the ear begin to take their final shape

The end of the eighth week marks the end of the “embryonic period” and the beginning of the “fetal period”.

Weeks 9 to 12

The fetus reaches a length of 3.2 inches

The head comprises nearly half of the fetus’ size

The face is well formed

Eyelids close and will not reopen until about the 28th week

Tooth buds appear for the baby teeth

Limbs are long and thin

The fetus can make a fist with its fingers

Genitals appear well differentiated

Red blood cells are produced in the liver

Weeks 13 to 16

The fetus reaches a length of about 6 inches

A fine hair develops on the head called lanugo

Fetal skin is almost transparent

More muscle tissue and bones have developed, and the bones become harder

The fetus makes active movements

Sucking motions are made with the mouth

Meconium is made in the intestinal tract

The liver and pancreas produce their appropriate fluid secretions

Week 20

The fetus reaches a length of 8 inches

Lanugo hair covers entire body

Eyebrows and lashes appear

Nails appear on fingers and toes

The fetus is more active with increased muscle development

“Quickening” usually occurs (the mother can feel the fetus moving)

Fetal heartbeat can be heard with a stethoscope

Week 24

The fetus reaches a length of 11.2 inches

The fetus weighs about 1 lb. 10 oz.

Eyebrows and eyelashes are well formed

All the eye components are developed

The fetus has a hand and startle reflex

Footprints and fingerprints forming

Alveoli (air sacs) forming in lungs

Weeks 25 to 28

The fetus reaches a length of 15 inches

The fetus weighs about 2 lbs. 11 oz.

Rapid brain development

Nervous system developed enough to control some body functions

Eyelids open and close

Respiratory system, while immature, has developed to the point where gas exchange is possible

A baby born at this time may survive, but the possibilities for complications and death remain high

Weeks 29 to 32

The fetus reaches a length of about 15-17 inches

The fetus weighs about 4 lbs. 6 oz.

Rapid increase in the amount of body fat

Rhythmic breathing movements occur, but lungs are not fully mature

Bones are fully developed, but still soft and pliable

Fetus begins storing iron, calcium, and phosphorus

Week 36

The fetus reaches a length of about 16-19 inches

The fetus weighs about 5 lbs. 12 oz. to 6 lbs. 12 oz.

Lanugo begins to disappear

Increase in body fat

Fingernails reach the end of the fingertips

A baby born at 36 weeks has a high chance of survival, but may require some medical interventions

Weeks 37 to 40

Considered full-term at 37 weeks

May be 19 to 21 inches in length

Lanugo is gone except for on the upper arms and shoulders

Fingernails extend beyond fingertips

Small breast buds are present on both sexes

Head hair is now coarse and thicker

Want to learn more about fetal development in preparation for exams? Discuss it with other experienced and prospective nurses.



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