Fetal Development

An unborn baby is referred to as an embryo through the first eight weeks following fertilization; it is then referred to as a fetus until delivery.

Week 1

Last menstrual period.

Week 2


Week 3

Implantation into the wall of the uterus

Week 4

Brain, nervous system, spinal cord begin to form. Organogenesis begins: the formation and development of organs

Week 5

Heart begins beating.  External features begin to form

Week 6

Eyes and ears begin to form; arm and leg buds begin to form; stomach forming; heart begins pumping blood through main vessels

Week 7

Pancreas begins to form and secrete hormones. Eyelid forming and partially covers the eye.

Small nostrils visible. The embryo is about the size of a pencil eraser.

Week 8

Sense of smell forming. Fingers and toes begin to lengthen.

Week 9

Hearing and balance abilities present. Gender was determined at conception but individual parts are not yet discernible.  Major joints like shoulders, knees, wrists, ankles, etc, move.

Week 10

Fingers are separated. Head is rounding out and neck is becoming distinguishable. Yolk sac begins to disappear. The “embryo” becomes a “fetus”.

Week 11

The fetus measures about 2 inches long. Eyelids fuse and won’t reopen again until week 28. Face is well formed. Bone beginning to harden.

Week 12

Fingernails are forming.

Week 13

Urine is being formed and discharged into the amniotic fluid. Arms and legs flex and kick.

Week 14

Red blood cells are being formed in the liver. Fetus can “make faces” using his/her facial muscles.

Week 15

Bones are continuing to harden and can be seen on ultrasound.

Week 16

Average length is 4 ½ inches. Heart pumps about 25 quarts of blood each day. Sucking motions

can be made.

Week 17

Fat begins to accumulate. Toenails are developing.

Week 18

Sense of hearing is developing. Average length is 5 ½ inches.

Week 19

Vernix, a protective cheesy-like substance, coats the skin. Hair may begin growing.

Week 20

Fetal movement can be felt.  Meconium, the substance seen in the first dirty diaper, is accumulating.

Week 21

Baby can swallow. Eyelids and eyebrows are fully developed.1

Week 22

A fine, downy, hair, called lanugo, covers the skin; this helps hold the vernix on the skin adding one more level of skin protection.

Week 23

Skin is red and wrinkled. Rapid eye movements.

Week 24

Regular sleep and wake patterns may be noticed. Average length is about 8 inches. Taste buds are developing.2

Week 25

The startle reflex begins to develop.

Week 26

Lungs are beginning to produce surfactant – a substance related to lung function. Average length is about 9 inches.

Week 27

End of second trimester. Lungs and nervous system continue to mature.

Week 28

Eyelids are partially reopened. Fetal weight gain starts to smooth out the wrinkled look. A 90%

survival rate if born early.[1. UNSW Embryology. (2015). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Main_Page]

Week 29

Bones are fully developed. Fetal movement is stronger.

Week 30

Eyes are often wide open. Average length is about 10 ½ inches, and average weight is about 3 pounds.3

Week 31

Central nervous system is mature enough to control body temperature. Reproductive system development continues.

Week 32

Toenails and fingernails fully formed. Fetal breathing movements may be noted. Vital minerals such as iron and calcium are being absorbed.

Week 33

Baby can tell the difference between light and dark.

Week 34

The lanugo covering is decreasing but the vernix covering is increasing. Average length is 12 inches.

Week 35

Rapid weight gain – about a half pound per week – begins.

Week 36

The head-down position is most common. Slightly decreased activity due to the crowded conditions inside the uterus.

Week 37

Fetal organs are ready to function on their own.

Week 38

Baby’s hand can firmly grasp. Average weight is about 6 ½ pounds.

Week 39

Antibodies are being formed by the placenta. Chest is more prominent. Fat stores increasing.

Week 40

Approximate due date.


  1. Baby Center.Retrieved from www.babycenter.com/fetal-development-images Retrieved Sept 25, 2015
  2. Mayo Clinic. (n.d.). Healthy Lifestyle-Mayo Clinic. Retrieved from http://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/fetal-development
  3. U.S. National Library of Medicine. (2013, September 30). Fetal development: MedlinePlus Medical Encyclopedia. Retrieved from http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm