An Overview of Fetal Development

Source: pbs.com

~Table of Contents~

Conception(Slide 3)

The First Trimester(Slide 6)

The Second Trimester(Slide 10)

The Third Trimester(Slide 13)

Birth(Slide 16)

Conception

We all know the story of the birds and the bees: daddy's sperm

meets mommy's egg, and nine months later a baby is made. But

the truth of the matter is, the miracle of life is much more

complex than that. Lets examine that first flicker of life in slightly

more detail:Source: The Pregnancy Center, Kailua

Kona, Hawaii

Fertilization and Implantation

The sperm and ovum unite in the uterine tubes to form a zygote, or fertilized ovum.

Contained within this single celled zygote are 23 chromosomes from each parent, and all the genetic material this person-in-the-making will ever need.

As the zygote makes its way down the fallopian tube it undergoes rapid mitotic cell division. During this stage of embryonic development(or cleavage, as it is called) the zygote splits into 2 cells, then 4, then 8, and so on.

The zygote enters the blasocyst stage as it develops an inner mass of cells (which will later become the embryo) as well as an outer, protective layer of cells, called the trophoblast(which will later become the placenta).

The blastocyst reaches the uterus around day 5, and implants itself into the uterine lining approximately 7 days after fertilization

Fertilization and Implantation cont.

• Once implanted in the uterine lining, the endometrium surrounds the blastocyst and provides it nourishment and oxygen.

• Meanwhile, the trophoblasts sprouts chorionic villi, which, in turn combines with the endometrium to form the placenta, which the fetus will use after the first trimester to deliver all of it’s nutritive substances from the mother, and to disposes of fetal waste products through the maternal kidneys.

• A special layer of cells forms the amnion or water-sac, which will surround the developing embryo except at the umbilical cord.

The First Trimester

The first three months of prenatal development make up the first trimester, and give way the most profound fetal growth and development.

Now that we have explored embryonic development during those first two weeks, lets shine the light on the rest of the first trimester, picking up where we left off at in the embryonic period:

A human embryo at 7 weeks gestation (Image: Ralph Hutchings / Visuals Unlimited / Getty)

Embryogenesis (The First and Second Months)

Gastrulation is the process in which 3 defined germ layers form in the embryo(which is now a blastula). These germ layers are the ectoderm, which will later form the sensory organs and nervous system, the mesoderm, which will

later form the circulatory, skeletal and muscular systems and the endoderm, which

will later form the digestive and some glandular systems.

Following gastrulation, the embryo’s major organ systems begin to form. This developmental period lasts the remaining 5 weeks of the embryonic

period, and is referred to as embryogenesis, or organogenesis.

At approximately 3 post-conception weeks the neural tube, which will later become the nervous system, begins to form through the process of

neurulation.

Around the same time, the mesodermal cells begin to form a heart

tube, which, by day 22, begins to beat

By the end of the first month the embryo has begun to generated an

intestinal tract, lungs, liver, kidneys, and limb buds.

By the end of the embryonic period(week 8), the embryo is approximately ½ inch long, is

clearly recognizable as human, and has developed all of it’s

internal organs.

The Beginning of the Fetal Period(The third month)

Beginning at the 8th week post-conception, and for the rest of the duration in the womb, the embryo will be referred to as a fetus.

All of the growth and development that takes place during the fetal period is building on what was started during the embryonic period.

By the end of the 12th week the fetus:• Weighs nearly one ounce• Has developed genitals distinguishable as either male of female(though they will not

be detectable through an ultrasound scan).• Has grown 20 temporary teeth buds• Has developed vocal folds that are capable of making sound• Has well-formed eyelids, which are now sealed shut(they will open again in the 28 th

week)• Her liver has begun to produce red blood cells• She is capable of involuntary movements of the various appendages, and facial

features. • Her kidneys are now working, and the fetus is now secreting urine into the amniotic

fluid (the placenta works to remove any other forms of waste products, which are then passed into mother's blood stream and discarded through the maternal kidneys).

• Her arms, legs, fingers, and toes are relatively long and spindly• Her face is well defined, and the features continue to develop

Source: http://robby.nstemp.com/catalog.html

The Second TrimesterPreparation for life outside the womb

The fourth through the six month post-

fertilization make up the second trimester. It

is a period of growth and development on

what was established in the first trimester.

The fourth monthDuring this month:

The fetus’s tail shrinks, her back straightens, and her head is no longer tucked into her chest.

Her hands, feet, and joints are well-formed. Her muscles have continued to develop, resulting in more acute reflexes. She can now ball her fists, suck her thumbs, and just generally wiggle about. The mother-to-be may now experience quickening(or the first perceptible fetal movements).

Her nerves have been coated with myelin, a fatty substance which enables faster nerve transmissions and insulates the nerves so that impulses can be sent throughout her body.

She now has her very own unique set of finger prints and toe prints

Her circulatory system is fully functional, and her heart pumps about twenty-five quarts of blood through it daily.

Her lungs continue to develop as she practices breathing amniotic fluid.

The various fetal waste products, or meconium, have begun to gather in her intestinal tract. This meconium will later become her first bowl movement.

The fifth and sixth monthsDuring these two months:

The fetus’s various organs and tissues continue to flourish The fetus has developed working sebaceous oil glands and sweat glands She is covered by a thin layer of lanugo(thin, white hairs that will fall out shortly

before birth), and vernix caseosa(a protective, waxy coating made up of dead skin cells, lanugo, and sebum)

A protective layer of brown fat covers her lymphatic system, as well as her neck, chest, and groin.

Accelerated brain development begins in the 21st week, and lasts the entire first five years of her life

Her skeleton ossifies, and her bone marrow takes over the production of red blood cells(a job previously held by the liver and spleen)

Her freshly ossified ear bones have brought sound into her world. She is soothed by the familiar sounds of her mother’s body and voice.

By the end of the sixth month she is still spindly, and her skin is still wrinkled and translucent(she will begin to fill out her skin with layers of fat next month).

She is about one foot long and weighs about one pound. If she were to be born now her chances of survival would be slim. According to

NewScientist.com contributor Gaia Vince “After 23 weeks' gestation, a baby has just a 16% chance of surviving(outside the womb) with intensive care, and a 64% risk of serious disability.”(Vince, newscientist.com) Image Source: www.fertilityc.com/

The Third Trimester

Source: A.D.A.M

The seventh monthDuring this month:

• Grooves and creases have formed on the surface of her formerly smooth brain. These wrinkles serve to maximize space for brain tissue and brain cells.

• The fetus’s nervous system is now capable of controlling some of her movements as well as her body temperature and breathing rate.

• Her eyelids open and close, and her eyes are sensitive to light.• The vascularization of her lungs will eventually aid in the

exchange of oxygen and circulate it to tissue• Alveoli are now present in the lungs, and have begun to produce

surfactant. • Her respiratory system is now capable of gas exchange, though it

will still be a few weeks before she is able to breath on her own without any difficulty.

• She has begun to fill out her skin with deposits of subcutaneous fat. Image Source: http://robby.nstemp.com/catalog.html

The eighth and ninth month• These last two months are spent putting the finishing touches on various organ

systems and preparing the fetus for life outside the womb.• The fetus is now plump an dimpled; subcutaneous fat deposits have smoothed

her skin and given it a pinkish hue. 15% of her body weight is now fat, which enables her to maintain her body temperature.

• Her heart rate is between 120 and 160 beats per minute(about twice that of the average adult heart rate).

• As her immune system begins to develop, the placenta begins to pump her full of antibodies to help her fight pathogens and disease during her first year of life.

• Her gastrointestinal system is rather underdeveloped; it will take another 3-4 years for it to fully develop.

• As the uterus is pushed to its limits towards the end of the prenatal period, she has precious little room to wiggle about. Her legs are forced up against the uterine walls into the fetal position, and movement in general becomes increasingly difficult.

• The lungs boost production of surfactant to keep the alveoli from collapsing when she takes her first breath.

• Sometime during the last month the mother-to-be should experience “lightening”(or the baby’s (ideally) head-first decent into her mother’s pelvis in preparation for birth). Source of background picture: Powhatan Chapter of the

Virginia Society for Human Life

BirthIn the final stage of

prenatal development the fetus prepares to

leave her mother’s womb and enter

the world through the process of labor

and delivery.Source: Home Birth Midwifery Service

The mechanisms of labor At the onset of labor the fetus is typically in the vertex

position, with her head pushed deep into the pelvis in a sideways position

Contractions of the mother’s uterine muscles stimulate the effacement (shortening) and dilation(opening)of her cervix.

As her head descends, her chin is pushed into her chest, which allows for the presentation of the smallest part of her head. This is called flexion.

The fetus’s head should be pressed against the cervix, helping to wedge the birth canal open(this is called engagement).

Once the cervix is dilated to 10 centimeters, and is 100% effaced, the fetus descends into the birth canal

Her head should rotate again in order to maneuver it through the pelvis in a movement called internal rotation.

Her head must lift from her chest , and she must extend her spine In order for her head to pass through the pubic arch.

Once her head is out it will rotate once more in order to align itself with the shoulders. This is called external rotation.

Her anterior and then posterior shoulders, as well as the rest of her body, are delivered shortly after her head. This is called expulsion.

Source: A.D.A.M

From Fetus to Neonate• Upon departing her mother’s womb, the neonate, or newborn, has to begin functioning on

her own with out the aid of the placenta.• Prior to the infant's first breath, the alveoli in her lungs are recumbent and retain fluid. Her

passage through the birth canal has squeezed out most of this fluid and liberated it into the circulatory system.

• Once her umbilical chord has been cut she will no longer receive oxygen from her mother.• A lower pH and consequent acidosis, resulting from a drop in oxygen levels and carbon

dioxide build up in her blood, rouses the respiration center of her brain, and she begins to breath.

• Sensory overload at the moment of birth usually prompts the newborn to cry. This helps to inflate the alveoli in her lungs, and kick-start her respiratory system.

• In utero, her circulatory system oxygenated her blood by sending it directly to the the umbilical cord and placenta through an opening in her heart called the foramen ovale. It completely bypassed her lungs.

• Pressure changes in her lungs after her first breath, forcing a flap of muscle over the foramen ovale to close the opening. As a result the blood starts to pump into her lungs, and away from the the blood vessels in the umbilical chord, which causes them to collapse.

• In the womb she relied on her mother to maintain her body temperature. Now she is faced with the problem of trying to maintain her own body temperature. This is where the brown fat she developed in the beginning of the third trimester comes into play. It is her main source of heat production, since she is unable to shiver, or move enough to produce significant body heat.

Background picture by Soul Prints Photography

Bibliography

• Deirdre, O’Reilly, ed. "Fetal development." Medline Plus. 19 Oct. 2007. U.S. National Library of Medicine. 23 June 2009 <http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm>.

• Hatfield, Nancy T., and Violet Broadribb. Broadribb's Introductory Pediatric Nursing. 2nd ed.

• Klossner, N. Jayne. Introductory Maternity Nursing. Philadelphia: Lippincott Williams & Wilkins, 2005.

• Max Planck Society. "Chapter 14. Gastrulation and Neurulation." Kenyon College Biology Department. Kenyon College. 23 June 2009 <http://biology.kenyon.edu/courses/biol114/Chap14/Chapter_14.html>.

• Pregnancy and Baby Information Index Baby2see.com duedate timeline weeks months trimesters info ovulation fertility conception calculators development contractions labor top names. 2009. 23 June 2009 <http://www.baby2see.com/index.html>.

• Vince, Gaia. "When premature babies should be allowed to die." Newscientist.com 15 Nov. 2000. 24 June 2009 <http://www.newscientist.com/article/dn10577-when-premature-babies-should-be-allowed-to-die.html>.

• "Visible Embryo Home Page." Welcome to The Visible Embryo. 2007. 24 June 2009 <http://www.visembryo.com/baby/>.