1. Stages of Development

2. Overview
Development of the embryo is broken into 3 Trimesters each with 12 weeks
Up to the 9th week=embryo
Beyond 9th week=fetus
At weeks, safe to deliver
40 weeks considered full term
Ovarian Cycle
Fetal Development Stages – Sutterhealth
Visible Embryo – PBS
Medline Plus

3. 1 Day Post-Ovulation
Sperm fertilizes egg
Creation of ZYGOTE

4. 1.5 – 3 Days Post-Ovulation Cleavage of zygote-picture
Development Picture
Cleavage of zygote-picture
First cell divisions repeat approximately every 20 hours
When cell division creates about 16 cells, the mass is called a MORULA
Morula leaves oviduct to uterus 3-4 days after fertilization

5. Day 4 (Week 1) Early Blastocyst phase
Development Picture
Early Blastocyst phase
In uterus, cell division continues
Cells move to outer areas leaving open cavity inside—this is called the BLASTOCYST (Blastula)

6. Day 5-6 (Week 1) Implantation Begins Considered pregnant
Development Picture
Implantation Begins
Considered pregnant
Progesterone produced to maintain thickness of uterine lining

7. Day 7-12 (Week 2) Implantation complete Growth of placenta begins
Development Picture
Implantation complete
Growth of placenta begins
Inner cells divide rapidly forming a two layered disc
Top layer will become embryo and amniotic cavity
Bottom layer becomes the yolk sac

8. (Week 2)
Gastrulation and formation of chorionic villi (“fingers” that form placenta anchor to uterus)
Formation of blood and blood vessels
Gastrulation, a process that gives rise to all three tissue layers of the embryo: ectoderm, mesoderm and endoderm.
Gastrulation Picture
DAY 16

9. Tissue Layers
Endoderm form the lining of lungs, tongue, tonsils, urethra and associated glands, bladder and digestive tract.
Mesoderm form the muscles, bones, lymphatic tissue, spleen, blood cells, heart, lungs, and reproductive and excretory systems.
Ectoderm form the skin, nails, hair, lens of eye, lining of the internal and external ear, nose, sinuses, mouth, anus, tooth enamel, pituitary gland, mammary glands, and all parts of the nervous system.

10. Week 3 Beginning development of the brain, spinal cord, and heart
Nervous system first to develop
Beginning development of the gastrointestinal tract
                                
1st Month Diagram

11. Week 4-5 Beginning of vertebra and bones
Development of the heart which now beats at a regular rhythm
Movement of basic blood through the main vessels
Beginning of the structures of the eye and ears
Brain develops into five areas and some cranial nerves are visible
Arm and leg buds are visible

12. Week 6 Beginning of formation of the lungs
Further development of the brain
Arms and legs have lengthened with foot and hand areas distinguishable
Hands and feet have digits, but may still be webbed

13. Week 7-8 Nipples and hair follicles form Elbows and toes visible
All essential organs have at least begun to form
Facial features continue to develop
Eyelids are more developed
External features of the ear begin to take their final shape

14. Week 9-12 Fetus reaches a length of 3.2 inches
Head comprises nearly half of the fetus' size
Development of four-chambered heart
Fingernails formation begins
Tooth buds appear for the baby teeth
Fetus can make a fist with its fingers
Genitals appear well differentiated
Vocal cords can make sounds
Bone hardening begins
First detectable brain waves-Involuntary movements possible

15. Week 13-16 Fetus about 6 inches
Fine hair develops on the head called lanugo
More muscle tissue and bones have developed, and the bones become harder
Fetus makes active movements
Sucking motions are made with the mouth
Muconium is made in the intestinal tract

16. Week 17-20 Fetus reaches a length of 8 in
Lanugo hair covers entire body
Eyebrows and lashes appear
Nails appear on fingers and toes
Fingerprints formed
More active with increased muscle development-mom can feel the fetus moving
Taste buds formed

17. Week 21-24 Fetus length of 11.2 inches Weighs about 1 lb. 10 oz.
All the eye components are developed
Hand and startle reflex
Air sacs forming in lungs
Thumb sucking
Rapid brain growth
Can sense light/sound

18. Week 25-28 Length of 15 inches; about 2 lbs. 11 oz.
Nervous system developed enough to control some body functions
Eyelids open and close
Respiratory system, while immature, has developed to the point where gas exchange is possible
Baby born at this time may survive, but the possibilities for complications and death remain high

19. Week 29-32 Length of about 15-17 inches-about 4 lbs. 6 oz.
Rapid increase in the amount of body fat
Rhythmic breathing movements occur, but lungs are not fully mature
Bones are fully developed, but still soft and pliable
Eyes open when active and shut when asleep
Fetus begins storing minerals

20. Week 33-36
Length of about inches; 5 lbs. 12 oz. to 6 lbs. 12 oz.
Lanugo begins to disappear
Increase in body fat
Fingernails reach the end of the fingertips
Baby born at 36 weeks has a high chance of survival, but may require some medical interventions

21. Week 37-40 19 to 21 inches in length
Lanugo is gone except for on the upper arms and shoulders
Fingernails extend beyond fingertips
Small breast buds are present on both sexes
Head hair is now coarse and thicker
Lungs finalize development

22. Development Comparison

24. 1st Month
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25. Cleavage
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