Stages of Pregnancy - Essay Example

Running head: STAGES OF PREGNANCY. Stages of Pregnancy: Development of the Baby Institution: Abstract Conception begins with the fertilization of the female gamete, the ovum, by the male gamete, the sperm, in the fallopian tube. The fertilized ovum is called the conceptus. The fusion of the genetic material of the ovum and the sperm forms the zygote, which moves down into the uterus. Cleavage of the zygote leads to the formation of the blastocyst, which implants itself in the endometrium. Cell differentiation proceeds, with the blastocyst developing into the embryo and the placenta. Pregnancy is divided into three stages of three months each, called the three trimesters. The first trimester is crucial for the development of the various organs, tissues and muscles. The second trimester is a period of rapid growth. In the third trimester, there is fat accumulation and consolidation of growth and development. The full term fetus is born at the end of the third trimester. Stages of Pregnancy: Development of the Baby. The miracle of life begins with the momentous encounter between the male and female gametes, each with half the chromosomal content of a normal human cell. The mature ovum or egg is released by the ovary in the female reproductive system, while the sperm is deposited in the vagina by the male during intercourse. Millions of tadpole-like sperm, with heads and tails, swim to the fallopian tube, where the ovum waits, facilitated by the secretion of a white, slippery mucous lining by the uterus. One sperm penetrates the thick outer covering, or the zona pellucida, of the ovum with its’ head, by the secretion of an enzyme acrosin. Immediately, a chemical reaction makes the zona pellucida impenetrable to other sperm. The tail of the successful sperm detaches, leaving only the head within the ovum. This ovum with the sperm head, or the fertilized egg, is called the conceptus. Conception, as a rule, occurs in the outer third of the fallopian tube. The genetic material of the ovum and the sperm fuses, restoring the chromosomal number to 46, creating a single-celled zygote. The ovum has only the X chromosome, while the sperm carries either the X or the Y chromosome. If the ovum is fertilized by an X chromosome carrying sperm, the resulting baby will be a female; if the ovum is fertilized by a Y chromosome carrying sperm, a baby boy will be the result. The zygote travels down the fallopian tube and moves into the uterus, where it floats. Within two days of zygote formation, cleavage of the zygote begins, in which it divides into two cells or blastomeres. As there is no synthesis of cytoplasm, the divided cell volume decreases. Further cell division of the zygote leads to the formation of a sixteen celled solid ball of cells, called the morulla, with a mulberry-like appearance, which is 1/1000 inch wide. The cells in the centre divide more slowly than those on the periphery, which are flattened against the zona pellucida. The compact morulla of day 3 – 4 develops a central cavity called the blastocoel, which is filled with fluid, and changes into the blastocyst by day 5. The blastocyst has an inner cell mass called the embryoplast, which will form the embryo and an outer shell of cells, called the trophoplast, which will become the placenta. Uterine secretions break down the zona pellucida (Hill, 2006). The uterine wall forms a dense, spongy endometrium, ready to receive the blastocyst. The implantation of the blastocyst occurs on day 6, when the trophoplastic cells adhere to the endometrium and the mouth of the cervix is plugged with mucous. Abnormal implantation in the fallopian tube or outside the uterus results in an ectopic or tubal pregnancy, which is not viable. The blastocyst receives nourishment from the mothers’ bloodstream. Earlier, it derived its’ nutrients from the glucose secreted by the fallopian tube and the uterine walls. The embryo secretes the human gonadotrophic hormone, which halts the mothers’ menstrual cycle. During the third week of development, implantation is complete. The embryo is the size of a pinhead, roughly 0.0006 inches. The embryoplast differentiates into the inner endoderm, middle mesoderm and outer ectoderm. This is called gastrulation. The endoderm will develop into the baby’s lungs, liver and digestive organs, the mesoderm into the bones, muscles, kidneys, sex organs and heart and the ectoderm into the baby’s tissues, skin, hair, eye and nervous system. The trophoplast divides into two layers: the outer layer or chorion, develops finger-like villi which penetrate the endometrium to secure nourishment. The inner layer or amnion, develops into the amniotic sac, filled with the amniotic fluid which facilitates fetal movement as well as acting as an effective shock absorber and regulator of temperature (birth.com.au. 2004). The embryonic stage begins on the fifteenth day after conception and continues up to the eighth week, after which it enters the fetal stage which lasts till the birth of the baby. The stages of pregnancy are divided into the first, second and third trimesters, each denoting a three-month period. It is during the crucial first trimester that there is a rapid multiplication of cells, accompanied by cell differentiation, in which the cells are specialized in structure and function. It is now that the embryo is most vulnerable to the adverse effects of teratogens: external stimuli like alchohol and specific drugs that cause birth defects, viral infections such as rubella, radiation from X rays or radiation therapy and nutritional deficiencies which can stunt development (Rauch, 2006). On the eighteenth day after conception, the ectoderm, which is in the form of a flat disc at the back of the embryo, folds to form the neural tube, which will later hold the spinal chord. The upper end of the tube bulges to form the beginnings of the brain. By the twenty-first day, the edges of the tube fuse. The endoderm simultaneously enlarges to form a body stalk and a yolk sac, which initially provides nourishment and later develops into the lungs, liver and digestive system. The yolk sac is also the site of the embryo’s individual blood production. The baby’s bloodstream remains completely separate from that of the mother throughout pregnancy, mother and child often having different blood groups (birth.com.au., 2004). Weeks 4 – 5 see the formation of tissue which will later develop into the vertebra, further development of the heart and the beginnings of a regular heartbeat, a primitive circulatory system, initial development of the ear and eye, division of the brain into five areas, along with a few cranial nerves and the start of arm and leg buds. The embryo is about ¼ inch in length. As the spinal chord grows faster, it appears to have a ‘tail’. The placenta and the umbilical chord develop from the body stalk and serve as the baby’s lifeline to the mother, bringing oxygen and nutrients from the mother and removing waste products and carbon dioxide. The wastes are filtered by the placenta (Rauch,2006. birth.com.au., 2004). The development of the lungs begins in week 6, along with further development of the brain. The limb buds lengthen and the hand and feet areas are distinct, with webbed digits. The tail recedes and the development of the heart is complete, with 120 – 180 beats per minute. The embryo is ½ inch long and weighs 1/1000 of an ounce. The lenses of the eye and the nostrils make an appearance and the pancreas start secreting digestive enzymes. The intestines, initially located outside the embryo’s body, inside the umbilical chord, begin to grow ( Rauch, 2006). In week 7, all essential organs begin development, the nipples and hair follicles form and teeth buds begin growth under the gums. The elbows and toes are now visible and spontaneous movement begins as the hands may be flexed at the elbows. The appendix, inner ear and tongue form and the upper jaw and palate fuse. If this fails to occur, a cleft palate will be the result. A fine, translucent layer of skin covers the embryo. The eighth week marks the end of the embryonic period. The rotation of the intestines, which move out of the umbilical chord into the abdomen, continued development of the facial features and eyelids and the shape of the external ear are all seen now. The cheek, mouth, lips and chin are defined, along with the nasal passage. There are immature taste buds on the tongue, tiny blood vessels under the skin and five ridges each on the hands and feet. The embryo now weighs 0.04 ounces and measures 0.61 inches. The fetal period begins with the ninth week, during which the baby measures about 1 inch and weighs 0.07 ounces. Sex organs begin to form, fingers and toes are now separate digits, the lungs show bronchial development, muscle tissue grows along with elbows, knees, ankles and wrists. The anus is also formed. The fetus moves and can curve its’ fingers. With this, the critical development of the baby is more or less complete and the following stages see more of rapid growth. In week 10, the baby is about 1.22 inches long and weighs about 0.14 ounces. The eyelids fuse and the iris, with a determined eye color, forms. Primitive reflexes are seen and muscles develop, along with the ribs. The baby almost doubles in growth in the next week, weighing about 0.28 ounces and measuring 2 inches. The taste buds mature, the sucking and swallowing instincts develop and the kidney begins functioning, secreting amniotic fluid into the bladder, which excretes the fluid. The fetus breathes amniotic fluid into the lungs, strengthening the diaphragm. The stomach secretes gastric juice and the gums hold about 20 teeth. Movement is now regular and coordinated, being essential for growth and development. The sex organs develop. Week 12 marks the end of the first trimester, with the baby’s weight at 1 ounce and length at 3.04 inches. Peristaltic movements of the bowel begin and meconium (the first bowel movement after birth) begins to accumulate. The bones begin to harden: ossification. Fine hair is visible on the upper lip and eyebrows and the placenta is fully functional. The vocal chords form and the liver functions. The second trimester begins with week 13, when the fetus measures 3 inches and weighs 1.75 ounces. The nasal passage is fully developed and the gag reflex is seen. The neck is longer. Week 14 sees the growth fine hair called lanugo all over the body as a protective layer. The fetus is now 1.52 ounces and 3.42 inches. The thyroid gland begins hormone production. Movement of the limbs increases in the 15th week, finger and toe nails grow and the baby’s dimensions are 3.98 inches and 2.47 ounces. The grasping reflex and thumb sucking may be seen. Fat development under the skin is seen in week 16. Heart beats can be heard with an external monitor and the baby’s weight is approximately 3.53 ounces and length 4.57 inches, increasing to 4.97 ounces and 5.12 inches at week 17. A human appearance is now seen. Week 18 sees the formation of the greasy vernix over the skin as a protective layer. Alvioli are formed in the lungs. The fetus measures 5.59 inches and weighs 6.7 ounces. The umbilical chord is fully mature and is covered with the protective, slippery ‘Wharton’s jelly.’ The fetus exhibits regular patterns of rest and activity in the 19th week and is now 6.02 inches long and weighs 8.47 ounces. The myelin sheath of the nerve cells forms and the male genitals or the uterus develop and hair growth on the scalp begins. The rapid growth stage is over by week 20. The fetus is 6.46 inches and 10.58 ounces, receives antibodies from the mother for immunity and develops senses of taste, smell, hearing, sight and touch. The startle reflex is seen. By week 21 - 24, mothers can feel the ‘quickening’ movements of the fetus in the womb. The liver functions, sweat glands appear and the lungs secrete a greasy substance called surfactant to facilitate inflation. The testes descend to the scrotum and primitive sperm develop. Dimensions are about 11.2 inches and 1 lb. 10 ounces. Eyelashes and eyebrows are distinct. This is the end of the second trimester. In the third trimester, or weeks 25- 36, there is rapid accumulation of fat and development of the respiratory and nervous system is completed. Eyelids open and close and eyes follow light sources. The fetus responds to the mother’s voice. The bone marrow takes over the production of red blood cells and iron, calcium and phosphorous are stored in the body. Weight reaches about 5 – 6 lbs. and length 16 – 19 inches. Lanugo and vernix disappear and breast buds appear. The fetus is ‘full term’ at week 37. It settles into the pelvis. The amniotic sac breaks and the fluid lubricates the birth canal. During labor, the mother’s hormones stimulate the fetus’ bloodstream to absorb the amniotic fluid in the lungs. And now, the cervix dilates and the uterus contracts pushing out the perfect miracle of a fully formed baby (Rauch, 2006. birth.com.au., 2004. Snyder, 2005). References. birth.com.au: Pregnancy week by week: pregnancy calendar, 2004. Retrieved 15 November, 2006 from http://www.birth.com.au/classmenu.asp Hill, M. UNSW Embryology – Cardiovascular System Development, 2006. Retrieved 14 November, 2006 from University of New South Wales Web site http://embryology.med.unsw.edu.au/wwwhuman/stages/stages.htm Rauch, D. MedlinePlus Medical Encyclopedia: Fetal development, 5 October, 2006 Retrieved on 15 November, 2006 from http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm Snyder, J. Pregnancy.org: Fetal development, 2005. Retrieved 14 November, 2006 from http://www.pregnancy.org/pregnancy/fetaldevelopment1.php Read More