HUMAN GROWTH AND DEVELOPMENT
PRE-EMBRYONIC PERIOD
the first 2 weeks of human development after
conception.
EMBRYONIC PERIOD
third week after fertilization and continuing until the end of the
eighth week. This period is known as the organogenetic period
that denotes the formation and differentiation of organs and
organ systems
week 2: developing of The yolk sac and the amniotic cavity, and
the embryoblast differentiates into the Implantation of the
blastocyst is completed by the end of the second week.
week 3: differentiation of the three germ layers:
1-ectoderm (outer layer, gives rise to skin, teeth, and glands of the
mouth and nervous system)
2-endoderm (inner layer, gives rise to epithelium of the respiratory,
digestive and genitourinary tracts)
3-mesoderm (lies between the ectoderm and endoderm; gives rise
to the connective tissue)
week 8: a clear distinction between the upper and lower limbs,
the external genitals are well developed, not well enough to
distinguish the gender, and the embryo has a human appearance
and main organ systems have also begun to develop
THE FETAL PERIOD
Weeks 9 to 12:
1-face is recognizably human at 10 weeks
2-Ossification appears
3-intestines leave the umbilical cord and enter the abdomen
4-external genitalia differentiate and are distinguishable by week 12
Weeks 17 to 20:
1-fetal movements (quickening)
2-skin is now covered with a thick, cheese-like material called
vernix caseosa
3-hair appears on the eyebrows and head.
4-Subcutaneous deposits of brown fat, used by the newborn for
heat production
Weeks 21 to 25: By 24 weeks
1-fingernails
2-lungs have begun to secrete surfactant
Weeks 26 to 29:
fetus may survive if born
1-lungs can breathe air
2-central nervous system can regulate body temperature and direct
rhythmic breathing
Weeks 30 to 34:
At 30 weeks the pupillary light reflex is present.
Weeks 35 to 40:
At 35 weeks, the fetus has a strong hand grasp reflex and
orientation to light