Stages of Development

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 36-37 weeks, safe to deliver• 40 weeks considered full term

Ovarian Cycle

Fetal Development Stages – Sutterhealth

Visible Embryo – PBS

Medline Plus

1 Day Post-Ovulation

• Sperm fertilizes egg• Creation of ZYGOTE

1.5 – 3 Days Post-Ovulation

• 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

Development Picture

Day 4 (Week 1)

• Early Blastocyst phase• In uterus, cell division

continues• Cells move to outer

areas leaving open cavity inside—this is called the BLASTOCYST (Blastula)

Development Picture

Day 5-6 (Week 1)

• Implantation Begins• Considered pregnant• Progesterone

produced to maintain thickness of uterine lining

Development Picture

Day 7-12 (Week 2)

• 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

Development Picture

(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

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.

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

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

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

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

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

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

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

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

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

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

Week 33-36• Length of about 16-19 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

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

Development Comparison

1st MonthBACK

Cleavage

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