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Fetal Development

Pregnancy is a time of very fast growth and organ development for your baby. The growth of your baby (also called a fetus) during pregnancy usually follows a very predictable pattern. Learn about the different stages of fetal growth and development during each trimester. All measurements are averages.

First Trimester (0 – 12 Weeks)

CR Length = The length measured from the top of the fetus’ head (crown) to the fetus’ bottom (rump).

Gestational Age

Weight / CR Length



 4 weeks  
fetus 4 weeks
  • A sperm and egg have united and created a small ball of cells called a blastocyst.
  • The blastocyst is implanted in the lining of the uterus.
  • The placenta begins to form. It will provide oxygen and nutrients to the growing baby and removes waste from the baby’s blood through the umbilical cord.
 6 weeks  
fetus 6 weeks
  • Growth is very rapid in the developing embryo.
  • Eyes, ears, spine, digestive tract, and nervous system begin to develop.
  • Heart is beating.
 8 weeks

1.3 cm (½ in.)

fetus 8 weeks
  • All systems and organs are in place and continue to develop.
  • Fingers begin to form.
 12 weeks

14 gm
(½ oz.)

6 cm
(2 ½ in.)

fetus 12 weeks
  • Fetus looks more like a human baby.
  • Toes and eyelids have formed.
  • Can clearly see the umbilical cord on ultrasound.
  • Genitals (penis or clitoris and labia) are developing.
  • Fingernails starting to grow.
  • Intestines are in the abdomen.
  • Next week will begin to make urine and release it into the amniotic sac to make amniotic fluid.


Second Trimester (13 – 27 Weeks)

The fetus can’t live outside your body during this trimester because their breathing and blood circulation systems are not developed enough.


Weight / CR Length



16 weeks

110 gm
(4 oz.)

12 cm
(4 ½ in.)

fetus 16 weeks
  • Head looks large in proportion to the body.
  • Bones developing and hardening.
  • Fetal movement can be seen on ultrasound.
  • Eyes can move.
  • Baby’s sex is clear.
20 weeks

320 gm
(11 oz.)

16 cm
(6 1/3 in.)

fetus 20 weeks
  • Halfway into the pregnancy!
  • Toe nails have appeared.
  • Begins to hear.
  • Digestive system has started to work.
  • Skin is thin and delicate and is protected by a creamy protective layer called vernix.
  • Mother may feel baby’s movements.
24 weeks

630 gm
(1 1/3 lbs.)

21 cm
(8 in.)

fetus 24 weeks
  • Body is covered with a fine downy hair called lanugo. The lanugo helps keep the protective layer of vernix on the skin.
  • Sucking reflex is developing and may suck thumb.
  • Hair grows on the head.
  • Able to swallow fluid.
  • Skin is red and wrinkled.
  • Fat has begun to form under the skin. This fat will help to keep the baby warm when born.
  • Fingerprints and footprints begin to form.
  • Hiccupping might begin and feel like small jerking movement.


Third Trimester (28 – 40 Weeks)

During these last 3 months, your baby could survive if born before full term. They would, however, need some level of special care. Your baby gains weight during this time.



CR Length


28 weeks

1000 gm
(2 1/4  lbs.)

25 cm
(10 in.)

fetus 28 weeks
  • May respond to your voice with movement.
  • Lungs are getting stronger. The lungs being to produce surfactant. Surfactant is a protein that helps the lungs to expand after birth.
  • Eyes can partially open and eyelashes have formed.
  • Central nervous system can direct rhythmic breathing and control body temperature.
30 weeks

1300 gm (1.3 kg)
(3 lbs.)

27 cm
(10 ½ in.)


  • Baby can kick, stretch and hands can grasp.
  • Eyes can open wide.
  • Time of fast weight gain for the baby begins.
32 weeks

1700 gm (1.7 kg)
(3 ¾ lbs.)

28 cm
(11 inches)

fetus 32 weeks
  • The lanugo hair that protects the skin begins to fall off.
  • Toenails are visible.
  • Baby practices breathing.
34 weeks

2100 gm (2.1 kg)
(4 ½ lbs.)

30 cm
(12 in.)

  • Eyes respond to light.
  • Fingernails have reached fingertips.
  • Bones are hardening but the skull stays soft and flexible.
36 weeks  
fetus 36 weeks
  • Baby's skin is pink and smooth and takes up most of the amniotic sac.
  • Arms and legs have a chubby appearance.
38 weeks 2900 gm (2.9 kg)
(6 ½ lbs.)
  • Toenails have reached the tips of their toes.
  • Shed most of their downy body hairs (lanugo).
  • Adding fat all over the body to keep them warm after birth.
  • Moving into position for birth and head may descend into mother’s pelvis.

Full term

40 weeks

3400 gm (3.4 kg)
(7 ½ lbs.)

36 centimetres
(14 in.)

fetus 40 weeks
  • Antibodies from mother are transferred to the baby providing protection against diseases.
  • At 40 weeks baby’s due date arrives.
  • Remember:
    • A due date is only an estimate. Many babies are born before or after their due dates.
    • Healthy babies come in many different sizes. The numbers given here are only an average.


Related Links

It’s important to be aware that what you are exposed to during your pregnancy can affect the health of your growing baby. The following may be harmful to the fetus at any time during your pregnancy:

Date of creation: December 6, 2012
Last modified on: October 23, 2019


1Mayo Clinic. (2017 July 12). Fetal Development: The First Trimester. Retrieved from
2Mayo Clinic. (2017 July 8). Fetal Development: The Second Trimester. Retrieved from
3Mayo Clinic. (2017 July 6). Fetal Development: The Third Trimester. Retrieved from