Ch 4 Amniotic Fluid, Normal Placenta + Cord Flashcards
What is amniotic fluid?
A dialysate of maternal serum
List 3 reasons why amniotic fluid is essential?
-Maintains even temperature + homeostasis
-Allows fetal movement + growth
-Develops tracheobronchial tree
S/F of amniotic fluid?
Usually anechoic, but can see milky debris typically in 3rd trimester due to vernix caseosa
What is vernix caseosa?
-White, creamy, milky biofilm that naturally covers the skin of the fetus during the 3rd trimester
-Baby cheese
(it comes off and goes into amniotic fluid)
At 12 weeks gestation how much fluid volume should there be?
60 mL
At 20 weeks gestation how much fluid volume should there be?
500 mL
From week 12-16, how much does the fluid volume increase?
20-25 mL per week
From week 16-20, how much does the fluid volume increase?
50-100 mL per week
How does the fetus contribute to fluid volume?
-Transudation through skin surfaces (includes skin, cord, chorion + amnion)
-Fetal urine (begins at 12 wks, not significant until 18-20 wks)
When does fetal urine start entering amniotic fluid + when does it become significant?
-Starts week 12 (insignificant)
-Becomes significant b/w week 18-20
How much urine does the fetus produce each day in the 3rd trimester?
450 mL urine per day
Do we actually measure the fluid during exams?
1st/2nd trimester:
-eye ball it subjectively
-report SDP if abnormal b/w 18w - 26w + 6d
3rd trimester:
-measure it objectively
-SDP + AFI (amniotic fluid index)
-semiquantitative measurements
By week 20 the fetus can’t supply fluid through transudation through skin surfaces anymore, how is fluid volume maintained now?
-By swallowing + fetal urination
-Can be maintained with only 1 functioning kidney
What is oligohydramnios + polyhydramnios?
Oligo: low amniotic fluid volume (<2cm)
Poly: excessive amniotic fluid volume (>8cm)
List SDP measurements for normal, oligohydramnio + polyhydramnio?
Normal: b/w 2-8cm
Poly: over 8cm
Oligo: under 2cm
How do we find + measure the SDP?
-Check 4 quadrants in uterus to determine which is deepest pocket free of cord + body parts
-Use CD to ensure we are not measuring “invisible” cord
-Measure perpendicularly (b/c fluid is gravity dependent) from anterior uterine wall down to 1st structure encountered (ex. leg, body, placenta, posterior uterine wall, etc)
-Imagine 1cm wide box + make sure nothing invades that box
(pocket should fit 2x1cm rectangle with long axis at right angle to uterine wall)
What should we do next if we measure an abnormal SDP?
Repeat measurement again to ensure we did not accidentally include cord, etc.
How do we measure AFI?
Measure fluid in 4 quadrants of uterus + add all measurements together (in cm)
Normal AFI value?
5-25 cm
AFI value that indicates oligohydramnios + polyhydramnios?
Oligo: under 5cm
Poly: over 25cm
What is the median AFI value?
14cm (from week 20-35 b/c fluid volume begins to reduce)
T/F: It is a very common reason to do an u/s if they think the mom is leaking fluid.
True
List reasons that cause oligohydramnios?
-Uteroplacental insufficiency
-Drugs
-Postterm pregnancy
-IUGR
-Fetal death
-Fetal chromosomal abnormalities
-Rupture of membranes
-Idiopathic
-Fetal malformations (m/c ones that decrease urine production)
List reasons that cause polyhydramnios?
-Fetal malformations
-Multiple gestation
-Maternal diabetes
-Fetal anemia (includes Rh incompatibility)
-Idiopathic
-Other fetal disorders/infections or genetic abnormalities
How many vessels make umbilical cord?
3 (2 arteries + 1 vein)
In fetuses, the umbilical arteries + vein carry what kind of blood?
Arteries: deoxygenated
Vein: oxygenated
(remember opposite from adults)
Where do the umbilical arteries + vein carry blood to?
Arteries: to placenta (from fetus)
Vein: to fetus (from placenta)
What 4 things should we be assessing when viewing the placenta?
-Size
-Shape
-Consistency
-Location
What 4 things do we document when evaluating the umbilical cord on u/s?
-Confirm # of vessels
-Document length (subjective, never measure)
-Appearance of cord
-Image fetal + placental cord insertion sites!!
The umbilical cord originates from fusion of YS stalk + omphalomesenteric duct at what week?
7 weeks gest
The umbilical arteries + vein are surrounded by what?
Wharton jelly (mucoid connective tissue), all enclosed in a layer of the amnion
At term, what is the average length of the cord?
51.5-61 cm
(circumference = 3.8 cm)
Is it easy to scan the entire cord?
No, is hard due to length
What is a velamentous cord?
Cord away from main body of placenta, highest risk b/c can kill baby + mom during birth
What 4 things contribute to the placental structure?
-Maternal portion (arises from endo)
-Fetal portion (arises from section of chorionic sac)
-Trophoblasts invade decidua + becomes part of placenta
-Villi proliferates into the villous chorion / chorion frondosum (which is fetal side of placenta)
What are the metabolic functions of the placenta?
Synthesizes sugar, fats + hormones (hCG, estrogen + progesterone)
How much does the placenta weigh?
480-600 grams at term
The fetal side of placenta is a fused layer of what 2 structures?
Amnion + chorion, with underlying fetal vessels
The maternal side of placenta has how many functioning lobes/cotyledons?
20
(composed of maternal sinusoids + chorionic villous structures)
Where is the normal cord insertion site into the placenta?
Central
What is it called when the cord inserts eccentrically near the margins of the placenta?
Battledore placenta
(eccentric = away from center, but still on edge)
What is it called when the cord inserts below the edge of the placenta?
Velamentous insertion
Normal placental thickness prior to 24 weeks gestation?
Less than 4cm
(not routinely measured)
Are the maternal + fetal circulations separate?
Yes
Explain placental circulation?
-Oxygenated maternal blood is pumped through spiral arterioles within decidua basalis + enters intervillous spaces surrounding/bathing the villi
-Deoxygenated fetal blood circulates through capillaries in chorionic villi within the placental lobes
-Resulting oxygenated blood within villous capillaries returns to fetus via umbilical vein
Where do gases + nutrients exchange in placental circulation?
Across walls of villi
The relationship of the placenta + cervix can be best seen with a full or empty bladder?
Full, be careful it is not overdistended tho as this can cause a false-positive appearance of placenta previa
(empty bladder may not be able to see cervical os)
Bladder is considered adequately full when cervical length is what?
B/w 3-5 cm
If the cervix is measuring long, what is the m/c cause of it?
B/c bladder is too full
(this causes close position of anterior + posterior walls of lower uterus, producing a falsely superior cervical os)
How would we measure the placenta?
Use thickest part of central placenta, excluding uterine wall
(remember not routinely done)
What are placental lakes?
-Normal finding
-Hypoechoic structures within anterior placenta
How does placental texture change from early pregnancy to end of 1st trimester?
Early pregnancy: echogenic focal thickening of wall of gest sac
End of 1st: fine, granular + homogeneous
Increased levels of serum alfa fetoprotein is associated with what?
Findings of large vascular spaces in placenta
In late 3rd trimester, the placenta may have nonvascular cystic areas centrally within delineated lobes, what do these areas represent?
Necrosis
Is it common for placentas to have some degree of macroscopic calcifications after 33 weeks?
Yes, over 50% do
(as the placenta ages they often get hypoechoic areas + calcifications)
What is the retroplacental complex composed of?
-Decidua basalis
-Portions of myometrium (includes maternal veins draining placenta)
At what week is the retroplacental complex visible?
Week 14
SF of retroplacental complex (RPC)?
Hypoechoic area 10-20mm deep to placenta
What can an anterior RPC lead to?
Excessive bleeding during invasive procedures
(ex C section or amniocentesis)
Large venous channels can be seen in the RPC, m/c seen in what location?
Posterior placenta
(pressure from gravity can overdistend the veins)
What is placental grading?
Grades degree of placental calcifications
Are placental calcifications still considered a marker for lung maturity?
No
What factors affect degree of placental calcifications?
-Smoking
-Low maternal age
-Parity
-The season
Premature placental calcifications can occur in maternal hypertensive states + are associated with what?
IUGR
What conditions can delay the rate of placental calcifications?
-Gest diabetes
-Fetal cardiopulmonary disorders
Do we use placental grading on the island?
No
List the 3 gradings on placental calcifiactions?
Grade 0: none
Grade 1: scattered calcifications
Grade 2: basal calcifications with increase in lobulations
Grade 3: basal + interlobar septal calcifications
(basal = base closest to uterus)
