PREGNANCY
Stages of Fetal Development
Some Reminders…
To become pregnant, an egg (female gamete) must be
fertilized by a sperm (male gamete).
An egg is released once each menstrual cycle
(approximately 28 days).
Day 1 of the menstrual cycle is the beginning of
menstruation (period).
Every woman’s menstrual cycle is different, and therefore,
the days of the important events are only estimates.
Gestation
The time between fertilization and birth is called
gestation.
Gestation is divided into 3 trimesters.
– each trimester lasts about 3 months
– several major changes occur during each trimester
The length of gestation is 38 weeks, however, it is more
common to think of it as 40 weeks long.
• This is because doctors usually time it with the
beginning of the woman’s menstrual cycle.
– therefore 2 weeks from start of menstruation to ovulation
st
1
TRIMESTER
WEEK 1 – WEEK 13
Some of the important events that take place during
this time are:
• Ovulation
• Fertilization
• Implantation of the egg
• Cell Division & Specialization
Ovulation
WEEK 2
This refers to the stage during the menstrual cycle
where a mature ovum from a follicle (in the ovary) is
released into the fallopian tube.
This occurs on day 14 of menstrual cycle.
#1
One ovum matures
Possibility of fertilization
#2
Ovultaion:
Ovumm is entering Fallopian Tube
Uterus lining begins to thicken
Fertilization
This is the union of a sperm nucleus (23 chromosomes)
with an ovum nucleus (23 chromosomes)
– Occurs in the fallopian tube
– Many sperm will swim toward an ovum but only one will
fertilize the ovum
– Upon fertilization the head of the sperm will enter the
ovum; however the tail portion will remain outside the
ovum
The fertilized egg is called a ZYGOTE.
– zygote = a single cell with a complete set (46 chromosomes)
F.Y.I.
The greatest chances to get pregnant is when a
couple has sexual intercourse between day 10 and
day ___ of the woman’s menstrual cycle.
You might consider this the most risky time to have sex when you do
not wish to become pregnant…
Pregnancy has
officially begun…
Cell Division
WEEK 3
As the zygote travels through the fallopian tube
towards the uterus it undergoes a series of cell
divisions.
• This is similar to what we saw with binary fission (asexual
reproduction)
When it reaches the uterus it has become a clump of
about 16 cells
– Therefore, there have been 4 cell divisions
1st cell division
3rd cell division
2nd cell division
4th cell division
Implantation
WEEK 4
The zygote enters the uterus & implants in the
endometrial lining.
– This happens 10 days after the egg has been fertilized
The zygote now has 2 layers of cells:
– EXTERNAL LAYER  develops into the placenta
– INTERNAL LAYER  the embryoblast which becomes
the EMBRYO
Gastrula Formation
WEEK 5
Once the zygote is implanted in the uterus, the next 4 days are filled
with some important developments and changes.
The unborn baby is now considered an EMBRYO.
1. Development of the amniotic sac
The amniotic sac contains the embryo & amniotic fluid.
– amniotic fluid: protects the embryo from injury & dehydration,
and regulates baby’s body temperature
2. Development of the placenta
• Membrane that forms between the mother’s uterus and the developing embryo
• Allows for exchanges of nutrients and oxygen between mother and embryo (it
serves as a filter)
• N.B. Mother’s blood does not enter the embryo
3. Differentiation of embryonic cells
Cells that were identical become specialized to perform
specific functions in the body
– The egg is now considered to be “gastrula” because of
its layers.
The gastrula is composed of 3 layers of cells:
• ectoderm: skin + nervous system
• mesoderm: kidneys, skeleton, muscles, blood vessels,
glands
• endoderm: lungs, walls of digestive system
Finding out you are
pregnant
Week 5 is when you might find out you are pregnant.
What are the signs that someone is pregnant?
• Missing menstrual period
– This can also be for other reasons such as increased
stress levels
• Pregnancy tests
– These tests detect the presence of hCG (human
chorionic gonadotrophin) in the urine
– hCG is produced by the developing embryo and is
present is a woman’s urine only when she is pregnant
Weeks 5 to 13
Week 5
• Head and tail are now distinguishable
• Central nervous system (brain & spinal cord) start developing.
Week 6
• Growing quickly…. Now looks like a tadpole
• Now has a brain and a tiny beating heart
• Other major organs start developing
Week 7
• Starting to look more like a human (losing the tail)
• Facial features are beginning to form (eyes, nose, etc)
Week 8
• Most of internal organs formed (but not matured)
• Bones start forming
Week 9
• Limbs start developing
Week 10
• Baby is now considered a FETUS
• Brain has grown a lot… huge head compared to body
• Genitals begin to form
Week 11
• The riskiest part of baby’s development has passed
• Body doubles in length this week
• Vital organs begin to operate
Week 12
• Baby is fully formed, but still needs to develop & mature
• Can move his limbs
• Can possibly tell the sex of the baby
Week 13
• Eyes starting to move to the right position
• Can start sensing sounds (through vibrations)
Ultrasound at 12 weeks
nd
2
TRIMESTER
WEEK 14 – WEEK 27
Week 14
• Placenta plays a vital role in supplying the fetus with all the
nutrients
• Movements are starting to get more controlled
Week 15
• Skin starts to be covered by lanuga (extra-fine hair)
Week 16
• Bones are hardening and joints are starting to work
• Nervous system is starting to function
• Lots of movement (rolling over, somersaults and kicking)
Week 17
• Fat is starting to be laid under skin
• Practices breathing
Week 18
• Super active
• Lungs developing quickly
Week 19
• Digestive system is developing more
Week 20
• Development of 5 senses
• If it’s is a girl… approx. 2 million eggs have formed
Week 21
• Taste buds have formed
• Might start sucking thumb or touching face
Week 22
• Sweat glands formed
• Skin is becoming less transparent
• If it’s a boy… testes have started to descend towards scrotum
Week 23
• More well-proportioned (body getting bigger compared to
head)
• Much better hearing (can recognize voices)
Week 24
• Lungs continue to develop
• 20-25% chance of survival if born now (but with
complications)
Week 25
• Components of the spine begin to form
• Adult teeth begin to form high up in gums
Week 26
• Spine is getting stronger
• Can respond to touch
Week 27
• Getting rounder and plumper (more fat under skin)
• Eyelids begin to open (can detect changes in light)
Ultrasound at 20 weeks
rd
3
TRIMESTER
WEEK 28 – WEEK 40
Week 28
• Survival rate has jumped to 90% if born now
Week 29
• Some babies begin to change to the head-down position
• Head is now in proportion to body
Week 30
• Lanugo starts disappearing
• Kicks are getting stronger and movement is more controlled
Week 31
• Overall growth starts to slow, mainly putting on weight
• Brain growing rapidly
Week 32
• The baby has now reached a major point in its development… it
would most likely be able to survive if born, with only minor
complications
• Sleeps 90-95% of the time
• Can turn head from side to side
Week 33
• Amniotic fluid is at its highest level
• Baby is starting to have less room
• You would feel less kicking and more rolling
Week 34
• Immune system is developing
• Has sharp fingernails
Week 35
• Central nervous system is maturing
• Digestive system almost complete
Week 36
• Lungs are usually fully mature
• Little space for baby too move… you might see an
elbow of heel stick out of the belly when moving
around
Week 37
• Baby is now considered full term
Week 38
• Clinically mature (ready to be born at any time now)
• Intestines are accumulating waste material
• Mother’s placenta starts aging now (becomes less
efficient)
Week 39
• Most of lanugo is gone now
• Has at least 300 bones (some will fuse together once
baby is born)
Week 40
• It’s time to come out!!
Ultrasound at 35 weeks
Overview of fetal development and critical periods
Important Factors To
Consider During Pregnancy
1. Nutrition
2. Physical exercise
3. Tobacco
– harmful chemicals pass through the placenta to the fetus
4. Alcohol
– Fetal alcohol syndrome is characterized by growth,
mental, and physical birth defects associated with the
mother's high levels of alcohol use during pregnancy
– Alcohol ingested by a pregnant woman easily passes
across the placental barrier to the fetus
Twins
Identical vs Fraternal
Identical twins (30% of the time)
• From a single egg and sperm
• Great deal of resemblance
• Same chromosomes
• Same sex
• Same blood type
Fraternal twins (70% of the time)
• Two ova, two sperms
• Little resemblance (like any other sibling)