Placenta and Umbilical Cord Imaging

Definition

The placental insertion of the umbilical cord (PCI) may occur centrally into the placental disk, eccentrically at the margin, or into the membranes beyond the margin of the placenta (so-called velamentous cord insertion, or VCI).

Incidence and Pathogenesis

• The developmental dynamics that determine the directions of growth of the placental disk and the relative point of insertion of the umbilical vessels as the placental disk expands are poorly documented and understood.

• Based on first-trimester ultrasound observations, VCI appears to be a very early occurrence.

• VCI occurs in approximately 1%–3% of singletons.

• In twin gestations, VCI is more common, occurring in 8% or more. Marginal insertion occurs in approximately 20% of multifetal gestations.

• VCI is more frequent with placenta previa (incidence, 7.5%).

Key Diagnostic Features

• The PCI is determined using color Doppler and gray-scale imaging to identify the location on the placental surface where the umbilical cord and its vessels insert.

• The location of the PCI should be confirmed in two right-angle planes.

• The sensitivity of second-trimester ultrasound in diagnosing VCI is 60%–70%, and specificity is >99%. However, the sensitivity is only 50% if VCI is on the posterior uterine wall and 40% if it is on the lateral fundus.

• With lower uterine placental implantations, transvaginal imaging may be necessary to accurately characterize the location of PCI.

Differential Diagnosis

• To differentiate marginal PCI from VCI, at least 1 cm of umbilical vessels should be seen traveling in the membranes beyond the placental edge.

• If VCI is suspected, care should be taken to exclude vasa previa (velamentous umbilical vessels coursing within 1 cm of the endocervical os).

Associated Anomalies

• VCI is associated with multifetal gestation and placenta previa.

• VCI is not associated with fetal structural anomalies.

• In multifetal gestations with VCI, serial assessment of growth is indicated.

Imaging

Management

Prognosis

• Central and marginal PCI have no associations with abnormal fetal outcome.

• VCI has been associated with intrauterine fetal demise, low birth weight, prematurity, and abnormal fetal heart patterns in labor.

• However, if twin gestations and vasa previa are excluded, there is no current evidence that VCI has a significant adverse effect on perinatal or subsequent outcome of the offspring.

• VCI in the lower uterine segment has been associated with nonreassuring fetal heart rate patterns and emergency cesarean deliveries.

Definition

A molar gestation arises when only the spermatic, but not the ovarian, chromosomes replicate in a fertilized ovum. The result is a hydatidiform intrauterine mass.

Incidence and Pathogenesis

• Molar gestation occurs in 1 of every 1000 pregnancies in the United States. In Asia, the frequency is as high as 1 in 100 pregnancies.

• A complete mole is caused when a single (90%) or two (10%) sperm combine with an egg that lacks nuclear DNA.

• Molar pregnancy genotype is typically 46,XX (diploid), with both X chromosomes of paternal origin, but can also be 46,XY.

• A partial mole occurs when an egg is fertilized by multiple sperm, yielding genotypes of triploidy (69,XXY) or quadraploidy (92,XXXY).

• In complete moles, all chorionic villi are avascular and vesicular, with no embryonic tissue present.

• In partial moles, some villi are vesicular and others are vascularized. Fetal development may occur, but the fetus is almost always affected by severe abnormalities and is nonviable.

Key Diagnostic Features

• The classic appearance of the endometrial cavity presents a “snowstorm” or uniformly multicystic pattern marked by numerous diffuse echolucent and echodense structures smaller than 5 mm.

• Bilateral multicystic ovaries.

Differential Diagnosis

• Missed abortion

• Endometrial carcinoma

• Ovarian neoplasm

Associated Anomalies

• Vaginal bleeding

• Uterine size greater than indicated by dates

• Bilateral ovarian masses (theca lutein cysts)

• Extremely elevated levels of human chorionic gonadotropin (hCG)

• Hyperthyroidism, thyroid storm

• Partial mole: severe fetal growth restriction

Imaging

Management

Prognosis

• Recurrence risk, 1%

• Invasive mole risk, 10%

• Choriocarcinoma risk, 2%–5%

Definition

Placenta accreta is the abnormal invasion of trophoblast into or through the myometrium and into adjacent extrauterine tissues.

Incidence and Pathogenesis

• The incidence of placenta accreta is rising. Recent estimates indicate 1 of every 300 to 500 pregnancies have placenta accreta.

• Risk factors for placenta accreta include prior uterine curettage or fundal surgery, prior cesarean delivery, and placenta previa ( Table 27.1 ).

  TABLE 27.1

  Definitions of Markers of Placenta Accreta Sequence in Second and Third Trimesters of Pregnancy

  From Shainker SA, Coleman B, Timor-Tritsch IE, et al. Special report of the Society for Maternal-Fetal Medicine Placenta Accreta Spectrum Ultrasound Marker Task Force: consensus on definition of markers and approach to the ultrasound examination in pregnancies at risk for placenta accreta spectrum. Am J Obstet Gynecol. 2021;224(1):B2–B14. https://doi.org/10.1016/j.ajog.2020.09.001 .

  Marker	Definition
  Placental lacunae	Irregular, hypoechoic spaces within the placenta containing vascular flow (which can be seen on gray-scale and/or color Doppler imaging)
  	The following lacunae findings are associated with high risk of PAS: Large size Irregular borders High velocity a and/or turbulent flow within
  Abnormal uteroplacental interface	Loss of the retroplacental hypoechoic zone between the placenta and myometrium b
  	This marker is often located along the posterior bladder wall resulting in partial or complete interruption or irregularities of the uteroplacental interface
  	Thinning of the retroplacental myometrium (previously described as myometrial thickness of < 1 mm)
  Abnormal uterine contour (placental bulge)	Placental tissue distorting the uterine contour resulting in a bulge-like appearance
  Exophytic mass	Placental tissue extruding beyond the uterine serosa
  Bridging vessel	Vessel that extends from the placenta across the myometrium and beyond the uterine serosa

  PAS , Placenta accreta spectrum.

  a Some studies suggest a velocity of >15 cm/s as the threshold for high peak systolic velocity.

  b This space represents the uterine decidua and has been described as the “clear zone.”

  TABLE 27.2

  Percent Risk of Placenta Accreta

  From Publications Committee, Society for Maternal-Fetal Medicine. Quality of evidence: placenta accreta. Am J Obstet Gynecol. 2010;203:430–439.

  Pregnancy No.	Placenta Previa	No Placenta Previa
  First (primary)	3.3	0.03
  Second	11	0.2
  Third	40	0.1
  Fourth	61	0.8
  Fifth	67	0.8
  ≥Sixth	67	4.7

• Types of invasive placentation

  • Accreta (82%): invasion into the uterine muscle but not full thickness

  • Increta (12%): invasion into the full thickness of the uterine muscle but not beyond the serosa

  • Percreta (6%): invasion beyond the uterine serosa

Key Diagnostic Features

• Loss of hypoechoic space between placenta and myometrium on ultrasound examination.

• Multiple vascular lacunae within the placental mass, giving a “Swiss cheese” appearance.

• Blood vessels bridging the uterine-placental or myometrial-bladder interface.

• Retroplacental myometrial thickness of less than 1 mm.

• “Bulging” of the placental-myometrial tissues into adjacent spaces (e.g., into the urinary bladder) seen on ultrasound or laterally/posteriorly on MRI imaging.

Differential Diagnosis

• Normal placenta

• Chorangioma

• Uterine myoma

• Uterine sarcoma

Associated Anomalies

• Placenta previa

Imaging

Management

Prognosis

• Prognosis and complications are significantly reduced if the invasive placentation is recognized prior to delivery, emphasizing the importance of detailed imaging of women at risk.

• Although the mortality rate with invasive placentation in most settings is low, complications of delivery for women with an invasive placenta can be severe and include maternal shock, injuries to urinary tract and gastrointestinal systems, and recurrent laparotomy.

• Women managed by a multidisciplinary team are less likely to require large-volume blood transfusion, reoperation within 7 days, or prolonged maternal admission to ICU.

Definition

• Placental lakes are enlarged intervillous vascular spaces containing maternal blood.

• Placental cysts are fluid-filled blebs near the fetal surface of the placenta, often near the umbilical cord insertion.

• Chorangioma is a nonmalignant vascular placental tumor.

Incidence and Pathogenesis

• Placental lakes are identified in 20% of second-trimester ultrasound examinations.

• Placental cysts occur in fewer than 1 of 2000 pregnancies.

• Chorangiomas (chorioangiomas) occur in 1 of every 3500 to 9000 pregnancies.

Key Diagnostic Features

• Placental lakes

  • Variable size and location over serial examinations

  • May completely resolve

  • Color Doppler shows no arterial flow in lakes, but swirling, venous flow is seen on real-time scanning.

  • Numerous lakes have been traditionally associated with placental insufficiency and fetal growth restriction, but recent data do not support this.

• Placental cysts

  • Originate from cord or placental surface

  • Increased morbidity with large cysts (>5 cm), including fetal growth restriction and hemorrhage

• Chorangioma

  • Appears as a well-defined mass contained within the placenta or on its fetal surface

  • Appears hypoechoic but may be heterogeneous if hemorrhage, infarction, or degeneration is present

  • Variable amount of blood flow on color Doppler scanning

Differential Diagnosis

• Placenta accreta

• Placental lakes, cysts

• Chorangioma

• Uterine fibroid