The Growing Fetus
n The fetus is the infant during intrauterine life.
n Nursing Process:
n Assess - predictable stages of development that are used for guidelines for the expected date of birth.
n Plan - goals and outcome criteria
n Implement - teaching parents about fetal growth and development at their level
n Evaluation - if changes are made
Stages of Fetal Development
n In 38 weeks a fertilized egg matures from a single cell carrying all necessary genetic material to fully developed fetus ready to be born.
n Terms:
n Ovum - from ovulation to fertilization
n Zygote - from fertilization to implantation
n Embryo - from implantation to 5-8 weeks
n Fetus - from 5-8 weeks until term
n Conceptus - developing embryo or fetus and placental structures throughout pregnancy
n Fertilization: The beginning of pregnancy.
n Fertilization is the union of the ovum and a spermatozoon.(conception, impregnation or fecundation).
n Fertilization occurs in outer 3rd of a fallopian tube, the ampullar portion.
n Fertilization has a span of 72 hours (48 hours before ovulation plus 24 hours afterwards).
n Ovum and surrounding cells are propelled into fallopian tube by cilia.
n One ovum reaches maturity each month.
n Ovum is capable of fertilization for only 24 hours (48 at the most).
n Sperm reach the ovum and clusters around the protective layer of corona cells.
n Hyaluronidase (proteolitic enzyme) is released by sperm and acts to dissolve the layer of protective cells.
n One sperm is able to penetrate the cell membrane of the ovum.
n Once it penetrates the zona pellucide the membrane becomes impervious to sperm. (Except in the formation of hydatidiform mole > abnormal growth).
n Next the chromosomal material of ovum and sperm fuse and become a zygote.
n 23 chromosomes each = 46 total
n Fertilization depends on:
n maturation of both sperm and ovum
n ability of sperm to reach the ovum
n ability of sperm to penetrate the zona pellucida and cell membrane and achieve fertilization.
n From the fertilized ovum (zygote) the future child and accessory structures needed for support during intrauterine life, such as the placenta, fetal membranes, amniotic fluid, and umbilical cord, are formed.
n Implantation:
n zygote migrates toward body of the uterus.
n takes 3 to 4 days
n during this time mitotic cell division, or cleavage, begins.
n 1st cleavage occurs at about 24 hours.
n continues at a rate of one every 22 hours.
n the zygote reaches the body of uterus with about 16 to 50 cells.
n bumpy appearance termed morula.
n Morula continues to multiply as it floats free in the uterine cavity for 3 to 4 days.
n Large cells collect leaving a fluid space surrounding an inner cell mass and is termed blastocyte.
n This attaches to the uterine endometrium.
n Trophoblast - cells in the outer ring.
n Embryoblast - inner cell mass later forms the embryo.
n Implantation occurs in 8 to 10 days after fertilization.
n The blastocyte brushes against the rich endometrium (in secretory phase of menstrual cycle) termed - apposition
n attaches to the surface of the endometrium - adhesion
n proteolytic enzymes dissolve tissues
n settles down (burrows) into soft folds - invasion
n usually high posterior side of the uterus
n if low = placenta previa
n 50% of zygotes never achieve this.
n A pregnancy ends in 8 to 10 days after fertilization, before the woman is aware of a pregnancy.
n Vaginal spotting may occur with adhesion / invasion due to rupture of capillaries by the implanting trophoblast cells.
n Once implanted the zygote is called an embryo.
Embryonic and Fetal Structures
n Decidua: (falling off)
n uterine endometrium grows in thickness and vascularity.
n Will be discarded after the birth of the child.
n 3 areas:
n decidua basalis - directly under the embryo
n decidua capsularis - stretches or encapsulates the surface of trophoblast
n decidua vera - remaining portion of lining
n Embryo grows and pushes decidua capsularis like a blanket, enlargement, contacts opposite uterine wall, fuses with endometrium.
n At birth this entire inner surface of the uterus is stripped away, leaving the organ highly susceptible to hemorrhage and infection.
n Chorionic Villi:
n Once implantation is achieved, trophoblastic layer of cells of blastocyst begins to mature rapidly.
n 11th to 12th day miniature villi reach out from trophoblast into endometrium.
n Central core of connective tissue contain fetal capillaries.
n Syncytial layer produces hormones, hCG, HPL, estrogen, progesterone.
n Placenta: (latin for pancake)
n arises out of trophoblast tissue.
n serves as fetal lungs, kidneys, GI, endocrine
n growth parallels fetal growth: 15 to 20 cm in diameter and 2 to 3 cm in depth at term.
n covers about 1/2 the surface of internal uterus
n Circulation:
n 12 th day of pregnancy maternal blood begins to collect
n by 3rd week-oxygen, nutrients, fluid diffuse from mother through chorionic villi to villi capillaries to the developing embryo.
n no direct exchange of blood between embryo and mother during pregnancy.
n exchange is by selective osmosis through the chorionic villi.
n Minute breaks do occur
n membrane is affected by maternal B/P, pH of fetal and maternal plasma.
n Cotyledon:
n in a mature placenta there are about 30 separate segments. (networked)
n 100 maternal uterine arteries supply the mature placenta.
n blood flow through the placenta is about 50 mL/min. at 10 weeks to 500 to 600 mL/min at term.
n To accommodate increased blood flow arteries increase in size.
n Mothers heart rate, total cardiac output, and blood volume increase to supply the placenta.
n Uterine perfusion:
n placental circulation is most efficient when mother lies on her left side.
n This lifts the uterus away from the inferior vena cava,preventing blood from being trapped in lower extremities.
n If the mother lies on her back the weight of the uterus on vena cava causes supine hypotension.
n At term a placenta weighs 400 to 600g 1 lb.
n A small or enlarged placenta suggests circulation to the placenta is compromised.
n Women with diabetes may develop a larger than usual placenta from fluid collected between cells.
Endocrine Function
n Human Chorionic Gonadotropin:
n 1 st hormone to be produced.
n found in maternal blood and urine shortly after implantation or first missed period for 100 days from trophoblast.
n analyzed with urine pregnancy test.
n negative within 1 to 2 weeks post delivery.
n hCG functions to keep corpus luteum producing progesterone, if this fails or progesterone falls the endometrium will slough,
n until 8th week outer layer of cells of placenta begins to produce progesterone.
n hCG suppresses maternal immunologic response to not reject the placenta.
n Estrogen:
n primarily estriol is produced as a second product of syncytial cells of placenta.
n contributes to mammary gland development in preparation for lactation.
n stimulates uterine growth to accommodate fetus.
n assessing amount of estriol in maternal serum was used to test fetal well being.
n Progesterone:
n maintains endometrial lining of the uterus during pregnancy.
n present in serum 4th week of pregnancy.
n reduces contractility of uterine muscle during pregnancy(prevents premature labor)
n Human Placental Lactogen: (HPL)
n both growth-promoting and lactogenic properties.
n produced by the placenta by 6th week of pregnancy and increases to peak at term.
n it promotes mammary gland growth in preparation for lactation in the mother.
n regulates maternal glucose, protein, and fat levels so adequate amounts are available to the fetus.
Umbilical Cord
n Formed from the amnion and chorion and provides a circulatory pathway connecting the embryo to the chorionic villi.
n Function is to transport oxygen and nutrients to the fetus from the placenta and to return waste products from the fetus to the placenta.
n 21 inches (53cm) long and 3/4 inch (2cm) thick.
n One vein - carries blood from placental villi to the fetus.
n 2 arteries - carrying blood from the fetus back to the placenta villi.
n Wharton’s jelly - gelatinous mucopolysaccharide gives cord body and prevents pressure on the vein and arteries.
n The outer surface is covered with amniotic membrane.
n Blood can be withdrawn from the umbilical vein or transfused into the vein during intrauterine life for fetal assessment or treatment.
n Rate is rapid 350 mL/min. at term.
n Blood flow (blood velocity) can be determined by ultrasound.
n The rapid rate of blood flow through the cord makes it unlikely that it will twist or knot enough to interfere with O2 supply.
n 20% of births a loose loop of cord is found around the fetal neck (nuchal cord).
n Smooth muscle is abundant in the arteries of the cord.
n Constriction of muscles after birth
n contributes to hemostasis and helps prevent hemorrhage of the newborn through the cord.
n The cord contains no nerve supply, so it can be cut at birth without discomfort to child or mother.
Membranes and Amniotic Fluid
n The chorionic villi on medial surface of the trophoblast gradually thin and leave the medial surface smooth this becomes chorionic membrane, the outer most fetal membrane - next to baby.
n Once it becomes smooth, it offers support to the sac that contains the amniotic fluid.
n The amniotic membrane (amnion) forms beneath the chorion and becomes adherent to the fetal surface of the placenta, and give that surface a typically shiny appearance.
n no nerve supply: no pain when it ruptures.
n Amniotic membrane acts to support and produce amniotic fluid.
n It produces a phospholipid that initiates the formation of prostaglandins which cause uterine contractions and maybe the trigger to initiate labor.
n amniotic fluid is constantly being newly formed and reabsorbed, so it is never stagnant within the membranes.
n Fetus continually swallows the fluid, it is absorbed across the fetal intestine into the fetal bloodstream.
n Umbilical arteries exchange it across the placenta. Also by direct contact with fetal surface of the placenta.
n At term amniotic fluid is 800 to 1,200 mL.
n Excessive amniotic fluid-hydramnios (>2,000mL) this occurs in women with diabetes R/T hyperglycemia (fluid shift into amniotic space).
n Reduction in the amount of amniotic fluid - oligohydramnios (<300) a disturbance of kidney function).
n alkaline pH 7.2
n Protective:
n shields against pressure or blow to mother’s abdomen.
n protects fetus from changes in temperature.
n aids in muscular development with allowing movement.
n protects cord from pressure, protecting fetal oxygenation.
Origin and Development
n From the beginning of fetal growth, development proceeds in a cephalocaudal (head to toe) direction.
n Head first then middle and then lower body parts. This continues after birth also.
n Body organ systems develop from specific tissue layers called germ layers.
n Primary Germ Layers:
n At the time of implantation, the blastocyte has separated to two cavities in the inner structure.
n Amniotic cavity (large) - lined with a distinctive layer of cells called - Ectoderm.
n Smaller cavity yoke sac which is lined with entoderm which supplies nourishment only until implantation. After that, it provides a source of red blood cells until the hematopoietic system is mature.
n Ectoderm - CNS, PNS, skin, hair ,nails, sebaceous glands, sense organs, mucus membranes of the mouth, anus, nose, tooth enamel, mammary glands
n Mesoderm (middle layer)
n support structures - bone cartilage, muscle, ligament, tendon. Dentin of teeth, kidneys, ureters, reproductive system, heart, circulatory system, blood cells, lymph cells.
n Entoderm (yolk sac)
n lining of pericardia, pleura peritoneum, GI tract, respiratory tract, tonsils, parathyroid, thyroid, thymus, bladder,and urethra.
n Each germ layer of primary tissue develops into specific body systems.
n One reason rubella infection is so serious in pregnancy is because the virus is capable of affecting all the germ layers.
n All organ systems are complete at 8 weeks’ gestation (end of the embryonic period).
n Organogenesis: (organ formation)
n The growing structure is most vulnerable to invasion by teratogens.
Cardiovascular System
n One of the 1st systems to become functional in intrauterine life.
n Simple blood cells joined to the walls of the yolk sac progress to a network of blood vessels and to a single heart tube forming as early as the 16th day of life, beating at 24th day, the septum divides during the 6th or 7th week, heartbeat may be heard with a doppler at 10th to 12th week.
n After the 28th week the heart rate begins to show a baseline variability of 5 beats/min.
n Fetal Circulation:
n As early as the 3rd week of intrauterine life, fetal blood has begun to exchange nutrients with maternal circ across the chorionic villi.
n Fetus derives O2 and excretes CO2 from the placenta (not lungs).
n Blood enters the cells of lungs.
n Specialized structures in the fetus shunt blood flow to brain, liver, heart,and kidneys.
n Blood from the placenta is highly oxygenated.
n Blood enters through the umbilical vein (called vein because the direction of blood flow is toward the fetal heart).
n Carries blood to inferior vena cava through accessory structures - ductus venosus.
n It receives O2 blood from the unbilical vein to supply the fetal liver.
n Then, empties into the inferior vena cava.
n From the inferior vena cava blood is carried to the right side of the heart.
n As blood enters right atrium the bulk is shunted into the left atrium through an opening in the atrial septum the foremen ovale.
n From the left atrium it follows normal circ. into the left ventricle and into the aorta.
n Deoxygenated blood from the body is returned to the heart by the vena cava.
n The blood enters the right atrium and leaves by the normal circ. route.
n A large portion of this blood is shunted away from the lungs through an additional structure - ductus arteriosus which is directly into the aorta and then the descending aorta.