Civil Services Blog: Human Growth and Development

Prenatal development refers to the process in which a baby develops from a single cell after conception into an embryo and later a fetus.The process of prenatal development occurs in three main stages. They are germinal stage, the embryonic stage, and the fetal stage.The first two weeks after conception are known as the germinal stage,the third through the eighth week are known as the embryonic period, and the time from the ninth week until birth is known as the fetal period.The average length of time for prenatal development to complete is 38 weeks from the date of conception.

Three Stages in Prenatal Development :

Germinal stage

Conception occurs when the female egg (ovum) is fertilized by a male sperm. Under normal circumstances, one egg is released approximately once a month from a woman's ovary during a process called ovulation. The egg makes its way into a fallopian tube, a structure that guides the egg away from the ovary toward the uterus. For fertilization to occur, sperm ejaculated during sexual intercourse (or introduced during artificial insemination) in a substance called semen must have made their way from the vagina into the uterus and subsequently into the fallopian tube where the ovum has been released. This process can take up to ten hours after ejaculation. For fertilization to occur, a sperm must penetrate the tough outer membrane of the egg called the zona pellucida. When one sperm successfully binds with the zona pellucida, a series of chemical reactions occurs to allow only that sperm to penetrate. Fertilization occurs when the sperm successfully enters the ovum's membrane. The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of prenatal development commences.

The zygote soon begins to divide rapidly in a process called cleavage, first into two identical cells called blastomeres, which further divide to four cells, then into eight, and so on. The group of diving cells begins to move along the fallopian tube toward the uterus. About sixty hours after fertilization, approximately sixteen cells have formed to what is called a morula, still enclosed by the zona pellucida.Three days after fertilization, the morula enters the uterus. As cell division continues, a fluid-filled cavity called a blastocoele forms in the center of the group of cells, with the outer shell of cells called trophoblasts and an inner mass of cells called embryoblasts. The zona pellucida disappears and the morula becomes a blastocyst. At this stage the blastocyst consists of 200 to 300 cells and is ready for implantation. The blastocyst is made up of three layers. The three layers also known as germ layers are formed by a process called gastrulation.

Those three layers are :

The ectoderm (which will become the skin and nervous system)
The endoderm (which will become the digestive and respiratory systems)
The mesoderm (which will become the muscle and skeletal systems)

Finally, the blastocyst arrives at the uterus and attaches to the uterine wall, a process known as implantation.

Implantation, the process in which the blastocyst implants into the uterine wall, occurs approximately six days after conception. Hormones secreted from the mother's ovaries and a chemical secreted by the trophoblasts begin to prepare the uterine wall. The blastocyst first adheres to the wall then moves into the uterine tissue.Implantation is not always an automatic and sure-fire process. Researchers estimate that approximately 58 percent of all natural conceptions never become properly implanted in the uterus, which results in the new life ending before the mother is ever aware she is pregnant.When implantation is successful, hormonal changes halt a woman’s normal menstrual cycle and cause a whole host of physical changes. Implantation marks the end of the germinal stage and the beginning of the embryonic stage.

Embryonic stage

The embryonic stage begins after implantation and lasts until eight weeks after conception. Soon after implantation, the cells continue to rapidly divide and clusters of cells begin to take on different functions (called differentiation). A process (gastrulation) leads to the formation of three distinct layers called germ layers: the ectoderm (outer layer), the mesoderm (middle layer), and the endoderm (inner layer). As the embryo develops, each germ layer differentiates into different tissues and structures. For example, the ectoderm eventually forms skin, nails, hair, brain, nervous tissue and cells, nose, sinuses, mouth, anus, tooth enamel, and other tissues. The mesoderm develops into muscles, bones, heart tissue, lungs, reproductive organs, lymphatic tissue, and other tissues. The endoderm forms the lining of lungs, bladder, digestive tract, tongue, tonsils, and other organs.The process of differentiation takes place over a period of weeks with different structures forming simultaneously.

Some of the major events that occur during the embryonic stage are as follows:

-Week 3: Beginning development of the brain, heart, blood cells, circulatory system, spinal cord, and digestive system.
-Week 4: Beginning development of bones, facial structures, and limbs (presence of arm and leg buds); continuing development of the heart (which begins to beat), brain, and nervous tissue.
-Week 5: Beginning development of eyes, nose, kidneys, lungs; continuing development of the heart (formation of valves), brain, nervous tissue, and digestive tract.
-Week 6: Beginning development of hands, feet, and digits; continuing development of brain, heart, and circulation system.
-Week 7: Beginning development of hair follicles, nipples, eyelids, and sex organs (testes or ovaries); first formation of urine in the kidneys and first evidence of brain waves.
-Week 8: Facial features more distinct, internal organs well developed, the brain can signal for muscles to move, heart development ends, external sex organs begin to form.

By the end of the embryonic stage, all essential external and internal structures have been formed. The embryo is now referred to as a fetus.

Fetal stage

This period of develop begins during the ninth week and lasts until birth. Although all of the organ systems were formed during embryonic development, they continue to develop and grow during the fetal stage.The fetus continues to grow in both weight and length.This stage of prenatal development lasts the longest and is marked by amazing change and growth. During the third month of gestation, the sex organs begin to differentiate and by the end of the month all parts of the body will be formed.

When an embryo becomes a fetus at eight weeks, it is approximately 3 centimeters (1.2 inches) in length from crown to rump and weighs about 3 grams. By the time the fetus is considered full-term at 38 weeks gestation, he or she may be 50 centimeters (20 inches) or 3.3 kilograms .

Examples of some of the major features of fetal development by week are as follows:

-Weeks 9–12: The fetus reaches approximately 8 cm in length; the head is approximately half the size of the fetus. External features such as the face, neck, eyelids, limbs, digits, and genitals are well formed. The beginnings of teeth appear, and red blood cells begin to be produced in the liver. The fetus is able to make a fist.
-Weeks 13–15: The fetus reaches approximately 15 cm in length. Fine hair called lanugo first develops on the head; structures such as the lungs, sweat glands, muscles, and bones continue to develop. The fetus is able to swallow and make sucking motions.
-Weeks 16–20: The fetus reaches approximately 20 cm in length. Lanugo begins to cover all skin surfaces, and fat begins to develop under the skin. Features such as finger and toenails, eyebrows, and eyelashes appear. The fetus becomes more active, and the mother can sometimes begin to feel fetal movements at this stage.
-Weeks 21–24: The fetus reaches approximately 28.5 cm in length and weighs approximately 0.7 kg . Hair grows longer on the head, and the eyebrows and eye lashes finish forming. The lungs continue to develop with the formation of air sac (alveoli); the eyes finish developing. A startle reflex develops at this time.
-Weeks 25–28: The fetus reaches approximately 38 cm in length and weighs approximately 1.2 kg . The next few weeks mark a period of rapid brain and nervous system development. The fetus gains greater control over movements such as opening and closing eyelids and certain body functions. The lungs have developed sufficiently that air breathing is possible.
-Weeks 29–32: The fetus reaches approximately 38–43 cm in length and weighs approximately 2 kg . Fat deposits become more pronounced under the skin. The lungs remain immature but breathing movements begin. The fetus's bones are developed but not yet hardened.
-Weeks 33–36: The fetus reaches approximately 41–48 cm in length and weighs 2.6–3.0 kg . Body fat continues to increase, lanugo begins to disappear, and fingernails are fully grown. The fetus has gained a high degree of control over body functions.
-Weeks 36–38: The fetus reaches 48–53 cm in length is considered to be full-term by the end of this period. Lanugo has mostly disappeared and is replaced with thicker hair on the head. Fingernails have grown past the tips of the fingers. In a healthy fetus, all organ systems are functioning.

While prenatal development usually follows this normal pattern, there are times when problems or deviations occur . (Details --> Click Here)

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Prenatal Development - Videos

Fertilization (Conception)