Growth and Development

Growth and Development

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Growth and Development...

Growth and Development:

Many physiological processes last only a fraction of a second, a heartbeat, respiration, nervous stimulation etc. But some important processes in life are measured in months, years or decades. A human being develops in the womb for nine months, grows to maturity in 15 -20 years and may live for the better part of a century.

Birth, growth, maturation, aging and death are all parts of a single, continuous process. This process does not end with the individual, for human beings can pass at least some of their characteristics on to a new generation that will repeat the same cycle.

The process of development is the gradual modification of physical and physiological characteristics during the period from conception to physical maturity. These changes are truly remarkable-----what begins as a single cell slightly larger than the period at the end of this sentence becomes an individual.

Let's review:

Fertilization involves the joining of two gametes or sex cells to form a zygote containing the normal number of chromosomes (46).

Fertilization usually occurs in the upper one third of the uterine tube within a day of ovulation. Of approximately 200 million spermatozoa introduced into the vagina around 10,000 make it past the uterus and fewer than 100 actually reach the ovum for attempts at fertilization. This is why a man with a sperm count of 20 million per milliliter will usually be sterile because too few sperm survive to reach the ovum.

Just think of how amazing it is that at the time of fertilization, we have determined the sex of the soon to be embryo, body build and the skin color. During the 10 days from fertilization to implantation within the uterine lining, there are no nutrients available from the mother. This implanted embryo can also be known as the blastocyst. This blastocyst will form the amniotic cavity and the chorion of the placenta.

The amniotic cavity is the fluid filled chamber that the fetus develops within. It enlarges and continues to expand. The fluid within the chamber cushions the developing fetus and embryo.

The chorion is the rapid transit blood vessel system that links the embryo with the nutrients and oxygen needed for growth and development. The placenta appears via ultrasound by the third week of development. Finger like projections called chorionic villi continue to enlarge and become more intricate until the maternal and embryonic circulation occurs by diffusion through this blood system network.

As you are aware there are many things that can be passed on from the mother to the developing fetus due to this uniquely shared circulatory system and not all of them are good for the development of the fetus. While the maternal and fetal bloods do not mix, the very thin layer of tissue that separates the two circulatory systems may offer some protection, while certain toxic substances may penetrate the barrier-causing harm to the fetus.

Can you think of some of these toxic substances or infectious organisms?

PERIODS OF DEVELOPMENT

The time spent in prenatal development is known as gestation. This is anywhere from 39- 40 weeks and for convenience is usually divided into three integrated trimesters:

The first trimester is the period of embryonic and early feral development. During this period, the basic components of all the major organ systems appear.

The second trimester, the organs and organs systems complete most of their development. The body proportions change, and by the end of the second trimester the fetus looks distinctly human.

The third trimester is characterized by rapid fetal growth. Early in the third trimester most of the major organ systems become fully functional, and an infant born even 2 months prematurely has a reasonable chance for survival.

FIRST TRIMESTER

The events that occur in the first trimester are complex and vital to the survival of the embryo. Because accidents often happen, the first trimester is the most dangerous period in life. Only about 40 percent of conceptions produce embryos that survive the first trimester, and some surviving fetuses enter the second trimester already doomed or deformed by some developmental mistake.

In the first trimester, the process of a series of cell divisions is initiated along with the implantation and formation of the placenta. Embryogenesis also takes place during this term. This is where there is formation of a viable embryo. The first trimester is a critical period for development because events in the first trimester establish the basis for organ formation and developmental milestones.

SECOND TRIMESTER

By the start of the second trimester, the beginnings of all the major organ systems have formed. Over the next three months, the fetus will grow to over 1 pound, fetus starts to move and there is rapid expansion of the cerebrum.

THIRD TRIMESTER

The basic components of all the organ systems appear and most become ready to fulfill their normal functions. The maternal weight gain in this semester is usually the largest of all the semesters. In this semester the testes start to descend and the eyelids are open and sensitive to light.

The endocrine system kicks into high gear and the estrogen levels in the maternal body start to rise, which increases the sensitivity of the uterine smooth muscles and make contractions more likely. The rising levels make the uterus more sensitive to stimulation. There is also rising oxytocin levels stimulate an increase in the force and frequency of uterine contractions. Last but not least there is a prostaglandin production, which stimulates smooth muscle uterine contractions.

After nine months of gestation, multiple factors interact to produce labor contractions in the muscle of the uterine wall. The goal of labor is the forcible expulsion of the fetus, a process known as parturition or birth.

STAGES OF LABOR

STAGE ONE- the dilation stage or the period from onset of contractions until dilation of the cervix is complete. This stage may last 8 or more hours. Many times late in this stage the aminiotic membranes may rupture of the water may break.

STAGE TWO- the expulsion stage or the period from the time of maximum cervical dilation until the baby exits through the vagina. This may last less than two hours. The arrival of the newborn to the outside world represents the birth or the delivery. This may be where making an incision or episiotomy to assist with the delivery temporarily enlarges the perineum. Also in this stage if there is failure to progress or CPD, which is cephalopelvic disproportion, a C-section may be performed. There are many other reasons, which may represent the need for a C-section.

STAGE THREE- the placental stage or the process of the expulsion of the placenta through the vagina. Muscle tension builds in the walls of the uterus as it is decreasing in size after the delivery of the newborn. This uterine contraction tears the connection between the lining of the uterus and the placenta. The loss of the placenta may be accompanied by a potential loss of blood of 500-600 ml.

MULTIPLE BIRTHS

The term multiple births refers to the birth of two or more infants from the same pregnancy. Multiple birth babies are often born prematurely and therefore can be at greater risk of complications in infancy.

Twinning or double births can result from two different processes:

Identical twins- result from the splitting of embryonic tissue from the same zygote in early development. They have the same genetic code. Despite this, identical twins may not be identical in terms of structures and functions.

Fraternal twins- result from the fertilization of two different ova by two different spermatazoa. This is more common in response to fertility drugs. Because 2 separate fertilizations must occur, it is even possible for fraternal twins to have different biological fathers.

DISORDERS OF PREGNANCY

IMPLANTATION DISORDERS- proper implantation does not always occur and many offspring are lost before implantation occurs and most for unknown reasons. Implantation outside of the uterus can result in an ectopic pregnancy. This may be an implantation in a region of endometriosis or peritoneal membrane. A tubular pregnancy is considered potentially life threatening, as the fallopian tube cannot stretch to accommodate the growing fetus.