OB Fetal Development
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What is the difference between Mitosis and Meiosis?
- Mitosis creates diploid cells and is the process by which the fertilized embryo grows (called cleavage)
- Meisosis produces haploid cells (half the genetic material), such as in oogenesis and spermatogenesis
Where does fertilization occur in the uterus and how many chromosomes does the new cell have?
- When the sperm enters the ovum, a chemical reaction seals the ovum and the new cell then contains 46 chromosomes (23 pairs)
- Fertilization occurs in the outer one third of the fallopian tubes (the ampulla)
What occurs in the first week after fertilization?
- Synchronized development of endometrium & zygote in preparation for implantation
- Two phases of development: cellular multiplication occurs right at fertilization and cellular differentiation (organogenesis) begins around day 14
- Progesterone is secreted by the corpus luteum to stimulate the endometrium, of the uterus which becomes thicker and more vascular in preparation for implantatio
When is the fertilized egg called a zygote, a marula, a blastocyte, an embryo, and a fetus?
- At Fertilization: Zygote
- 3 Days: Morula (12-16 cells)
- 5 Days: Blastocyte
- 3 Weeks: Embryo
- 9 Weeks: Fetus
Once the Morula becomes a Blastocyst, what to two distinct cell masses form and what do they do?
- Embryoblast: Inner cells surrounded by fluid
- -One layer develops into the amnion
- -One layer develops into the embryo
- Trophoblast: Outer layer of cells
- -develops into the chorion part of the placenta
When does implantation (Nidation) occur, where does it occur, and what is the next step?
- Implantation occurs at the beginning of day day 5-6 at the posterior fundus
- The endometrium thickens at the site of implantation and is called the decidua
- Trophoblast grows into the thickened lining forming the chorionic villi
What is the embryonic period and why is it so important?
- weeks 3-8
- The embryoblast differentiates into the ectoderm, mesoderm, and endoderm
- Cell organization begins and organogensis begins
- *THIS IS MOST VULNERABLE TIME WHEN THE FETUS IS SUSCEPTIBLE TO VIRUS, RUBELLA, TERATOGENS, RADIATION & DRUGS
- (this is scary because most women do not yet know they are pregnant)
What occurs during the third week of fetal development?
- A singular tubular heart is formed
- The neural tube is formed
- Folic acid can prevent neural tube defects like spina biffida
- Folic acid supplements of 0.4mg should be taken daily for 6mo prior to conception
- The embryo is 1.5mm and is growing considerably before the woman even suspects she is pregnant
What occurs during the fourth week of fetal development?
- The heart pumps blood
- The esophagus & trachea separate; stomach forms
- Neural tube closes on each end
- The upper and lower limn buds form
- Eyes and ears begin to form
- Embryo is 4-6 mm long
When does the yolk sac develop and what is its purpose?
- Yolk sac develop about 8-9 days after conception
- Forms the primitive red blood cells during the first 6 weeks until fetal liver takes over and then it is incorporated into the umbilical cord
During the 5th week of development, what occurs?
- Brain has differentiated and cranial nerves are present
- Developing muscles has innervation
- Atrial division occurs in the heart
- Embryo is 8mm crown to rump
What is important about the 7th week of development?
- Fetal heartbeat can be detected with an ultrasound (fetal heartbeat cannot be heard with a doppler until 8-12 wks)
- The head is rounded and nearly erect the eyes have shifted forward and are closer together- the eyelids begin to form
What is the fetal period and what is the importance of the 9th week?
- Weeks 9-40
- Every organ system and external structure is prevent by week 9, the remainder of gestation is devoted to refining structures & perfecting function
During the 14th week of development, what occurs?
- This is a period of rapid growth
- Skin is transparent and blood vessels are visible beneath it
- More muscle and skeletal tissue has developed and this holds the fetus more erect
What happens during the 16th week of development?
- Teeth are forming
- Meconium forming in the intestines
- Sucking motions are present
Describe the fetus during the 20th week of development
- Mother feels the fetal movement (quickening)
- Subcut fat deposits of brown fat make the skin less transparent
- "Wooly" hair covers the fetal head
- Nails have developed on the fingers & toes
- Length is about 19cm and weighs about 1lb (435-465 grams)
What occurs during the 24th week of development?
- The lungs begin to secrete surfactant and the fetus is now medically viable with respiration, however most fetuses die if born at this time
- Mother feels even stronger movement (quickening)
- Wrinkled skin & lean body result from lack of subcut fat
- Lenght is 23-24 cm and weight is 780 grams
What is the difference between the fetus being legally viable and medically viable?
- The fetus is legally viable at 20 weeks, but the fetus does not have a chance of live until it is medically viable at 24 weeks when surfactant is produced
- The legally viable measurement of 20 weeks is important when parsing out the mother's para
What occurs during the 36th week of gestation?
- There is a surge in lung surfactant production in preparation for birth
- Skin is smooth and pink with subcut fat
- Length is 48 cm (19in) and weight is 2500 grams or 5lb, 12 oz
What path does MOST blood take through fetal circulation?
- Oxygenated blood from the placenta moves up the umbilical vein into the inferior vena cava through the ductus venosus
- Some blood goes to the liver and is shot up to the brain
- Once blood enters the right atrium, most of it goes through the foramen ovale into the left atrium into the left ventricle
- From the left ventricle into the aorta and through the fetal body
- Deoxygenated blood leaves through the 2 umbilical arteries exiting the fetal femoral arteries
What are the other pathways through the fetal heart?
Blood that goes into the right ventricle goes through the pulmonary artery and then into the ductus arteriosus. This takes the blood through the aorta and back into the body
Describe the differences in the fetal heart and when these changes in circulation occur after birth
- Foramen ovale: open hole between the right and left atrium that closes within 2 hrs from birth. It closes permanently with 30 months
- Ductus arteriosis: connection between the pulmonary artery and the aorta. Closes within 15 horus and permanently within 4 weeks. Becomes a ligament.
- Ductus venosus: closes functionally when the umbilical cord is clamped and cut. Permanently closes within 1 week and turns into a ligament by 2 mo of age
Describe the placenta and when it becomes fully functioning
- Formed from both maternal and fetal tissue
- Placenta begins to fully function at 8-10 wks (prior to that, the corpus luteum fills this job)
- Fetal side (shiny): develops from the trophoblast; chorionic villi are projections from the chorion that embed into the decidua basalis (endometrium)
- Maternal side (beefy): decidua basalis is the endometrium at the site of implantation and forms the maternal side of the placenta. It contain cotyledons with branching villi. This is the site of exchange between the maternal & fetal blood across the placental membrane
What are the functions of the placenta?
- Endocrine: produces hormones such as progesterone, estrogen, human chorionic gonadotropin (hCG), and human placental lactogen (hPL)
- Metobolic: removes respiratory and renal waste
- Immune properties
How are nutrients and waste products transferred across the placenta?
- Fetal & Maternal blood do not mix
- Oxygenated, nutrient-rich blood spurts nto the intervillus space from spiral arteries into the decidua
- Fetal blood releases carbon dioxide and waste products from the umbilical arteries
- Fetal blood takes the oxygen and nutrients before before retuning to the fetus through the veins in the umbilical cord
- Many harmful substances can cross the placental membrane and those drugs the FDA categorizes C,D, or X should not be taken during pregnancy
What is the role of progesterone during pregnancy?
- First produced by the corpus luteum to maintain the uterine lining for implantation of the zygote
- Produced by the placenta to reduce uterine contraction to prevent spontaneous abortion
What is the role of estrogen in pregnancy?
- Stimulates uterine growth and increases the blood flow through the uterine vessels
- Stimulates the development of the breast ducts to prep for lactation
- Effects of estrogen not related to pregnancy includes increased skin pigmentation, vascular changes in the skin and mucous membranes, increased salivation
What role does hCG play?
- Causes the corpus luteum to persist & continue the production of estrogen and progesterone to sustain the pregnancy
- hCG is detectable in maternal blood as soon as implantation occurs (usually 7-9 days after fertilization)
- Rapid rise in first trimester and then decreases
- Basis for pregnancy test
What is human placental lactogen (hPL) and what does it do?
- Decreases insulin insensitivity and utilization of glucose by mother, which helps to make more glucose available to fetus to meet growth needs
- Stimulates breast development for lactation
- Related to gestational diabetes
- Also known as human chorionic somatomammotropin (hCS)
What are the two embryonic membranes and from what do they develop?
- Chorion & amnion form the amniotic sac
- Chorion: develops from the trophoblast and is the outermost membrane
- Amnion: originates from the embryoblast. Its a thin protective membrane that surrounds the fetus. It contains the amniotic fluid and expands as the embryo grows until the fetus comes in contact with the chorion
Describe amniotic fluid, its development, and its circulation
- Slightly alkaline (shows up on nitrazine paper)
- Secreted from the developing embryo in early pregnancy
- Water & Solutes move freely across the fetal skin until about 23-25 weeks
- Fetal kidneys produce the amniotic fluid after the first trimester
- Amniotic fluid is absorbed by fetal blood as it perfuses the surface of the placenta
- Fluid moves in and out of fetal lungs
What is the purpose of amniotic fluid?
- Cushions the developing fetus and umbilical cord
- Temperature control for the fetus
- Symmetrical growth of the fetus
- Prevents adherence to the amnion for the developing fetus
- Acts as a wedge during labor
- Can be analyzed to determine fetal health
Describe the normal amount of amniotic fluid and what occurs when the amount is abnormal
- Peaks at 800-1000ml during the 34th wk and then decreases to 500-600ml at term
- Polyhydraminos: is excess amniotic fluid from 1500-3000ml. It is associated with chromosomal GI, cardiac & neural tube defects
- Oligohydraminos: <500ml amniotic fluid or a 50% reduction. Associated with congential renal disorders or placental dysfunction
Describe the umbilical cord
- 2 arteries and 1 vein covered in Wharton's Jelly
- Formed from the amnion and links the fetus to the mother
- The umbilical cord should be centrally located in the placenta
- It does not have any sensory or motor innervation
- At term it is about 22in (55cm) in length
What immunologic property does the placenta have?
- It is not a well understood process
- Progesteron and hCG may suppress maternal cellular immunity so that the immune system does not recognize the baby as an intruder
What is the difference between monozygotic and dizygotic twins or multiples?
- Monozygotic twins are from a single fertilized egg (identical & same sex): may have two amnions with a common chorion & placenta
- Dizygotic twins are from two separate fertilized eggs: two amnions with two placentas which may fuse, no more alike genetically than 2 siblings
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