Fetal development
Dr. Carmen Rexach
Physiology
Mt San Antonio College
We all start as
one fertilized
oocyte!
Gestation lasts
≈266 days
8 weeks
5 months
1st trimester = 1st 12 weeks
2nd trimester = 13-24 weeks
Trimesters:
Full term
Clinical Approach
3rd trimester = 25 weeks to birth
Stages of Development
Pre-embryonic
Zygote
0-30 hr
A single diploid cell
Cleavage
30-72 hr
Identical blastomeres
Morula
Blastocyst
3-4d
Ball of 16+ blastomeres
4-16d
Trophoblast + inner mass
Embryonic
16d-8wks
Differentiation of 1o germ layers
Fetal
8-40wks
Growth and maturation
First trimester
7 weeks
8 weeks
12 weeks
30 days post fertilization (3 weeks)
• Four limb buds are visible
• Lens disc & optic vesicle are formed and you can
tell future eye color
• body is covered with first thin skin layer
• brain differentiates into forebrain, midbrain,
hindbrain
• thyroid development in progress
• Lymphatic system forms
• hearing development begins
30 days post fertilization
• Primitive mouth and tongue have formed
• heart chambers are full of plasma and
blood cells
• liver has formed and receives blood from
the umbilicus
• blood circulation has established
• gall bladder, stomach, intestines,
pancreas and stomach form
7 to 8 weeks
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First detectable brain waves
head begins to become erect
semicircular canals form
gonads form
– sex is visibly detectable by 8 weeks
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Legs formed with recognizable knees and ankles
Toes are notched and toe nails begin to appear
trunk elongates
muscles continue to develop and grow strong
12 weeks (2nd trimester begins)
• Fetus begins to move around
• Neck and shoulders are well-defined
• salivary glands function and sucking muscles
of mouth are present
• tooth buds still developing
• hair pattern can be detected on the scalp
• heartbeat can be detected externally
3 months
12 weeks (2nd trimester begins)
• fetus inhales and exhales amniotic fluid
which aids in lung development
• 25 quarts of blood are pumped by heart
each day
• Fully functional spleen under the direction
of the liver
• Limbs are more proportional
– Hands and thumbs show increased function
• sweat glands appear
• body hair begins to grow
• muscle and nerves continue to advance
Second trimester
4 months
5 months
Second trimester
• 14 weeks
– increased flexibility
– eyes move forward in the
face and ears are in
correct position
– torso grows rapidly
12 weeks
22 weeks
– fetus occupies more space in uterus -- becomes
crowded
– fetus recognizes maternal sounds -- mother’s
voice, heartbeat, breath, digestion
• auditory ossicles have hardened
– continued development of respiratory system
– continued growth of bones, muscles and organs
– skin is pink, wrinkled, with blood vessels, bones,
and organs visible through the transparent
surface
3rd trimester
6 months
Third trimester
• 28 weeks
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brain waves function like full term baby
forebrain enlarges
eyelashes present and eyes partially opened
improved sucking and swallowing
lungs can breathe air
2-3% body fat
testes descend in male
40 weeks = Full term
– 15% of body is fat
(80% under skin, 20%
holding organs in place)
– 300 bones -- some will
fuse postpartum to
make 206 in adult
– over 70 reflex
apparent
Amniocentesis
• Diagnostic test used to:
– Chromosomal abnormalities
• Down’s syndrome
• Spina bifida
• anencephaly
– Rh abnormalities
– Lung maturity