From the time of conception until the
eighth week, the developing baby is
known as an embryo. At conception, the
fertilized egg consists of a single cell, the
zygote, which contains genetic material
from the sperm and the ovum. The
zygote divides several times to form a
ball of cells, which then implants into
the lining of the uterus. At the point of
attachment, the outer layer of cells
forms the placenta, while a group of
cells within one area of the cell ball
develops into the embryo. A sac filled
with amniotic fluid forms around the
embryo to protect it. As the embryo
grows, it begins to form features and,
by the fifth week, it has developed a
recognizable head and limb buds.
Embryo at about six weeks
The embryo is floating in the amniotic sac.
The smaller sac above (the yolk sac) provides
nourishment for the early embryo.
Three weeks
The embryo now becomes pear-shaped, with
a rounded head, pointed lower spine, and
notochord running along its back.
Four weeks
The embryo now becomes C-shaped and a tail
is visible. The umbilical cord forms and the
forebrain enlarges.
Six weeks
Eyes are visible and the mouth, nose, and ears
are forming. The limbs grow rapidly from initial
tiny buds.
All the internal organs (such as the liver, pancreas, stomach, kidneys, heart,
lungs, and sex organs) have begun to form by the fifth week. During this critical
stage of development, the embryo is highly vulnerable to harmful substances
that are consumed by the mother (such as alcohol and medication), which may
cause birth defects.
S t o m o d e u m
( m o u t h )
H e a r t b u l g e
L iv e r
G a l l b l a d d e r
U m b i lic a l c o r d
U r i n a r y b l a d d e r
P h a r y n g e a l g u t
L u n g b u d
S t o m a c h
P a n c r e a s
I n t e s t in a l l o o p
H in d g u t
Eight weeks
The face is more “human”, the head is more
upright, and the tail has gone. Limbs become
jointed and digits appear.
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