Stages of Prenatal Development: From Conception to Birth, Slides of Human Development

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STAGES OF PRENATAL

DEVELOPMENT

College of William and Mary Students for Life 2013

From conception to implantation

Germinal Period

Fertilization – A Unique Human

 When the sperm meets with the egg, a

new human, with a DNA structure

unique from that of either parent is

formed

 The zygote may have a different blood

type than the mother

 This is not a small or arbitrary

difference

 If the mother has a negative blood

type and the zygote has a positive

blood type, the mother’s body will

fight against the foreign blood,

which may cause a miscarriage (in

order to ensure that this does not

happen, the woman must receive

regular injections before conception

occurs)

 If the zygote were simply another part

of the woman’s body, it would

necessarily be more similar in structure

to her DNA and blood

The Rest of the Germinal Period

 In total, the germinal

period lasts for 8 days

(ending well before a

woman knows that she is

pregnant); she has most

likely not yet missed a

period

 Once the morula attaches

to the uterine wall, it is

considered an embryo and

the germinal period ends

 This would be the “clump

of cells” appearance that

many abortion clinics may

refer to; however, this

stage ends long before a

woman would begin to

seek an abortion

Terms to Know

 Cephalocaudal : the body

develops from the head

down

 Proximodistal : the body

develops from the inside

out

 Gonadal ridges : where

sex organs will eventually

develop

 Embryo : the stage of

human development from

implantation until all

organs begin to develop

(roughly 8 weeks)

Development of the Embryo*

 Development is both cephalocaudal and proximodistal; this means that the central nervous system and the heart are the first things to begin developing

 3 weeks after conception

 The first heartbeat occurs (despite the heart not being fully formed)

 Three major divisions of the brain begin to differentiate (hindbrain, midbrain, and forebrain)

 5 weeks after conception: bronchial buds (which will become lungs) are formed

 6 weeks after conception:

 Lower jawbone fuses and upper jawbone begins to form

 The external ear begins to form

 The three main parts of the brain are distinct

 7 weeks after conception:

 Although the sex of the embryo has been decided since conception (when the sperm donated either a second X chromosome – female – or a Y chromosome – male), it is only now that the distinct physical differentiation begins to take place and gonadal ridges begin to form

 Some people seem to believe that it is only at this point that the embryo has a sex, but this is incorrect! They have had the chromosomal make-up to become either a male or female since conception, it is only now that those physical characteristics take form

 Muscles begin to assume their final shapes and relationships

 The brain develops thousands of nerve cells per minute

 8 weeks after conception:

 The external, middle, and inner ears assume their final form – the embryo can hear its mother’s voice and the sounds of its environment

 By the end of this week, the fetus is able to move around slightly and responds to stimulation around its mouth

*These are just the major developments; each week has many

more milestones not compiled here

Terms to Know

 Fetus : the stage in human development from

approximately 8 weeks until birth

 Retroflexion : bending, curving, or turning

backward

 Anteflexion : bending, curving, or turning

forward

Milestones of the Fetal Period

 The fetal period officially begins when all the

basic tissues and organs are in place in a

rudimentary form and when the skeleton

begins to ossify

 10 weeks after conception: intestines are in

place and the fetus has the ability to breathe

 12 weeks after conception:

 Sexual characteristics fully defined

 Sucking and swallowing movement

 20 weeks (5 months) after conception: fetus

has almost as many nerve cells as it will ever

have

 24 weeks (6 months) after conception:

olfactory cells are functional and fetus has the

ability to smell (only minutes after birth, an

infant will show preference for the smell of its

mother and for the smell of its own amniotic

fluid)

 28 weeks (7 months) after conception: lungs

fully formed

 Over the course of the fetal period, all organs

continue to grow and develop

Sensory Capacities (Cont’d)

 At 26 weeks, if a bright light is held

against the mother’s abdomen, the

fetus may respond with changes in

heart rate and movement

 It has been postulated that a fetus

sees light similarly to the way a

flashlight looks when covered by a

hand

 It is important to note that the slow

development of sight in a fetus is not

something that differentiates it

significantly from a newborn; a

newborn can only see approximately

one foot in front of it; farther than that,

the infant can only distinguish

shadows and basic shapes

Sight

Sensory Capacities (Cont’d)

 20-24 weeks (5-6 months) after

conception: the fetus responds to

sounds

 24-28 weeks (6-7 months) after

conception: the fetus can recognize

changes in sounds (begins to

differentiate the sounds present in the

language its mother speaks from the

sounds present in other languages)

 The sound level of the outside world to

a fetus is roughly equal to the level at

which one would hear the outside

world when sitting in a car with the

windows up

 The mother’s heartbeat and voice are

able to be heard the best

 Language Acquisition Experiments

 Infants were played sounds from their own language as well as sounds only present in other languages

 The infant was then played two sounds, one of which was present in its native language and one of which was not; they responded more strongly to the novel sound, indicating that they recognized it was not in the language they were used to hearing

 In another study, fetuses still in the womb were shown to prefer sounds in their native language

 Evidence for Hearing in the Womb

 Infants were read two stories by their mothers, one that had been read aloud to them while they were in the womb and one they had never heard before  The infants were then conditioned to be able to control what story would be read to them in a lab experiment (in one group, if they sucked faster, their mothers would read them the familiar story – slower sucking would cause their mothers to read the unfamiliar story

• and in the other group, they had to suck slower to hear the familiar story and faster for the unfamiliar)  The infants showed a clear preference for the familiar story and wanted their mothers to read it to them

 This indicates that they were not only able to hear the stories read to them in the womb, but remember them as well

Hearing Experimental Evidence

The prenatal stage is simply one of many stages in the development of

every human being.

In Conclusion...

Note: How could you expect me to end with anything other than an adorable picture of a baby?