PREGNANCY rd 3 Trimester PREGNANCY 3rd Trimester I. Uterine Changes A. Uterus experiences greatest period of growth B. Reaches into epigastric region by 8th month C. Endometrium 1. Layer thickens 2. Glands enlarge, may penetrate myometrium PREGNANCY 3rd Trimester, con’t… Location of uterine fundus by developmental week Uterine changes, continued … D. Myometrium: arranged into three distinct layers 1. External layer: beneath peritoneum a. Fibers pass transversely across fundus b. converge @ superior angles of uterus Uterine changes, continued … c. Fibers extend onto 1. fallopian tubes 2. round ligaments 3. ovarian ligaments 2. Middle layer: a. thickest b. meshwork of random fibers c. many blood vessels Uterine changes, continued … Myometrium 3. Inner layer: a. fibers arranged circularly b. form cones 1. Apices surround fallopian tubes 2. Fibers form sphincters around internal ossa of tubes Uterine changes, continued … Myometrium 4. Smooth muscle fibers hypertrophy a. >800% (8x) larger b. muscle fibers undergo mitosis 1. only during pregnancy 2. Lose fibers post-partum Uterine changes, continued … D. Wall of uterus thins as pregnancy progresses E. Placental “ascension” or “migration” 1. During the last part of the 3rd trimester 2. Differential growth of lower uterine segment 3. Artifact of development PREGNANCY 3rd Trimester, con’t… Early fetal period Middle fetal period 8-16 weeks 17-30 weeks Late fetal period 13-38 weeks PREGNANCY 3rd Trimester, con’t… • II. Placental Changes • A. Chorionic Plate • 1. Strong interface visible • 2. Between amniotic cavity and fetal surface of placenta • 3. After 12th menstrual week = Grade 0, mature placenta • 4. Placenta attains a diffuse echogenic pattern PREGNANCY 3rd Trimester, con’t… Normal Mature Placenta Maternal side Fetal side Microscopic view Placental Changes, con’t…. • B. Early in 3rd trimester • • • 1. changes occur 2. Detectable sonographically 3. Grade I changes: • 30-32 3.8 cm a. first noted @ weeks b. Placenta is Placental Changes, continued … d. Chorionic plate develops “wavy” appearance e. Homogeneous look is lost f. See increased glands, calcifications g. see more scattered echogenic regions Placental Changes, continued … • 2. Grade II changes • a. 34-36 weeks • b. Chorionic plate develops marked indentations • Placental changes, continued… c. Comma-shaped septa extend from chorionic plate toward basalis d. Placenta becomes more echogenic e. Placental “mottling” appears Placental Changes, continued … • 3. Grade III changes: a. 38-40 weeks (3.5 cm) • b. Septa extend thru placenta • c. To decidua basalis • d. Divide placenta into “cotyledons” • e. Calcium deposits create Placental changes, continued … f. May find “cysts” between chorionic plate and placenta g. Areas of excessive fibrin deposition h. placenta starts deteriorating i. Venous lakes develop Placental changes, continued … C. Thickness gradually decreases after 32 weeks D. Growth ceases after 38 weeks E. Changes normal for senescent placenta 1. At birth: weighs ~ 1 lb 2. Size: ~6-8” diameter 3. Surface area: ~ 16 m2 Pregnancy, 3rd Trimester, con’t… F. Amount of amniotic fluid 1. Increases during pregnancy 2. May reach one gallon in late 3rd trimester PREGNANCY 3rd Trimester, con’t… Placental variations Succenturiate placenta Circumvallate placenta Battledore placenta Velamentous placenta May result in vasa previa PREGNANCY 3rd Trimester, con’t… III.Fetal Structures A. Fetal Head 1. First visualized at 5-6 weeks 2. After 12 weeks, details can be seen: a. b. c. d. Falx cerebri Ventricles Thalami Corpus callosum Fetal Structures: Fetal head e. f. g. h. Septum pellucidum Lateral fissure Midbrain Basilar artery i. Cerebral hemisphere j. cerebellum 8.0 mm 6 months 10.5 mm 9 months PREGNANCY 3rd Trimester, con’t… B. Fetal Thorax • 1. Heart: evaluation after 10-12 weeks • a. Four chambers: “4 chambers @ 4 months” • b. Ventricular diameter: 34-41 weeks • c. Aorta: early on with Doppler Endocardial Cushion: green Fetal Thorax, Con’t… 2. Lungs: a. no clear boundaries between lungs, diaphragm, and liver b. unless fluid intervenes 3. Diaphragm: usually visible Fetal structures, continued … C. Abdomen 1. Stomach a. upper left quadrant b. by 12-15 weeks 2. Fluid in small intestine by 2nd trimester 3. Descending colon a. seen in late 3rd trimester b. filled with fluid and meconium G.I. Tract development at 3-4 months Fetal structures, Abdomen, cont… 4. Umbilcal vein a. visible in area of umbilicus b. coursing superior and posterior to portal sinus (vein) 5. Liver: a. in upper right quadrant b. with hepatic & portal veins c. largest organ by 8 weeks Fetal structures, continued … D. Pelvis & retroperitoneum 1. Kidneys: seen @ 15-20 weeks 2. Urinary Bladder: a. seen @ 13-15 weeks b. in 3rd trimester, urine-filled 6 weeks 7 weeks 9 weeks Fetal structures, continued … E. Other structures 1. Skeletal components a. Axial skeleton b. Long bones 2. Extremities 3. Genitalia by 10 weeks (+/-) IV. FETAL CIRCULATION A. General 1. Ovum and yolk sac provide initial nutrients to embryo 2. Other means must develop early on FETAL CIRCULATION, CON’T… 3. Blood vessel and blood formation a. begin15-16 days after fertilization b. in mesoderm of yolk sac, body stalk, chorion 4. Cardiovascular system first to develop Fetal circulation, continued … 5. Blood flow begins end of the third week 6. Heart beat begins about 22 days 7. REMEMBER: a. fetus is totally dependent on outside source for oxygen, nutrients, and waste disposal. b. This source is the placenta Fetal circulation, continued … B. Course of blood through fetal circulation 1. Always begin at placenta 2. Vessels named with respect to fetus 3. Blood leaves placenta via umbilical or placental vein (+ O2 blood) Fetal circulation, continued … 4. Travels in umbilical cord a. Through umbilicus b. Into baby’s body 5. Ascends along falciform ligament a. To inferior surface of liver b. Here, enters liver Fetal Circulation Umbilical Vein Fetal circulation, continued … 6. At porta hepatis, umbilical vein divides: a. Portal vein: carries blood through liver hepatic veins IVC b. Ductus venosus: goes directly to IVC Fetal Circulation Ductus Venosus Portal Vein Fetal Circulation, continued … 7. Blood in IVC is –O2 blood, and is a. mixes with +O2 blood from hepatic veins, ductus venosus b. is now “mixed” blood 8. IVC enters right atrium Fetal Circulation IVC Fetal Circulation, continued … a. some blood follows post-partum path b. Most blood is diverted 1. by valve of IVC (Eustachian valve) 2. through foramen ovale into left atrium Fetal Circulation, con’t… Foramen Ovale Eustachian Valve Fetal Circulation, continued … 9. From left atrium: a. blood is pumped to left ventricle b. out the aorta c. throughout systemic circulation Fetal Circulation, continued … 10. Blood from head and upper extremities a. returns to right atrium via SVC b. mixes with blood from IVC 11. Blood to lower body a. passes from aorta common iliac arteries internal iliac arteries Fetal Circulation SVC IVC Common Iliac Arteries Umbilical Arteries Fetal Circulation, continued … 12. Fetal lungs are non-functional a. require minimal blood supply b. shunt is present between pulmonary artery and aorta c. ductus arteriosus: by-passes lungs Fetal Circulation, con’t… Ductus Arteriosus Fetal Circulation, continued 13. Vessels in umbilical cord a. single large vein b. two smaller arteries Fetal circulation, continued … 14. Recap of fetal circulation: a. Oxygenated blood carried by 1. umbilical vein 2. portal vein 3. ductus venosus Fetal circulation, continued … b. Placenta is: 1. Fetal organ of respiration, nutrition & excretion 2. Most blood from placenta a. goes thru liver b. before entering IVC Fetal circulation, continued … c. Eustachian Valve 1. directs most blood from IVC to left atrium 2. directs blood from SVC to right ventricle Fetal circulation, continued … d. Blood from umbilical vein, IVC 1. go from L.A. to L.V. to aorta 2. then mostly to head, upper extremities e. Descending aorta contains mixed blood Fetal Circulation Descending Aorta Fetal circulation, con’t… C. Changes in Cardiovascular System at Birth 1. After pulmonary respiration begins a. the ductus arteriosus constricts b. blood flows to lungs Fetal circulation, con’t… 2. More blood returns to left atrium a. flow thru foramen ovale stops b. Seals to become fossa ovalis 3. Ductus arteriosus a. fills with fibrous C.T. b. becomes ligamentum arteriosum Fetal circulation, con’t… 4. Cutting umbilical cord causes vessels to collapse, seal a. Umbilical vein b. Ductus venosus ligamentum teres hepatis ligamentum venosum c. Proximal umbilical arteries d. Distal umbilical arteries ligaments internal iliac arteries lateral umbilical Fetal Vessels: Postnatal Changes Ligamentum arteriosum Fossa ovalis Ligamentum venosum Ligamentum teres hepatis Lateral umbilical ligaments Fetal circulation, con’t… Patent Urachus D. Urachus (median umbilical ligament) 1. Obliterated remains of allantois 2. Attaches urinary bladder to abdominal wall 3. Can form urachal cyst (rare) Urachal Cyst Fetal circulation, con’t… 3. Passes superiorly from apex of bladder to umbilicus 4. Composed of fibrous C.T. and smooth muscle 5. Lies between umbilical ligaments V. THIRD TRIMESTER BLEEDING A. Abruptio Placentae (Placental abruption): 1. Premature separation of a normally implanted placenta 2. May be mistaken for placenta previa 3. May be only a few mm separation or a complete detachment Third trimester bleeding, con’t. 4. Retroplacental bleeding occurs 5. May be external or concealed 6. Signs and symptoms vary a. depends on degree of separation b. amount of blood loss Third Trimester Bleeding, continued … B. Placenta Previa: 1. Placenta implants over internal os of cervix 2. Occurs in 1:200 cases 3. False positive rate of 10% Placenta Previa Marginal Previa Partial Previa Complete Previa Third Trimester Bleeding, continued … 4. Symptoms: a. sudden painless vaginal bleeding b. in late third trimester c. Followed by painless massive bright red bleeding Third Trimester Bleeding, continued … 5. May be mistaken for abruptio placentae 6. Ultrasound is diagnostic modality Third trimester bleeding, continued … C. Rh Incompatibility: [erythroblastosis fetalis, hemolytic disease of the newborn (HDN)] 1. lysis of fetal RBC’s 2. due to Rh- isoimunization 3. Rh- multiparous female 4. Rh+ father and fetus Third trimester bleeding, continued … 5. Leads to: a. hemolysis of fetal RBC’s b. anemia of fetus c. congestive heart failure d. may cause jaundice Erythroblastosis Fetalis, continued … 3. Caused by transplacental transmission of maternal anti-Rh antibodies (Ab) a. All cells contain antigens on cell membranes, so…. b. All RBC’s contain antigens cell membranes Erythroblastosis Fetalis, continued … c. ABO Antigens (Ag): 1. type A has A antigens 2. Type B has B antigens 3. type AB has both A & B 4. type O has neither Ag d. Rh: Rh+ has antigen Rh- lacks antigen (ABO & Rh: major antigens involved in antibody incompatibility) Erythroblastosis Fetalis, continued …. 4. At birth, baby’s blood may mix with mom’s (~40%) 5. Mom makes antibodies to Rh+ 6. During succeeding pregnancies, antibodies cross placenta 7. Cause baby’s blood to agglutinate Erythroblastosis Fetalis, continued … 9. Rhogam a. antibody formation may be prevented: “fools” mom’s immune system b. Rhogam given within 72 hours after fetus “leaves” c. Currently, Rhogam given to all Rh- moms every trimester Twinning mechanisms and Placentation Full-term Pregnancy