Maternal-Fetal Medicine Quiz

The Placenta and Umbilical Cord

• Accurate assessment of the placenta and umbilical cord is an important part of routine obstetric sonographic examinations.
• Placental evaluation usually includes assessment of size, shape, consistency, and location.
• Umbilical cord evaluation by ultrasound should confirm the number of vessels, document length, and appearance of the cord, and visualize the insertion points.

Normal Placenta

• A discoid organ that typically weighs 500-600 grams at delivery.
• Usually 2-4 cm in anteroposterior (AP) dimension.
• The maternal surface is irregular and divided into cotyledons.

Why Care About the Placenta?

• Placental pathology or abnormal development can lead to complications during pregnancy.
• Sonographers use tools to evaluate placental morphology, localization, and feto-placental hemodynamics.
• Placental pathology often requires further investigation to minimize fetal risks.

Development of the Placenta

• Composed of maternal and fetal portions:
  • Maternal portion arises from the endometrium.
  • Fetal portion arises from the chorionic sac.
• The functional layer of the endometrium in a pregnant woman is called the decidua.
  • Decidua basalis is the deeper portion and becomes the maternal side of the placenta.
  • Decidua capsularis overlies the embryo.
  • Decidua parietalis/vera encompasses the remaining decidua.

The Placenta

• Attachment to the uterine wall can occur anywhere within the uterine cavity.
• Fetal side is a fused layer of amnion and chorion (chorionic plate) with fetal vessels underneath.
• Maternal side has about 20 functional lobes (cotyledons) composed of maternal sinusoids and chorionic villous structures.
• Sonographically, the placenta is relatively homogeneous, but may show calcification and anechoic spaces (lacunae) in later pregnancy.

Placental Circulation

• Maternal and fetal circulations are separate.
• Oxygenated maternal blood is pumped through spiral arterioles and enters intervillous spaces (sinus) around the villi.
• Gas and nutrient exchange occurs across the villus walls.
• Oxygenated blood returns to the fetus by the umbilical vein.
• Deoxygenated fetal blood returns to the placenta by the umbilical arteries, circulating through capillaries in chorionic villi.
• Maternal blood volume increases during pregnancy to meet fetal needs. Many vascular channels in the placenta create a low-impedance system, at least until late in pregnancy.

Sonography of the Placenta

• By the fourth month, sonography can identify the placenta's final shape.
• Normal cord insertion is usually central, but can be eccentric (battledore or velamentous insertion).
• Thicker placental base usually means a thinner placenta.
• Placental length, volume, and thickness help correlate pregnancy outcomes.
• A placental thickness greater than 4 cm before 24 weeks is often abnormal.
• Transabdominal scanning with a curvilinear or linear transducer is used for evaluation.
• Endovaginal transducers allow for earlier visualization and are ideal for questionable cases of placenta previa.
• Transperineal ultrasounds are used in situations where endovaginal imaging is not possible.

A Full Bladder

• A full bladder enhances placental visualization in early and later pregnancy.
• A full bladder is crucial when considering a diagnosis of placenta previa.
• An empty bladder may hinder visualization of the cervix.
• Sonographers should encourage emptying of a portion of the bladder to prevent a false-positive diagnosis.
• A bladder is considered appropriately full when cervical length is between 3-5 cm.

Placental Texture

• Placental texture changes from an echogenic focal thickening—early pregnancy—to a homogeneous, granular texture later in the pregnancy.
• Intraplacental vascular spaces can be observed.
• Areas of varying echogenicity should be carefully documented.
• Increased AFP levels may correlate with larger vascular spaces.

The Retroplacental Complex

• The retroplacental complex is composed of the decidua basalis and portions of the myometrium.
• Deep hypoechoic area of 10-20mm is visible as early as 14 weeks.
• Proper identification is necessary.
• A prominent anterior retroplacental complex may lead to excessive bleeding during procedures.

Color Doppler image of the retroplacental vessels

Placental Grading

• Structural changes occur in the placenta as it ages.
• Placental grading, to determine gestational age, has poor correlation in practice.
• A grade II placenta in the second trimester or early third trimester could indicate placental insufficiency, especially with maternal complications.

Placental Variants

• Extrachorial: Membranous chorion does not extend to the edge of the placenta. This might be associated with hemorrhage.
  • Circumvallate: Fetal membranes fold back around the edge of the placenta.
  • Circummarginate: Flat transition of membranes inserted at some distance from the placental edge.

Accessory Types of Placentas

• Succenturiate: Accessory cotyledons connected to the main placenta.
• Bipartite: Placenta divided into two lobes but united by vessels and membranes.
• Annular: Ring-shaped placenta.

Abnormal Placental Thickness

• Normally, the placenta is less than 5 mm in AP measurement.
• Thickening (placentomegaly) over 5 cm can have various causes.
  • Gestational diabetes
  • Rh isoimmunization
  • Maternal infection
  • Chorioangioma
  • Multiple gestation
  • Hydrops fetalis
  • Sacrococcygeal teratoma
  • Partial moles
  • Chromosomal abnormalities
  • Abruption (not necessarily thicker, only appears that way due to clot)

Placentomegaly & Fundal Placenta

• Fundal placenta refers to a placenta that is implanted near the fundus of the uterus.

Decreased Placental Thickness

• Placenta measures less than 1.5 cm in AP measurement.
• Pre-eclampsia, IUGR, intrauterine infections, and polyhydramnios are possible causes.

Abnormal Placental Location - Placenta Previa

• Placental tissue encroaches on the cervix.
• Caused by implantation of the blastocyst.
• Painless vaginal bleeding during the third trimester is a classic symptom.
• Best diagnosed sonographically in the third trimester due to placental migration.

Classification of Previa

• Complete/total/central previa: Placenta completely covers the cervix internal os.
• Partial previa: Partially covers the cervix internal os.
• Marginal previa: Encroaches on the cervix but does not cross it.
• Low-lying placenta: Placenta is within 2 cm of the cervix internal os.

Placental Abruption

• Premature separation of part or all of a normally implanted placenta from the myometrium
• Maternal hypertension, advanced maternal age, multiparity, and uterine leiomyoma are risk factors.
• Abdominal pain and vaginal bleeding are common symptoms
• May be concealed or external

Placental Abruption - Sonographic Findings

• Elevation of the placenta from the uterine wall
• Retroplacental fluid collection, varying in echogenicity (usually hypoechoic) is indicative of hemorrhage
• Could appear normal or thickened due to hemorrhage.

Abnormal Placental Adherence

• Rare condition resulting from defective decidual formation.
• Associated with placenta previa.
• Could result in uterine scarring.
• Types include, Accreta (chorionic villi in direct contact with the myometrium, but do not invade) , Increta (chorionic villi invade the myometrium), and Percreta (chorionic villi penetrate the myometrium).

Intraplacental Lesions - Placental Lakes

• Intraplacental lesions which are the presence of large pools of maternal venous blood are usually insignificant.
• They appear as anechoic/hypoechoic rounded areas within the placenta.

Intraplacental Lesions - More Infarcts

• Ischemic necrosis of placental villi occurs due to spiral artery obstruction. Usually found at the periphery of the placenta.
• Usually found in eclampsia or pre-eclampsia cases with may also be associated with intervillous thrombus and retroplacental hemorrhage .
• Appear as anechoic or hypoechoic areas in the placenta, and may not have a specific sonographic appearance.
• Absence of blood flow with color and spectral Doppler signals may be suggestive.

Chorioangioma

• Vascular tumor of the placental tissue occurring in approximately 1 in 5,000 pregnancies.
• Complications arise when larger than 5cm, and may be associated with polyhydramnios and fetal hydrops..
• Appearance is a solid, well-circumscribed placental mass, often near the cord insertion site.

Placenta Variations

• Descriptions of the different ways a placenta can develop.

Umbilical Cord

• Umbilical cord originates from the yolk sac stalk and omphalomesenteric duct by the 7th week.
• Contains 2 arteries and 1 vein enveloped in Wharton's Jelly, which is mucoid connective tissue.
• Blood vessels carry oxygenated blood to the fetus and deoxygenated blood away from the fetus.
• Average length and circumference are around 51.5-61cm and 3.8cm respectively.

Umbilical Cord

• Visualization best during late second/early third trimester due to amniotic fluid volume.
• Scan the entire cord to identify possible abnormalities.
• Changing maternal positions can shift fetal position to get better visualization of the cord.
• Three vessels should be noted.

Normal Sonographic Appearance

• In early pregnancy, the cord appears as short, linear echoes extending from the fetus to the placenta.
• As pregnancy progresses, transverse images show three distinct circles with the vein being the largest.
• Longitudinal images may show a "stack of coins" appearance from the various portions of the cord folded together.
• Arterial pulsations are observed within the umbilical arteries.

Abnormal Umbilical Cord

• Single Umbilical Artery (SUA): One umbilical artery instead of two; associated with other congenital anomalies.
• Umbilical Cord Cysts: Developmental and asymptomatic focal accumulations of Wharton's jelly that appear as cysts.

Nuchal Cord

• Cord wrapping around the fetal neck.
• Common in 20% of pregnancies.
• Rarely associated with important complications. Differentiation between cord draping across the back of the neck vs. wrapped around the neck is essential.

Cord Prolapse

• Cord protruding through the cervix or adjacent to fetal presenting part.
• Considered an emergent situation.

Vasa Previa

• Fetal vessels cross over the cervix.
• Associated with velamentous insertion.
• Presence of fetal vessels crossing over the internal cervical os is an emergency situation.

Umbilical Vein Thrombosis

• Possible causes include torsion, knotting, or compression of the cord or after intrauterine transfusion or fetal blood sampling.
• Increased echogenicity within the umbilical vein lumen, and absence of color and Doppler signals may be suggestive.

Umbilical Cord Knots

• Rare occurrence, often associated with monozygotic twins.
• Appears as multiple cord loops in one image scan.

Placental Cord Insertion Sites

• Normal: Central insertion
• Marginal/Battledore: Peripheral cord insertion at the edge of the placenta.
• Velamentous: Cord insertion into the membranes before entering the placenta, which can be associated with fetal anomalies.

Amniotic Fluid

• Produced by the fetal kidneys, tissues, skin, and membranes.
• Cushions the fetus.
• Regulates temperature and pressure.
• Enables fetal development of the lungs.
• A lack or excess of amniotic fluid can cause complications.

Fluid Estimation (AFI) and Single Deepest Pocket

• AFI: Sum of the largest vertical pocket measurements in four uterine quadrants.
• Single Deepest Pocket (SDP): Measures the largest vertical pocket alone.
• Guidelines for categorizing levels of amniotic fluid:
  • Oligohydramnios (<5cm)
  • Decreased fluid (5.1-8cm)
  • Normal (8.1-25cm)
  • Polyhydramnios (>25cm)

Oligohydramnios

• Abnormally decreased amniotic fluid volume.
• Associated with fetal demise, renal abnormalities, IUGR, and post-dates.
• Also associated with preeclampsia or eclampsia.

Anhydramnios

• Complete absence of amniotic fluid.

Polyhydramnios

• Abnormally increased amniotic fluid volume.
• Possible causes include maternal diabetes, Rh incompatibility, fetal CNS or GI anomalies, and facial clefts.
• Associated with fetal hydrops, twin-twin transfusion syndrome, sacrococcygeal teratoma, and skeletal dysplasia.

Amniocentesis

• Procedure to obtain amniotic fluid for analysis.
• Rate of complications is 0.5%.
• Performed in the late second or third trimester.

Fetal Membrane Abnormalities

• Various intrauterine membranes, septations, and bands are found.
• Chorioamniotic separation can occur due to intervention such as amniocentesis. Or it can be sporadic, without an obvious reason; and it is associated with chromosomal or developmental abnormalities
• Intrauterine synechiae are linear, and external to the amniotic sac, tissues that project into the amniotic cavity. They are usually an incidental finding. This is not associated with fetal anomalies.
• Amniotic Band Syndrome: Occurs due to rupture of the amnion without rupturing the chorion. This can cause malformations, amputations, constriction rings, and facial clefts.