The embryonic stage of development begins at day 15 after conception and continues through week 8.
Basic structures of all major body organs and the main external features are completed during this
time period, including internal organs. By week three the brain spinal cord and heart begin to
develop along with the gastrointestinal tract. Here, the neural tube forms which later becomes
the spinal cord. Also, leg and arm buds begin to appear and grow out from the small body. In week
four the brain begins to differentiate and the limb buds grow and develop more. In week five,
the heart now begins to beat a regular rhythm. Beginning structures of the eyes and ears are
noticeable. Some cranial nerves are now visible and muscles connect to nerves. In week six,
the formation of the lungs begins and fetal circulation is established.
The liver begins producing red blood cells and the brain develops further. Here, the primitive
skeleton forms and the central nervous system is now detectable with brain waves. In week 7,
we see a straightening of the trunk. Nipples and hair follicles form, along with elbows and toes.
Arms and legs begin to move more frequently and the diaphragm is formed. The mouth and
lips become visible along with early tooth buds. In week 8, there is a rotation of the intestines
and facial features continue to develop. The heart development is now complete and the embryo
begins to resemble a human being. In weeks through 12, sexual differentiation continues.
Buds for all 20 temporary teeth are now laid down and the digestive system shows activity. In this
stage, the head makes up nearly half the size of the fetus. The face and neck are well formed and
the urogenital tract completes its development. Red blood cells are produced in the liver and
urine begins to be produced and excreted. The fetal gender can be determined by week 12 using
an ultrasound, which will also reveal limbs that are long and thin and digits that are well-formed.
In weeks 13-16, a fine hair called lanugo develops on the head. The fetal skin is almost transparent
as bones become harder. The fetus makes active movements and sucking motions are made with the
mouth. The fetus swallows amniotic fluid and external genitalia are recognizable at this
stage. Fingernails and toenails are also present as the fetus's weight quadruples.
It is usually during this phase that fetal movement, also known as quickening,
is detected by the mother. Weeks 17-20 show a rapid brain growth with fetal heart tones that
can be heard with a stethoscope. The kidneys will continue to secrete urine into the amniotic fluid.
The vernix caseosa, which is a white greasy film, now covers the fetus. The eyebrows and head hair
may appear and brown fat is deposited to help maintain the fetus's temperature. Muscles are
well developed and nails are present on both fingers and toes. During weeks 21-24,
eyebrows and eyelashes are well-formed. The fetus has a hand grasp and startle reflex.
Alveoli begin forming in the lungs and skin remains translucent and red. The eyelids
still remain sealed and the lungs begin to produce surfactant. During weeks 25-28, the fetus reaches
a length of about 15 inches. There is a rapid development of the brain and eyelids are able to
open and close. The nervous system is now able to control some of the functions of the fetus.
Fingerprints are also set. Subcutaneous fat is now visible under the skin. Blood formation shifts
from the spleen to bone marrow and the fetus usually assumes a head down position. In weeks
29 to 32, there is a rapid increase in the amount of body fat. There is an increased central nervous
system control over body functions and rhythmic breathing is now established. However, the lungs
are not fully mature. Pupillary light reflex is now present and the fetus stores iron, calcium,
and phosphorus. During weeks 33-38, the testes are in the scrotum of the male fetus. Lanugo
begins to disappear. The fetus has a good strong hand grasp reflex and increases in body fat.
Earlobes are formed and firm. Fingernails reach the end of the fingertips and small breast buds
are present on both sexes. The mother supplies the fetus with antibodies against disease. The fetus
is considered full-term at 38 weeks, when the fetus fills all the available space in the uterus.