Delineate the characteristics and periods in prenatal development.
Characteristics of Prenatal Period :
The prenatal period has some important characteristics, each of which has a long-lasting effect on development during the life span.
• The prenatal period is the most important and first period of development in the life span.
• This is the shortest period for the infancy or newborn baby, which starts from the beginning and ends at the birth time of the baby. This period is approximately 270 to 280 days or nine months.
• Heredity is one of the important factors for prenatal development. It serves as the foundation for later development. Favourable or unfavourable conditions may affect to some extent of the physical and psychological traits, both before and after birth that makes up this heredity endowment. The changes will be quantitative and not qualitative.
• Favourable and unfavourable conditions of the mothers body can foster the development of hereditary abilities. Sometimes the hereditary abilities are so influenced by environmental conditions. They affect the fetus or the embryo as the case may be affecting the development adversely.
• The sex of the baby is fixed at the time of conception. The sex of the individual, determined at the time of conception, remains the same and doesn’t change, except surgery is used for sex transformation.
• During the prenatal period, proportionately greater growth and development take place than any other time throughout the entire life of a human.
• Before birth (during nine-month) the child grows from a microscopically small cell to an infant who measures approximately twenty inches in length and weight, on overage 7 pounds. It is observed that during this time weight increases U million times.
• Many believe that this time is more hazardous than other periods of the life span. It definitely is a time when the environment or psychological hazards can have a marked effect on the pattern of later development.
• The attitudes of people towards the newly created individual have a significant impact on the development, during the prenatal period. For instance, the mother’s positive attitude is essential to the normal development of the newly created individual.
Periods of Prenatal Development :
The prenatal period is ten lunar months of twenty-eight days each in length or nine months. Though, the period can vary greatly in length, ranging between 180-334 days. There are roughly three times as many babies born prematurely as post maturely. Meredith has reported that the average length of the prenatal period is 266 days or 38 weeks. Though, 70% of babies vary from 36 to 40 weeks or 266 days and 98% range from 34 to 42 weeks or between 238-294 days. The prenatal period is divided into three stages. These are discussed below in detail –
(i) Period of the Zygote
(ii) Period of the Embryo and
(iii) Period of the Fetus.
(i) Period of Zygote (fertilisation to the end of the second week)
Half of a person’s genetic substantial comes from her/his mother and half comes from her/his father. These two halves come together to form a unique combination of genetic potentials when the sperm fertilises the egg. In the nucleus of the fertilised egg or zygote, are the substantials that stand the pattern for a new person. This first cell in which the male genes are joined with the female is called the Zygote. The zygote looks like an unfertilised egg. The egg is so much larger than the sperm, it can absorb the sperm without showing it. However, the unfertilised egg has only 23 chromosomes, whereas the zygote has 46. The 23 that were in the egg originally and the additional 23 contributed by the male. The zygote divides into two cells less than two days after the sperm unites with the egg. Then these two cells each divide again, and the process of division goes on, forming in nine months a new human being.
(ii) Period of Embryo (end of the second week to end of the second lunar month)
• The embryo develops into a miniature human being. This stage begins on the 15th day after conception and continues until about the 8th week, or until the embryo is 1.2 inches in length. The cells of the embryo aren’t only multiplying, but they are taking on specific functions during this period. This process is called Tissue Differentiation. It is during this critical period of differentiation (three-month period) that the growing fetus is most susceptible to damage from external sources (teratogens) including viral infections, for example, rubella, x-rays and other radiation, and poor nutrition.
• A child who has one developmental problem may have other problems that arose at the same time. Hearing problems and kidney problems, for example, are often found together because both the inner ears and kidneys develop at the same time. The formation of the heart, the beginning development of the brain and spinal cord, and the beginning of the gastrointestinal tract begin in 3 weeks.
• Teratogens introduced during this period may cause severe problems, for example, a heart that is outside of the chest cavity at birth or the absence of one or more limbs. • Beginnings of the Vertebra, the lower jaw, the larynx, and the rudiments of the ear and eye develop at weeks 4 and 5,1/4 inch long. The heart, which is still outside the body, now beats at a regular rhythm. Although arm and leg “buds- are visible with hand and foot “pads”. The embryo still has a tail and cannot be distinguished from rabbit, pig, chick or elephant embryo by an untrained eye.
• Teratogens may cause very serious problems involving the vertebrae, esophagus, and eyes. The baby could be born with missing hands or feet or severe facial cleft.
• Formation of the nose, jaw, palate, lung buds takes place at the 6th week (1/2 inch). The fingers and toes form, but may still be webbed. The tail is disappearing, and the heart is almost fully developed. Teratogens at this point may leave the baby with a cleft lip or deep heart problems.
• The eyes move forward on the face, and the eyelids and tongue begin to form in the 7th week (7/8 inch). All essential organs have begun to form. Teratogens may cause lung and heart problems, a cleft palate, and ambiguous genitalia (not quite female or male).
• At the 8th week (1 inch) embryo now resembles a human being. The facial features continue to develop and the external ear appears. We see the beginnings of external genitalia also. The circulation through the umbilical cord is well developed by now. The long bones begin to form and the muscles can contract. Teratogens may still cause heart problems and stunting of the fingers and toes.
(iii) Period of Fetus (end of the second lunar month to birth)
• The embryo is developed enough to call a fetus at this point. All organs and structures found in a full-term newborn are present. The head comprises nearly half of the fetus’ size and the face is well-formed at 9th to 12th weeks (3 inches). The eyelids close now and will not reopen until about the 28th week. The tooth buds for the baby teeth appear. The genitalia is clearly male or female now.
• 13th to 16th weeks (6 inches) mark the beginning of the second trimester. The skin of the fetus is almost transparent. Fine hair develops on the head called lanugo. The fetus makes active movements, including sucking, which leads to some swallowing of the amniotic fluid. A thin dark substance called meconium is made in the intestinal tract. The heart beats 120-150 beats per minute and brain waves are detectable. • During 17th to 20th weeks (8 inches) eyebrows, lashes appear on face and nails appear on fingers and toes. This is an exciting time for the parents. The mother can feel the fetus moving and as well hear the heartbeat with the help of a stethoscope. • At 21
• to 24th weeks (11.2 inches) all the eye components are developed. Footprints and fingerprints are formed. The entire body is covered in cream cheese-like vernix caseosa. The fetus now has surprise reflex action. Many reflexes, which are automatic and unlearned responses to specific stimuli. It appears as swallowing, coughing, and sucking.
• 25th to 28th weeks (15 inches) we can observe the rapid brain development of the fetus. The nervous system is developed enough to control some body functions. The eyelids open and close. At this time a baby born may survive. The chances of complications and death are high at this period.
• At 29th to 32nd weeks (15 to 17 inches) development occurs towards an independent life. For instance, respiration movements are predicted even though oxygen is being provided through the placenta. There is a rapid increase in the amount of body fat and the fetus begins storing its own iron, calcium, and phosphorus. The bones are fully developed but still soft and flexible. There are rhythmic breathing movements present. The fetal body temperature is partially self-controlled. There is increased central nervous system control over body functions.
• At the week of 33″1 to 36th (16 to 19 inches) the body hair begins to disappear. A baby born at 36th weeks has a high chance of survival.
• At 381^ week (19 to 21 inches) the fetus is considered full term. It fills the entire uterus, and its head is the same size around as its shoulders. The mother supplies the fetus with the antibodies which need to protect it against disease.