PREGNANCY 3 rd Trimester. I. Uterine Changes A. Uterus experiences greatest period of growth B. Reaches into epigastric region by 8 th month C. Endometrium

PREGNANCY 3rd Trimester

PREGNANCY 3rd Trimester

I. Uterine ChangesA. Uterus experiences greatest period of

growthB. Reaches into epigastric region by 8th

monthC. Endometrium

1. Layer thickens2. 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


c. Fibers extend onto 1. fallopian tubes2. 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


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


Early fetal period 8-16 weeks

Middle fetal period 17-30 weeks

Late fetal period 13-38 weeks


• 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


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:

• a. first noted @ 30-32 weeks

b. Placenta is 3.8 cm

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


• 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


• 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 echoes

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


Placental variations Succenturiate placenta

Circumvallate placenta

Battledore placenta

Velamentous placenta May result in vasa

previa

III.Fetal Structures

A. Fetal Head

1. First visualized at 5-6 weeks

2. After 12 weeks, details can be seen:a. Falx cerebrib. Ventriclesc. Thalamid. Corpus callosum


Fetal Structures: Fetal head

e. Septum pellucidumf. Lateral fissure g. Midbrainh. Basilar artery

i. Cerebral hemisphere

j. cerebellum

6 months

9 months

10.5 mm

8.0 mm


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. Abdomen1. 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


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


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


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)


4. Travels in umbilical cord a. Through umbilicus b. Into baby’s body

5. Ascends along falciform ligament a. To inferior surface of liverb. Here, enters liver

Fetal Circulation

Umbilical Vein


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


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


9. From left atrium:

a. blood is pumped to left ventricle

b. out the aorta

c. throughout systemic circulation


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


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

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


b. Placenta is:

1. Fetal organ of respiration, nutrition & excretion

2. Most blood from placenta a. goes thru liver b. before entering IVC


c. Eustachian Valve

1. directs most blood from IVC to left atrium

2. directs blood from SVC to right ventricle


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


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


4. Cutting umbilical cord causes vessels to collapse, seal

a. Umbilical vein ligamentum teres hepatis

b. Ductus venosus ligamentum venosum

c. Proximal umbilical arteries internal iliac arteries

d. Distal umbilical arteries lateral umbilical ligaments

Fetal Vessels: Postnatal Changes

Ligamentum arteriosum Fossa ovalis Ligamentum venosum Ligamentum teres

hepatis Lateral umbilical

ligaments


Patent Urachus

UrachalCyst

D. Urachus (median umbilical ligament)1. Obliterated remains of allantois

2. Attaches urinary bladder to abdominal wall

3. Can form urachal cyst (rare)


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


4. Symptoms:

a. sudden painless vaginal bleeding

b. in late third trimester

c. Followed by painless massive bright red bleeding


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


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


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

Documents

PREGNANCY 3 rd Trimester. I. Uterine Changes A. Uterus experiences greatest period of growth B. Reaches into epigastric region by 8 th month C. Endometrium