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Conception to Birth
Physical Development
Geminal Period
• Conception occurs when sperm successfully fertilizes
an ovum. From there the Germinal Period begins and
lasts two weeks.
• The fertilized ovum is now known as a zygote. This
period is characterized by rapid cell division, the
beginning of cell differentiation, and implantation of
the zygote into the uterine wall.
• Implantation usually occurs ten days after conception
and if it does not occur, the zygote is flushed out of the
woman’s system in her menstrual cycle.
• Not every zygote will implant, so medically speaking a
woman becomes pregnant when implantation occurs.
By the time implantation occurs the zygote is
composed of 100 to 150 cells.
Embryonic Period
• The embryonic period is the period of prenatal
development that occurs from two weeks to eight weeks
after conception. The zygote is now called an embryo.
• The embryo’s endoderm, the inner layer of cells, primarily
produces internal body parts such as the respiratory and
digestive systems.
• The middle layer of cells, called the mesoderm, will become
the circulatory system, bones, muscle, excretory system,
and reproductive system.
• The outer layer of cells, the ectoderm, will become the
nervous system, the sensory receptors (eyes, nose, and
ears), and skin.
• As the embryo develops so too does the placenta, the
umbilical cord and the amnion (fluid filled sac). It is at the
end of this period and the beginning of the fetal period that
most women become aware that they are pregnant as they
miss a menstrual cycle.
Fetal Period
• The fetal period is the period of development that begins
two months after conception and lasts for seven months on
average. The embryo is now called the fetus.
• Three months after conception the fetus is about 10cm
long and weighs 60g (0.13lbs or 2.11 oz). The fetus
becomes active, moving its arms and legs, opening and
closing its mouth, and moving its head. The face, forehead,
eyelids, nose, and chin are all distinguishable, as are the
arms, hands, and legs.
• At four months, a growth spurt occurs in the fetus’s lower
parts. By the end of the fifth month the toenails and
fingernails have formed and the fetus is more active.
• By the end of the sixth month the eyes and eyelids have
completely formed, a grasping reflex is present and
irregular breathing occurs. In the last two months, fatty
tissues develop and the functioning of organs such as the
heart and kidneys steps up.
• Pregnancy lasts 40 weeks and at 26 weeks the
fetus is considered viable, meaning the fetus
could survive outside of the uterus, but it is
very risky.
Body System
Critical Periods
Especially Sensitive
Development up to:
Central Nervous System
4th to 8th weeks
Postnatal, through to
5th to 9th weeks
12th week
Upper Limbs
6th to 10th weeks
12th week
6th to 10th weeks
Lower Limbs
6th to 10th weeks
12th week
9th to 11th weeks
9th to 11th weeks
16th week
External Genitalia
9th to 11th weeks
6th to 11th weeks
13th week
• As you can see, all the critical period occurs
during the first trimester (conception to 12
weeks, about three months). The first
trimester is also the highest risk for
miscarriage, as there is a correlation that if
something goes wrong during these critical
periods, the fetus will miscarry.
• Physical development will be influenced by
genetic and environmental conditions. But
more on that later…