Different Stages of Human Growth
Human, like other animals, begin life as a single cell, the fertilized ovum guided by the interaction of the genetic information provided by each parent and the environmental milieu. This cell divides and grows, differentiates and developed into the embryo, fetus, child and adult. Though growth and development occur simultaneously, they are distinct biological process. Babies grow within the womb of their mothers. The sperm of the father unites with the egg of the mother to start the creation of life. Broadly, the human growth is divided in two periods-
a) Pre-natal and
b) Post-natal.
Pre-natal period:-
Pre-natal period is the time from conception to birth or the course of pregnancy. It starts with fertilization. This period is divided into three trimesters.
First trimester-
First trimester is called embryonic stage, starts from the fertilization till the thirteenth week of pregnancy. During the first trimester, one of the major events is the multiplication of the single cell, the fertilized ovum, into tens of thousands of new cells.
Fertilization
Every human being begins life as a single cell, formed when father's sperm fertilizes mother's egg. Fertilization normally takes place in the mother's Fallopian tube, which connects the uterus (womb) with the ovary. The uterus is the size and shape of a large pear: it is made of muscle and it stretches to allow the baby's growth throughout the months of pregnancy. A woman ordinarily has two tubes and two ovaries, one at each side of her uterus. Every month one of the ovaries in turn releases an egg (ovum) which passes slowly along the tube towards the womb cavity. If the egg is not fertilized within 12 hours or so of being released, it dies; it cannot develop further. But if the woman has sexual intercourse during the days of her monthly cycle just before or at the time when an egg has been released from the ovary, then many sperm cells released by her partner may travel up to the Fallopian tube and one may fertilize the egg. When fertilization is completed and the nuclei of egg and sperm have combined, a new being comes into existence and is capable of further development. Because the parents are human - belonging to the species Homo sapiens - the new being is also human. So fertilization (by which we mean conception) marks the beginning of the human lifespan.
Implantation
After fertilisation the single cell splits into two, then the two cells double to four, four to eight, eight to sixteen and so on. Because the cell cluster looks superficially like a berry it is called the morula (Latin for "mulberry"), but the new life is always biologically human (species Homo sapiens).
The journey along the Fallopian tube continues slowly for about four days. Growth increases. By the time the womb cavity is reached, the cell cluster becomes hollow and fluid-filled, and is referred to as the blastocyst. However, this is not an inert clump of cells but a busily developing human individual: differentiation (organisation into different parts and functions) is already taking place. Meanwhile the uterus is forming a spongy lining within which the embryo will implant. To achieve this embryo burrows into the wall of the womb and is covered over by the lining of the womb. This begins 6 days after fertilisation and is completed within the next 7 days. If fertilisation has not taken place; the lining of the uterus comes away at the end of the monthly cycle as the woman's menstrual period. But once implantation occurs, the embryo sends out a hormonal signal which prevents the mother's period. This is usually her first indication of pregnancy.
First Week— During this period the ovum is in the uterine tube. Having been fertilized in the upper part of the tube, it slowly passes down, undergoing segmentation, and reaches the uterus. It was imbedded in the decidua on the posterior wall of the uterus and enveloped by a decidua capsularis, the central part of which, however, consisted merely of a layer of fibrin. The ovum was in the form of a sac, the outer wall of which consisted of a layer of trophoblast; inside this was a thin layer of mesoderm composed of round, oval, and spindle-shaped cells. Numerous villous processes—some consisting of trophoblast only, others possessing a core of mesoderm—projected from the surface of the ovum into the surrounding decidua. Inside this sac the rudiment of the embryo was found in the form of a patch of ectoderm, covered by a small but completely closed amnion. It possessed a minute yolk-sac and was surrounded by mesoderm, which was connected by a band to that lining the trophoblast.
Second Week — By the end of this week the ovum has increased considerably in size, and the majority of its villi are vascularized. The embryo has assumed a definite form, and its cephalic and caudal extremities are easily distinguished. The neural folds are partly united. The embryo is more completely separated from the yolk-sac, and the paraxial mesoderm is being divided into the primitive segments.
Third Week — By the end of the third week the embryo is strongly curved, and the primitive segments number about thirty. The primary divisions of the brain are visible, and the optic and auditory vesicles are formed. Four branchial grooves are present: the stomodeum is well-marked, and the bucco-pharyngeal membrane has disappeared. The rudiments of the limbs are seen as short buds, and the Wolffian bodies are visible.
Fourth Week.— The embryo is markedly curved on itself, and when viewed in profile is almost circular in outline. The cerebral hemispheres appear as hollow buds, and the elevations which form the rudiments of the auricula are visible. The limbs now appear as oval flattened projections.
Fifth Week— The embryo is less curved and the head is relatively of large size. Differentiation of the limbs into their segments occurs. The nose forms a short, flattened projection. The cloacal tubercle is evident.
Sixth Week.—The curvature of the embryo is further diminished. The branchial grooves—except the first—have disappeared, and the rudiments of the fingers and toes can be recognized
Seventh and Eighth Weeks.—The flexure of the head is gradually reduced and the neck is somewhat lengthened. The upper lip is completed and the nose is more prominent. The nostrils are directed forward and the palate is not completely developed. The eyelids are present in the shape of folds above and below the eye, and the different parts of the auricula are distinguishable. By the end of the second month the fetus measures from 28 to 30 mm. in length .
The third month
The head is extended and the neck is lengthened. The eyelids meet and fuse, remaining closed until the end of the sixth month. The limbs are well-developed and nails appear on the digits. The external generative organs are so far differentiated that it is possible to distinguish the sex. By the end of this month the length of the fetus is about 7 cm., but if the legs be included it is from 9 to 10 cm. Boyhood or girlhood is now obvious.
Sensitivity
From a simple, generalised response to stimulation at 6 weeks gestational age, the foetus develops an almost complete range of responses to touches on the skin by 12 weeks.
Feeling pain
Brain cells which are essential for consciousness in the adult are known to be present in the foetus by 10 weeks. Nerve fibres which transmit pain impulses are known to be present before fibres inhibiting pain are completed. Foetus has started to acquire a sentient capacity perhaps as early as six weeks, certainly by nine to ten weeks of gestation. Anatomical examination of such foetuses indicates the probability that differentiation sufficient for reception, transmission and perception of primitive pain sensation has already occurred.
Practising for life outside the womb
At 11 weeks after conception the foetus starts to swallow the surrounding amniotic fluid and to pass it back in his urine. He can also produce complex facial expressions and even smile. Swallowing prepares the baby for taking in milk at birth. Thumb-sucking has also been recorded in the foetus.
Foetal breathing movements have been detected as early as 11 weeks. Although the baby does not breathe air inside the fluid-filled amnion, these movements help develop the respiratory organs.
Second trimester- The duration of fourth to six lunar month.
By sixteen weeks the baby measures 140mm from crown to rump, just over one third of the size he or she will be at full term, and weighs around 200g. The heart now pumps 30 litres of blood a day. The uterus expands and changes shape to accommodate the growing baby; pregnancy begins to show externally. The doctor can tell approximately how advanced the pregnancy is by locating the fundus (the top part of the uterus between the Fallopian tubes, which stretches upwards towards the mother's chest as the uterus expands).
Hearing
There is evidence that from four months the foetus responds to sound. Doctors testing unborn children for deafness, while monitoring their reactions to noise with ultrasound (a technique for visualising the children in utero), have observed eye movements and "blink-startle" responses in foetuses of 16 to 32 weeks gestation.
The baby hears sounds from the outside world as well as from the mothers heart and digestive system: "In fact the inner ear of the foetus is completely developed by mid-pregnancy, and the foetus responds to a wide variety of sounds. He is surrounded by a constant very loud noise in the uterus - the rhythmical sound of the uterine blood supply punctuated by the noises of air passing through the mother's intestine. Loud noises from outside the uterus such as the slamming of a door or loud music reach the foetus and he reacts to them
Sensitivity to light
From the sixteenth week the foetus responds to light. If a blinking light is shone on to the mother's abdomen, the foetal heartbeat fluctuates.22 "In late pregnancy, some light penetrates through the uterine wall and amniotic fluid, and foetal activity has been shown to increase in response to bright light."23 The womb is a more stimulating environment than some people think; its occupant is alert and responsive.
Third Trimester
Seventh Month
Organism capable of independent life from this time on. Cerebral hemispheres cover almost the entire brain. Seven-month fetus can emit a variety of specialized responses. Generally is about 15 inches long and weighs about three pounds.
Eighth and Ninth Month
During this time, finishing touches are being put on the various organs and functional capacities. Fat is formed rapidly over the entire body, smoothing out the wrinkled skin and rounding out body contours. Dull red color of skin fades so that a firth pigmentation of skin is usually very slight in all races. Activity is usually great and he can change his position within the somewhat crowded uterus. Periods of activity will alternate with periods of quiescence. Fetal organs step up their activity. Fetal heart rate becomes quite rapid. Digestive organs continue to expel more waste products, leading to the formation of a fetal stool, called the meconium, which is expelled shortly after birth. Violent uterine contractions begin, though milder ones have been tolerated earlier, and the fetus is eventually expelled from the womb into an independent physiological existence.
Waking and sleeping
Foetal activity is affected when the mother is tired or under stress. The baby is usually most notably active when the mother is lying down at night. The mother feels the baby's kicking and may notice sharp movements when the baby gets hiccups after drinking the amniotic fluid or practises its breathing movements. In later pregnancy the foetus has been observed to show "behavioural states" - waking, calm sleeping, and "rapid eye movement sleep" which is associated with dreaming in adults.
The quest for comfort
The baby still has some room to manoeuvre inside the womb and seeks the position which feels most comfortable:
"It is very easy to demonstrate now with ultrasound that the babies make the most of all the space and room available to them ... We know that foetal comfort determines foetal position, that changes in maternal position provoke baby to seek a new position of comfort."
In the last weeks of pregnancy the baby lies head downwards, as the head is normally the first part to emerge at birth. Occasionally, if a baby's position of personal comfort is not changed to fit in with the normal birth process, there may be a "breech" presentation - rear end first - which needs medical attention.
The mother's labour begins as (following hormonal signals including that from the placenta) the muscular uterus contracts to expel the baby. The cervix (neck of the womb) gradually opens to allow the baby to pass into the vagina (birth canal). The amnion tears and releases its fluid (this is often referred to as "the waters breaking"). Contractions become more frequent as the baby is pushed through the cervix and vagina. After labour, which varies in length but usually last some hours, the baby is born. A gasp and a cry start the lungs working. The umbilical cord is cut and the baby is examined and weighed. Normal birthweight is approximately 3,400 grammes or about 7½ lb, but considerable variations sometimes occur because of genetic factors, health problems and outside influences such as the mother's smoking during pregnancy.Finally the membranes and placenta are expelled. The baby no longer needs a direct life support system as he or she can now breathe air and take milk.
Birth
Birth is the critical transition between life in utero and life independent of the support system provided by the uterine environment. The neonate moves from fluid to gaseous environment. The newborn is also removed from a source of supply of oxygen and nutrients provided by mother’s blood and passed through the placenta, which also handles the elimination of fetal waste products, to reliance his or her own system for digestion, respiration and elimination.
Fig: Pre-natal growth
The post-natal period-
The stages of post-natal growth and development are infancy, childhood and adolescence span the time when newborn potential achieve their mature realization. The approximate division between periods is: infancy – birth to three years of age, childhood- 3 to 12 years of age and adolescence 12 to 18 years of the age.
Infancy :-
It is very fast changing phase. The rate of growth never likes the prenatal growth. During the first year of the life infants may add 28cm in length and 7 kg in weight, which represents more than 50% of birth length and 200% of birth weight. The rate of decrease in velocity is also very steep, which makes growth during first year more rapid than any time in the post-anal life. It is nutrition dependent phase of life. The nervous system develops faster, learning capacity is larger. The eruption of milk teeth taking place and permanent teeth get calcified in sockets.
Childhood: -
Early childhood
The rate of growth slowed down in the early childhood. By 3rd year growth is slower but stable; neurological development is faster so psychological skill and learning ability enhance. Speech learning is taken place in this age and learning is about the physical, social and cultural environment. Pattern of growth in childhood is predictable but they are influence in amount and rate by several factors viz. heredity, nutrition social- economic status and psychological variables.
Middle childhood: -
This is most stable phase of the childhood period. By 9th body proportion of the child changes; growth of the brain is also enhance up to 95%. Milk teeth are replaced by the permanent with the exception of 2nd molar which erupts as 12 years of age. Mid growth spurt is also taken place in this phase. Research links the middle childhood spurts with an endocrine event called ‘adrenarche’ (on set of secretion of androgen hormones from adrenal gland).
Late childhood: -
This stage is influenced by growth and sex hormones which results to development of the reproductive system and secondary sexual characteristics take place. Linear growth is fast in last childhood and the spurt of the adolescence begins in this stage. The intensity of the spurt is more in male than female.
Adolescence
In pre pubertal phase there is differences in body size and shape between boys and girls. Changes in the velocity and acceleration of growth at adolescence affect almost all parts of body including long bones, vertebrates, skull and facial bones, heart, lung and other viscera, muscles mass but the exception of the adipose tissues. The boys are determined by broader shoulder and muscles unlike girl who have narrow shoulder, broader hips and more fat distribution in the body. Onset of oogenesis and spermatogenesis; sexual development with the clear appearance of the secondary sexual characters. During the post pubertal stage physique is differentiated in boys and girls. Body is filled with subcutaneous muscles and fat. In girls maximum growth in the length and the menarche follows within six week after adolescence spurt. Girls have adolescence spurt two year earlier than the boys but size of the spurt is smaller than boys. Peak growth velocity is almost 10.3cm/year in boys and 9 cm/year in girls (Tanner et al). Fig; Velocity curve of growth
After that adult stage come when body attain maximum maturation and development. Adult phase followed by senility comes where growth and development decline. This phase is also called aging.
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