Fetal Development PDF

1/16/24, 10:08 PM Realizeit for Student Introduction Human reproduction is one of the most intimate spheres of an individual’s life. Pregnancy is a dynamic and precisely coordinated process involving systemic and local changes that will support the fetus for growth in utero and in subsequent lacta...

1/16/24, 10:08 PM Realizeit for Student Introduction Human reproduction is one of the most intimate spheres of an individual’s life. Pregnancy is a dynamic and precisely coordinated process involving systemic and local changes that will support the fetus for growth in utero and in subsequent lactation. For conception to occur, a healthy ovum from the woman is released from the ovary, passes into an open fallopian tube, and starts its journey downward. Sperm from the male is deposited into the vagina and swims approximately 7 in to meet the ovum at the outermost portion of the fallopian tube, the area where fertilization takes place. This process occurs in about an hour (Webster et al., 2018). When one spermatozoon penetrates the ovum’s thick outer membrane, pregnancy begins. All this activity takes place within a 5-hour time span. Nurses caring for the childbearing family need to have a basic understanding of conception and prenatal development so they can identify problems or variations and can initiate appropriate interventions should any problems occur. This chapter presents an overview of fetal development, beginning with conception. It also discusses hereditary influences on fetal development and the nurse’s role in genetic counseling. Fetal Development Fetal development during pregnancy is measured in number of weeks after fertilization. The duration of pregnancy is about 40 weeks from the time of fertilization. This equates to nine calendar months or approximately 266 to 280 calendar days. The stages of human development during pregnancy are: 1. Zygotic stage: fertilization of sperm and egg through the second week. 2. Blastocyst stage: Zygote divides into a solid ball of cells which attaches to the uterus. 3. Embryonic stage: Major organs and structures begin to emerge by end of the second week through the eighth week. 4. Fetal stage: Differentiation and structures specialize by end of the eighth week until birth. Fetal circulation is a significant aspect of fetal development that spans all three stages. Zygotic Stage The zygotic stage begins with fertilization, also called conception. Fertilization is the fusing of ovum and sperm, which is the starting point of pregnancy. Fertilization typically occurs around 2 weeks after the last normal menstrual period in a 28-day cycle (Jordan et al., 2019). Fertilization requires a timely interaction between the release of the mature ovum at ovulation and the ejaculation of enough healthy, mobile sperm to survive the hostile vaginal environment through which they must travel to meet the ovum. All things considered; the act of conception is difficult at best. To say merely that it occurs when the sperm unites with the ovum is overly simple because this union requires an intricate interplay of hormonal preparation and overcoming an overwhelming number of natural barriers. A human being is truly an amazing outcome of this elaborate process. Fertilization takes place in the outer third of the ampulla of the fallopian tube. When the ovum is fertilized by the sperm (now called a zygote), a great deal of activity immediately takes place. Mitosis, or cleavage, occurs as the zygote is slowly transported into the uterine cavity by tubal muscular movements (Fig. 10.3). After a series of four cleavages, the 16 cells appear as a solid ball of cells, or morula, meaning “little mulberry.” The morula reaches the uterine cavity about 72 hours after fertilization. As fluid, which provides nutrients, from the uterine cavity enters the morula, the blastocyst is formed (Blackburn, 2018). Blastocyst Stage With additional cell division, the morula divides into specialized cells that will later form fetal structures. Within the morula, an off-center, fluid-filled space appears, transforming it into a hollow ball of cells called a blastocyst (Fig. 10.4). The inner surface of the blastocyst will form the embryo and amnion. The outer layer of cells surrounding the blastocyst cavity is called a trophoblast. Eventually, the trophoblast develops into one of the embryonic membranes, the chorion, and helps to form the placenta. FIGURE 10.3 Mitosis of the stoma cells. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zTYaQRcVeGjlcsXn1BnhV1BdrBMb3lqiFIhL1oNdUK7j0NCYTE… 1/5 1/16/24, 10:08 PM Realizeit for Student FIGURE 10.4 A. Fertilized human egg (zygote) having reached the blastocyst stage. A zygote contains 20 to 30 eggs and a fluid-filled blastocele is beginning to form. B. Implantation. Stylized image showing a frontal view of a uterus with a blastocyst about to implant into endometrium of the uterus. (Images from LifeART image copyright (c) 2011 Lippincott Williams & Wilkins. All rights reserved.) At this time, the developing blastocyst needs more food and oxygen to keep growing. The trophoblast attaches itself to the surface of the endometrium for further nourishment. Normally, implantation occurs in the upper uterus (fundus), where a rich blood supply is available. This area also contains strong muscular fibers, which clamp down on blood vessels after the placenta separates from the inner wall of the uterus. Additionally, the lining is thickest here so the placenta cannot attach so strongly that it remains attached after birth. The process of attachment and placental formation is termed implantation. This involves a complex interaction between the trophoblasts and the maternal decidua. From a medical perspective, a pregnancy has not occurred until successful implantation has taken place (Cunningham et al., 2018). Figure 10.5 shows the process of fertilization and implantation. FIGURE 10.5 Fertilization and tubal transport of the zygote. From fertilization to implantation, the zygote travels through the fallopian tube, experiencing rapid mitotic division (cleavage). During the journey toward the uterus the zygote evolves through several stages, including morula and blastocyst. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zTYaQRcVeGjlcsXn1BnhV1BdrBMb3lqiFIhL1oNdUK7j0NCYTE… 2/5 1/16/24, 10:08 PM Realizeit for Student Embryonic development starts with the fusion of oocyte and sperm cell. This single cell eventually differentiates into three germ layers. Concurrent with the development of the trophoblast and implantation, further differentiation of the inner cell mass occurs. Some of the cells become the embryo itself, and others give rise to the membranes that surround and protect it. The three embryonic layers of cells formed are: 1. Ectoderm—forms the central nervous system, special senses, skin, and glands. 2. Mesoderm—forms the skeletal, urinary, circulatory, and reproductive organs. 3. Endoderm—forms the respiratory system, liver, pancreas, and digestive system. These three layers are formed at the same time as the embryonic membranes, and all tissues, organs, and organ systems develop from these three primary germ cell layers (Chiras, 2019). Box 10.1 summarizes preembryonic development. Despite the intense and dramatic activities going on internally to create a human life, many women are unaware that pregnancy has begun. Several weeks will pass before even one of the presumptive signs of pregnancy—missing the first menstrual period—will take place. Embryonic Stage The embryonic stage of development begins at day 15 after conception and continues through week 8. Basic structures of all major body organs and the main external features are completed during this time period. Box 10.1 summarizes embryonic development. BOX 10.1 SUMMARY OF PRE-EMBRYONIC DEVELOPMENT Fertilization takes place in ampulla of the fallopian tube. Union of sperm and ovum forms a zygote (46 chromosomes). Cleavage cell division continues to form a morula (mass of 16 cells). The inner cell mass is called blastocyst, which forms the embryo and amnion. The outer cell mass is called trophoblast, which forms the placenta and chorion. https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zTYaQRcVeGjlcsXn1BnhV1BdrBMb3lqiFIhL1oNdUK7j0NCYTE… 3/5 1/16/24, 10:08 PM Realizeit for Student Implantation occurs 7 to 10 days after conception in the endometrium. Amniotic fluid surrounds the embryo and increases in volume as the pregnancy progresses, reaching approximately 1 L at term. Amniotic fluid is derived from two sources: fluid transported from the maternal blood across the amnion and fetal urine. Its volume changes constantly as the fetus swallows and voids. Sufficient amounts of amniotic fluid help maintain a constant body temperature for the fetus, permit symmetric growth and development, cushion the fetus from trauma, allow the umbilical cord to be relatively free from compression, and promote fetal movement to enhance musculoskeletal development. Amniotic fluid is composed of 98% water and 2% organic matter. It is slightly alkaline and contains albumin, urea, bile pigments, renin, glucose, hormones, uric acid, creatinine, bilirubin, lecithin, sphingomyelin, epithelial cells, vernix, and fine hair called lanugo. The composition of amniotic fluid changes with gestation. Adequate amniotic fluid volume is necessary for proper fetal growth and development (Resnik et al., 2019). The volume of amniotic fluid is important in determining fetal well-being. It gradually fluctuates throughout the pregnancy. Alterations in amniotic fluid volume can be associated with problems in the fetus. Too little amniotic fluid (<500 mL at term), termed oligohydramnios, is associated with uteroplacental insufficiency, fetal renal abnormalities, and a higher risk of surgical births and low–birth-weight infants. Too much amniotic fluid (>2,000 mL at term), termed hydramnios, is associated with maternal diabetes, neural tube defects, chromosomal deviations, and malformations of the central nervous system and/or gastrointestinal tract that prevent normal swallowing of amniotic fluid by the fetus. Hydramnios may threaten premature rupture of membranes due to uterine over distention (Scott & Lee, 2020). The structure of the placenta is usually completed by week 12. The placenta is not only a transfer organ but a hormone factory as well. Placental hormones have profound effects on maternal metabolism, initially building up her energy reserves and then releasing these to support fetal growth in later pregnancy and lactation postnatally. Several hormones are produced that are necessary for normal pregnancy: Chorionic gonadotropin (CG)—preserves the corpus luteum and its progesterone production so that the endometrial lining of the uterus is maintained; this is the basis for pregnancy tests. Prolactin—mediates maternal metabolic adaptations to pregnancy by regulating insulin production and sensitivity; and plays an important role in lactation. Human placental lactogen (hPL)—modulates fetal and maternal metabolism, participates in the development of maternal breasts for lactation, and decreases maternal insulin sensitivity to increase its availability for fetal nutrition. Estrogen (estriol)—causes enlargement of a woman’s breasts, uterus, and external genitalia; stimulates myometrial contractility. Progesterone (progestin)—maintains the endometrium, decreases the contractility of the uterus, stimulates maternal metabolism and breast development, provides nourishment for the early conceptus. Relaxin—is a potent vasodilator and regulates maternal hemodynamics. It acts synergistically with progesterone to maintain pregnancy, causes relaxation of the pelvic ligaments, softens the cervix in preparation for birth (Roberts & Myatt, 2019). The placenta acts as a pass-through between the mother and fetus, not a barrier. Almost everything the mother ingests (food, alcohol, drugs) passes through to the developing conceptus. This is why it is so important to advise pregnant women not to use unprescribed drugs, alcohol, and tobacco, because they can be harmful to the conceptus. During the embryonic stage, the conceptus grows rapidly as all organs and structures are forming. During this critical period of differentiation, the growing embryo is most susceptible to damage from external sources, including teratogens (substances that cause birth defects, such as alcohol and drugs), infections (such as rubella or cytomegalovirus), radiation, and nutritional deficiencies. Fetal Stage The average pregnancy lasts 280 days from the first day of the last menstrual period. The fetal stage is the time from the end of the eighth week until birth. It is the longest period of prenatal development. During this stage, the conceptus is mature enough to be called a fetus. Although all major systems are present in their basic form, dramatic growth and refinement of all organ systems take place during the fetal period (see Table 10.1). Figure 10.8 depicts a 12- to 15-week-old fetus. Fetal Circulation Fetal circulation differs from adult circulation due to the presence of certain vessels and shunts which support the unique physiologic needs of the developing fetus. These shunts will close after birth and most of the vessels will be seen as remnants in the adult circulation. The function of these shunts is to direct oxygen-rich venous blood to the systemic circulation and to ensure that oxygen-depleted venous blood bypasses the underdeveloped pulmonary circulation. The lungs finish their development after birth. Prior to birth, the lungs function is taken over by the placenta during fetal life (Blackburn, 2018). The circulation through the fetus during uterine life differs from that of a child or an adult. TABLE 10.1 EMBRYONIC AND FETAL DEVELOPMENT https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zTYaQRcVeGjlcsXn1BnhV1BdrBMb3lqiFIhL1oNdUK7j0NCYTE… 4/5 1/16/24, 10:08 PM Realizeit for Student FIGURE 10.8 Fetal development: 12- to 15-week fetus https://herzing.realizeithome.com/RealizeitApp/Student.aspx?Token=lqf9HhURQ5RqpgqAkzH2zTYaQRcVeGjlcsXn1BnhV1BdrBMb3lqiFIhL1oNdUK7j0NCYTE… 5/5