Definition
Human life begins with the fertilization of
egg by sperm. The period from the time of fertilization to birth is called
prenatal period. Generally, prenatal period is of nine lunar calendar months or
forty weeks or 280 days.
Fertilized
single cell is called a pair, which means "combination or mutual
formation". Approximately 24 to 30 hours after fertilization, the zygote
completes its first cell division. Then one cell splits into two, two into
four, and so on. At this stage the process of cell division moves at rapid
rate. Out of these divided cells some become reproductive cells while others
become body cells. Body cells forms muscle, tissues and other parts and organs
of the body.
The
zygote's 46 chromosomes represent the unique first edition of a new individual's
complete genetic blueprint. The master plan resides in tightly coiled molecules
called DNA. They contain the instructions for the development of the entire
body. DNA molecules resemble a twisted ladder known as a double helix
“If we could uncoil all of the DNA within an adult's
100 trillion cells, it would extend over 63 billion miles. This distance
reaches from the earth to the sun and back 340 times”
3 to
4 days after fertilization, the dividing cells of the embryo assume a spherical
shape. After traveling down the uterine tube, the embryo embeds itself into the
inner wall of the mother's uterus which is called implantation,
By
3rd weeks the brain divides into 3 primary sections called the forebrain,
midbrain, and hindbrain. Simultaneously
the development of the respiratory and digestive systems also takes place. The
heart begins beating 3 weeks and 1 day after the fertilization. The circulatory
system is the first body system to achieve a functional state.
Between 3rd and 4th weeks, the body plan emerges as
the brain, spinal cord, and heart of the embryo can be easily identified.
The
heart beats about 113 times per minute. The heart will beat approximately 540
lakh times before birth and over 320 crore times over the course of an 80-year
lifespan. Upper and lower limb development begins with the appearance of the
limb buds by 5th weeks.
The
skin is transparent at this point because it is only one cell thick. The head
is about 1/3rd of the embryo's total size.
Between
4th and 5th weeks, the brain continues its rapid growth and divides into five
distinct sections. The cerebral hemispheres appear, becoming the largest part
of the brain. The cerebral hemispheres controls thought, learning, memory,
speech, vision, hearing, voluntary movement, and problem-solving. The kidneys
appear by 5th weeks. The embryo begins to make spontaneous and reflexive
movements.
By
6th week, blood cell formation is underway in the liver where lymphocytes are
now present. The white blood cells are a key part of the developing immune
system.
By 6½
weeks, the elbows are distinct, the fingers are beginning to separate, and hand
movement can be seen.
Hiccups
have been observed by 7th week. Leg movements can now be seen. The
four-chambered heart is almost complete.
By 7½
weeks, the pigmented retina of the eye is easily seen and the eyelids begins a
period of rapid growth.
At
8th week the brain becomes complex and constitutes almost ½ of the embryo's
total body weight.
Although
there is no air in the uterus, the embryo displays intermittent breathing
motions by 8th week.
By
8th week the outer skin, becomes a multi-layered membrane, losing most of its
transparency. 8th week marks the end of the embryonic period. The embryo now
possesses more than 90% of the structures found in adults. The fetal period
continues until birth.
By
9th week, thumb sucking begins and the fetus can swallow amniotic fluid and
begin to distinguish as male or female.
By
11th week the nose and lips are completely formed. gender-dependent
developmental differences appear for the first time. For instance, female
fetuses exhibit jaw movement more frequently than males.
From
19th week foetal movements, breathing activity, and heart rate begin to follow
daily cycles called ‘circadian rhythm’.
Hair
begins to grow on the scalp in the 20th week.
By
21st to 22nd week the lungs gain some ability to breathe.
By
24th week the eyelids reopen and the foetus exhibits a blink-startle response.
This reaction to sudden, loud noises typically develops earlier in the female
foetus in comparison to male foetus.
By
26th week the eyes start producing tears and ability to detect odours also
develops. The foetus appears less wrinkled as additional fat deposits forms
beneath the skin. Fat plays a vital role in maintaining body temperature and
storing energy after birth.
By
28th week the foetus can distinguish between high- and low-pitched sounds.
By
30th week, breathing movements are more common and occur 30 to 40% of the time
in an average foetus.
At
35th week the foetus has a firm hand grasp. Foetal exposure to various
substances appears to affect flavour preferences after birth. For instance,
foetuses whose mothers consumed Masala Dosa showed a preference for Masala Dosa
taste after birth.
The
foetus initiates labour by releasing large amounts of a hormone called
oestrogen and thus begins the transition from foetus to new born. From
fertilization to birth and beyond, human development is dynamic, continuous,
and complex process. New research about this fascinating process shows the
vital impact of foetal development on lifelong health. The period before birth
has started to be understood during which the developing baby receives many
structures and learns various skills that are necessary for survival after
birth.
References:
1. http://www.ehd.org/resources_bpd_
documentation. php?language=37
2. NCERT
XI Psychology Text Book.
3. NCERT
XI & XII Biology Text Book.
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