What is fetal development?


Fetal development describes the scientific process of growth and development of the unborn from a single, fertilized egg to birth. 1

Fertilization:  Day 1:  Life Begins

  • Life begins when the sperm and ovum (mature egg) unite to form a single cell (zygote, or fertilized egg)¹.
  • 23 chromosomes are from the male and
  • 23 chromosomes are from the female.
  • The combined 46 chromosomes are now a zygote.
  • This zygote is unique.
  • It is a cell which never existed before.
  • This cell has the genetic blueprint for eye color, gender, intelligence, height, hair color, skin tone, facial features, body type and personality characteristics.
  • THE EMBRYO:  3-5 days after fertilization, the zygote becomes an embryo.
    • The embryo travels down the fallopian tube and
    • enters the uterus to implant into the uterine wall.

Week 2 – Embryo Growth

  • The embryo implants into the uterine wall by the seventh day.
  • The embryo begins to draw nutrients.
  • Now hormones prevent the menstrual period (for most pregnant women).

Week 3 – Embryo Growth

  • The brain, spinal cord, and nervous system begin to develop.
  • The growing backbone causes the body to curl.
  • The beginning of future organs are visible.
  • The heart is formed.
  • The heart beats around day 20.


Week 4 – Embryo Growth

  • Simple kidneys, liver, and digestive system form.
  • Arms, legs, eyes, and ears form.
  • At 28 days old, this embryo is 10,000 times larger than the original fertilized egg.





Month 2 (Week 8) The Fetus

  • The beating heart is visible on an ultrasound monitor.
  • Brain waves can be recorded as early as 40 days.
  • Impulses begin to control and regulate circulation, digestion, and waste elimination.
  • Facial features are beginning to form: eyes, ears, lips, tongue.
  • The jaw forms, including 20 tiny tooth buds in the gums.
  • The eyelids seal to protect the light-sensitive eyes.
  • At 8 weeks the embryo becomes a fetus (Latin word meaning “Young one.”)
  • Elbows and fingers are now evident.
  • The fetus is a half an inch in length.
  • Everything is now present that will be found in a fully-formed and developed adult. 
    • The heart has been beating for more than a month.
    • Immature lungs, kidneys, liver, brain, nervous system and digestive system are present.
    • Forty muscle sets have begun to operate in conjunction with the nervous system.
    • The fetus is capable of responding to touch.


Month 3  (Week 12)  Fetal Growth

  • The fetus is an average length of 2 inches.
  • The soft, bony limb tissue now begins to calcify.
  • The fetus has a routine: sleeps, wakes, exercises.
  • Other developments:
    • Curls its toes
    • Turns its head
    • Opens and closes its mouth
    • “Breathes” amniotic fluid
    • The iris of the eye is developing.
    • Fingers and toes are forming, and fingernails begin to appear.
    • The fetus has fingerprints, a unique identity that will identify him or her for the rest of its life.
    • The fetus will move away if touched.
    • Grasps fingers around an object
    • If the palm is stroked, the fetus will make a tight fist.
    • The fetus is able to experience pain.
    • Sexual differentiation is apparent.
    • All organic systems are functioning.
    • The fetus has skeletal structure, nerves, and circulation.
    • The fetus can now yawn and suck.
    • The fetus can cry, and tries to do this in the uterus at times.

Month 4  (Week 16) 

  • The heart is beating.  (It pumps 25 quarts of blood a day.)
  • The ears are functioning.
  • There is strong evidence that the fetus can hear quite a bit, including its mother’s voice and her heartbeat.
  • The pregnant woman reports feeling movement within.
  • The fetus is nourished an hour or two after the mother eats.
  • The fetus can now urinate and swallow.
  • By the end of this month the fetus is 7 inches in length.
  • At 16 weeks the gender of the fetus can be seen on ultrasound.

20-wksMonth 5 (20 Weeks) “Quickening”

  • The fetus is approximately 12 inches long and weighs approximately one and a half pounds.
  • The fetus curls when the mother moves and stretches out when the mother rests.
  • Some pregnant women report being able to feel and sense this movement.
  • Fetal movement felt by the pregnant women is called “quickening.”
  • The female fetus now has a uterus and ovaries containing eggs.
  • Loud noises may startle the fetus and cause him or her to jump in response.
  • At this point the fetus rests when the mother rests.
  • This is the halfway point of pregnancy.

Month 6  (Week 24)

  • Vernix (white, waxy ointment) covers the fetus’ skin as protection.
  • The skin is pink, translucent and wrinkly.
  • Rapid eye movements are noted.
  • The fetus has a blink-startle response if stimulated by loud noises.
  • If born now (prematurely), and given the proper care, survival is possible.
  • The lungs are well-developed and nearly ready to work.
  • Hair is now visible.
  • Unborn males have testicles which are ready to descend from the abdomen to the scrotum.
  • From week 22 to week 32, the fetus experiences pain more intensely than at any other time in development.
  • At 24 weeks, the fetus weighs slightly more than a pound.

Month 7 (Week 28)

  • Hearing, vision, taste, and touch are in process.
  • Eyelashes are clearly visible and the eyes begin to open.
  • The fetus has fully-functioning taste buds on her or his tongue.
  • Now the fetus can experience motion and do flips in the womb.
  • The fetus can suck its thumb.
  • The fetus now weighs approximately 3 pounds and is approximately 11 inches in length.
  • If born at this age, the fetus can breathe air on its own.
  • The brain is developed enough to coordinate rhythmic breathing and regulate its body temperature.
  • Pregnant women report Braxton-Hicks Contractions at this point.  These contractions occur in the third trimester (month 6-9 or weeks 24-36) and prepare the pregnant woman for childbirth.

Month 8 (Week 32)

  • The fetus’ skin thickens and stores a layer of fat for insulation and nourishment.
  • Antibodies form to give protection from disease and give immunity.
  • The heart is now pumping 300 gallons of blood a day.
  • The fetus absorbs a gallon of amniotic fluid per day and completely replaces that fluid every three hours.
  • The fetus now has definite, active cycles of sleep and wakefulness.

Month 9 (Week 36 weeks to Week 40)

  • The fetus is now 20 inches in length.
  • Its body is more plump.
  • The fetus has a firm grasp.
  • Approximately one week before the 260th day, the fetus stops growing and drops its head downward into the pregnant mother’s pelvic cavity, preparing for birth.
  • The average pregnancy is 280 days from the first day of the pregnant woman’s last period.
  • Most babies are born between 266 and 294 days.
  • They weigh between 6 and 9 pounds.
  • The fetus is capable of living outside of the womb.


Related topic:

See: Images of Early Development


¹ https://www.webmd.com/baby/guide/understanding-conception