Embryonic and Fetal Development

Conception includes

A. Fertilization

B. Pre-Embryonic period

C. Embryonic period

D. Fetal period

A. Fertilization

Fertilization occurs when one sperm enters

the ovum and the two nuclei containing the

parents’ chromosomes merge

i) Entry of one spermatozoon into the ovum ii) Fusion of the Nuclei of sperm and ovum

B. Pre-Embryonic Period

The Pre-Embryonic Period is the First 2

weeks after conception

It consist of Initiation of cell division, Entry

of the zygote into the uterus, Implantation

in the decidua, Maintaining the decidua,

Location and Implantation and Mechanism

of Implantation

C. Embryonic Period

Beginning of 3rd week through 8th week after conception

Basic structures of all major body organs are completed during embryonic period

Weekly Developments

Development of the Embryo and Fetus

* Cephalo caudal direction

* Central to peripheral direction

2nd Week

* Implantation completed by end of 2nd Wk

*Trophoblast grows fastly

*Inner mass becomes flattened into

embryonic disk

3rd Week

1st menstrual cycle missed

Embryonic disc develops 3 layers a. Ectoderm b. Endoderm c. Mesoderm

contd…

Derivatives of the three germ layers

Ectoderm

-Brain and spinal cord

-Peripheral Nervous system

-Pituitary gland

-Sensory Epithelium of the eye, ear and nose

-Epidermis, Hair & Nails

-Subcutaneous & Mammary glands

-Tooth enamel

MesoDerm

Cartilage

-Bone, Connective tissue, Muscle tissue

-Heart, Blood vessels, Blood cells, Lymphatic

system,

-Spleen, kidneys, Adrenal cortex

-Ovaries, Testes and Reproductive system

lining membranes

Endo Derm

Lining of G.I and Respiratory tracts

- Tonsils and Thyroid

- Parathyroid, Thymus

- Liver

- Pancreas

- Lining of urinary bladder and urethra

- Lining of Ear canal

contd…

CNS begins developing 3rd week

Thickened flat neural plate appears, extends toward end of Embryonic disk- becomes Head

Heart develops as a pair of parallel tubes and joins together

Primitive heart begins beating at 21-22 days

Primitive blood cells arise from the endoderm lining the distal blood vessels

Fourth WeekThe shape of the Embryo changes

It folds at the Head and Tail end and laterally. Remains in C- shape

Tail is apparent because brain and spinal cord develop more rapidly than other systems

Neural tube closed by 4th week. If not Anencephaly and Spinabifida results

contd…

4 - WEEKS

Formation of Face and U.R.T begins

Beginnings of internal ear and Eye apparent

Upper extremities appear as buds on the lateral body walls

Heart partition into four chambers completed by 6th week

L.R.T begins as a branch of upper digestive tract

Esophagus and trachea separate completely

Trachea branches to right and left bronchi &

Then forms Lobs, Alveoli rich capillary network that allows o2 &Co2 at birth

Fifth Week

Head is very large because the brain grows rapidly

Heart is beating and developing 4 chambers

Upper limb buds are paddle shaped with obvious notches of fingers

Lower limbs are paddle shaped but area between the toes is not well defined

Sixth Week

Prominent Head due to rapid development and is bent over the chestHeart reaches its final 4 chambered formUpper & Lower extremities become more definedEye continues to develop, External ear is apparent as 6 small bumps near each side of neckFacial development with eyes ,ears & nasal pits ( widely seperated )

Seventh Week

Growth in all systems & refinement occurs

Face becomes more human looking

Eye lids begin to grow, Extremities longer & better defined

Trunk elongates & straightens, C-shaped spinal curve present still birth

contd…

Intestines grows faster than abdominal cavity

Large liver & kidney occupies more space

Umbilical cord contains most of the intestines

At 10th week abdomen is large to contain all its contents

Eighth Week

Definite human form & refinement of all systems continue

Low-set ears but approaching their final location

Eyes are pigmented , not fully covered by eyelids

Fingers & Toes are stubby but well defined

External genitalia begin to differentiate but Male Female characteristics are not distinct

8 - WEEKS

D. Fetal Period

9 weeks after conception & ends with birth

Dramatic growth and refinement in the structure & function of all organ systems

Less likely teratogen may damage already formed structures

CNS is vulnerable for damage through out pregnancy

Weeks 9 - 12

Head is large ie half of the total length of the fetusBody grows fast than headFinal relative length of extremities but leg remain shorter than armsFirst fetal movements begin but too slightFace is broad, wide nose & Widely spaced eyesEyes closes at 9 weeks & reopen at about 26 weeks contd…

12 - WEEKS

Low-set ears because Mandible is still small

Abdominal capacity increases intestinal contents enters in to it

At 9th wk blood formation in liver that shifts to spleen by end of 12th wk

Urine produced & excrete into Amniotic fluid

contd...

Male & Female internal differences apparent in 7th week

External genitalia looks similar until end of 9th week

At 12th wk fetal sex can be determined by external genitalia appearance

Week 13 - 16

Rapid growth in length, so head becomes smaller in proportion to the total length

Strengthened Quickening mother able to find if it is 2nd gravida Resembles human face due to eyes face forward

Ears in final position, in line with the eyes

16 - WEEKS

Weeks 17 - 20Fetal movements like “Fluttering” or “Butterflies”Evident changes in skin and hairVernix Caseosa – protect from constant exposure to Amniotic fluidLanugo – help the vernix to adhere to the skinEye brows and head hair appearsBrown Fat – back of neck, behind sternum & around kidneys

20 WEEKS

Weeks 21 - 24

Growth continues & gaining weight but thin & little subcutaneous fatTranslucent skin & looks red Because capillaries are close to its fragile surfaceLungs produce surfactantCapillary network of alveoli increased but it is immature. although some gas exchange is possible 22 to 25 weeks fetus may survive with Intensive care

24 WEEKS

Weeks 25 - 28

More likely to survive if born after 24 wks Because of maturation of lungs, pulmonary capillaries & CNS

Subcutaneous fat deposition make fetus plumper & smoother- skinned

Skin becomes less red

Re-opening of eyes, abundant head hair

contd…

Blood formation shifts from spleen to bone marrow

Fetus assumes head-down position for 2 reasons

* Uterus is like inverted egg.

* Fetal head is heavier than feet

Weeks 29-32

Smooth skin, pigmented according to raceLarger vessels are visible over abdomen but not small capillariesToenails present, finger nails extended to the finger TipsMore Subcutaneous rounding the body contoursIf born chances of survival is good

31 to 32 WEEKS

Weeks 33-38

Until birth growth continues but rate slows as full term approaches.

Gaining weight

Matured pulmonary system enables efficient & unlabored breath after birth

Well-nourished term fetus is round with abundant subcutaneous fat

Lanugo in forehead, upper back, upper arms contd…

Vernix remain in groin and Axilla

Full term ranges from 36 to 40 weeks of fertilization or 38 to 42 weeks of gestational age