1. Stages of Fetal Development
PREGNANCY
Stages of Fetal Development

2. 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.

3. Gestation The time between fertilization and birth is called
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

4. 1st 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

6. WEEK 2
Ovulation
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.

7. #1 #2 One ovum matures Possibility of fertilization Ovultaion:
Ovumm is entering Fallopian Tube
Uterus lining begins to thicken

8. 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)

9. 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…

10. Pregnancy has officially begun…

11. Cell Division As the zygote travels through the fallopian tube
WEEK 3
Cell Division
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

12. 1st cell division
2nd cell division
3rd cell division
4th cell division

13. Implantation The zygote enters the uterus & implants in the
WEEK 4
Implantation
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

15. Gastrula Formation 1. Development of the amniotic sac
WEEK 5
Gastrula Formation
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

17. 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

18. 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

19. 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)

20. 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

21. 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)

22. Ultrasound at 12 weeks

23. 2nd TRIMESTER
WEEK 14 – WEEK 27

24. 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

25. 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

26. 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)

27. 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)

28. Ultrasound at 20 weeks

29. 3rd TRIMESTER
WEEK 28 – WEEK 40

30. 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

31. 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

32. 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

33. 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!!

34. Ultrasound at 35 weeks

36. Overview of fetal development and critical periods

37. Important Factors To Consider During Pregnancy
Nutrition
Physical exercise
Tobacco
harmful chemicals pass through the placenta to the fetus
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

38. 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)