OBGYN2 - Placenta, Umbilical Cord, Amniotic Fluid Notes

Placenta

  • The placenta, umbilical cord, and amniotic fluid are vital for fetal development.

Normal Placental Anatomy

  • The placenta is a discoid organ, typically weighing 500-600 grams at delivery.
  • Its AP dimension is usually 2-4 cm.
  • The maternal surface is irregular and divided into cotyledons by septae.
  • The fetal surface is smooth and covered by chorionic and amniotic membranes.
  • Functions of the placenta include:
    • Conversion of fetal steroids to estrogen.
    • Secretion of progesterone.
    • Secretion of human chorionic gonadotropin.
    • Exchange of oxygen, waste products, and nutrients between fetus and mother.

Placentation

  • The placenta contains both maternal and fetal tissue.
  • The maternal component is derived from the decidua basalis.
  • The fetal component is derived from the trophoblastic tissue.
  • By 5 weeks LMP (Last Menstrual Period), the trophoblast develops into chorionic villi.
  • Chorionic villi in contact with the decidua basalis rapidly proliferate to become the chorion frondosum, the fetal part of the placenta.
  • The placenta is formed by proliferation of the chorionic villi (chorion frondosum) and the maternal decidua basalis.
  • Intervillous spaces serve as the site of exchange of oxygen, nutrients, and wastes.
  • Each functional unit of the placenta is known as a cotyledon (12 to 20 per placenta).

Placental Thickness

  • Normal placental thickness is less than 5cm AP.
  • Increased placental thickness (placentomegaly) is defined as AP placental measurement > 5 cm.
  • Causes of increased placental thickness:
    • Gestational diabetes mellitus
    • Rh isoimmunization
    • Maternal infection
    • Chorioangioma
    • Multiple gestation
    • Maternal anemia
    • Hydrops fetalis
    • Sacrococcygeal teratoma
    • Partial mole
    • Chromosome abnormalities
    • Abruption (appears thick due to retroplacental clot)
  • Causes of decreased placental thickness (<1.5 cm):
    • Pre-eclampsia
    • IUGR (Intrauterine Growth Restriction)
    • Diabetes mellitus predating pregnancy
    • Intrauterine infection
    • Polyhydramnios (appears thinner)

Placenta Previa

  • Placental tissue encroaching upon the cervix and/or crossing the internal cervical os.
  • Caused by abnormally low implantation of the blastocyst.
  • More common in multiparous women, patients with previous C-section or myomectomy, or multiple abortions with instrumentation causing uterine scarring.
  • Classic symptom is painless vaginal bleeding during the third trimester.
  • Best diagnosed sonographically in the third trimester due to "placental migration".
  • An overdistended maternal urinary bladder or focal myometrial contraction can lead to a false positive diagnosis.
  • Classification of placenta previa:
    • Complete previa/ total previa/ central previa - placenta completely covering internal cervical os. May be symmetric or asymmetric.
    • Partial placenta previa - placenta partially covering internal cervical os
    • Marginal placenta previa - placenta encroaching on, but not crossing the os
    • Low lying placenta (not a type of previa) - placenta in lower uterine segment within 2 cm of internal os

Abruptio Placenta/Placental Abruption

  • Premature separation of all or part of a normally implanted placenta from the myometrium.
  • Predisposing conditions/factors:
    • Maternal hypertension
    • Advanced maternal age
    • Multiparity
    • Maternal vascular disease
    • Cigarette smoking
    • Trauma
    • Cocaine use
    • Uterine leiomyomas
  • Symptom is abdominal pain, with or without vaginal bleeding, depending on the location of the abruption.
  • Retroplacental bleeding occurs in all cases, but two types exist:
    • Concealed: Occurs in 20% of cases and hemorrhage is confined to uterine cavity. The detachment may be complete and the consequences are severe. May be diagnosed sonographically.
    • External: Blood drains through the cervical os, and patient presents clinically with painful vaginal bleeding. Detachment is usually not as severe. If no blood remains in the retroplacental space, sonographic diagnosis is difficult.
  • Sonographic Findings:
    • Elevation of the placenta from the uterine wall
    • Retroplacental fluid collection of varying echogenicity (depending on age) - most likely hypoechoic
    • Placenta may appear normal
    • Placenta may appear thickened

Abnormal Adherence

  • Group of relatively uncommon conditions resulting from defective decidual formation, causing abnormal attachment of the placenta to the uterine wall.
  • Uterine scarring is a predisposing factor (the villi may grow into a C-section scar), and two-thirds are associated with placenta previa.
  • Due to associated maternal hemorrhage, hysterectomy may be necessary.
  • MRI may be useful in cases of suspected accreta/increta/percreta to ascertain extent of invasion into myometrium.
  • Three classifications, based on villi extension:
    • Placenta accreta - chorionic villi are in direct contact with the myometrium but do not invade
    • Placenta increta - chorionic villi invade the myometrium
    • Placenta percreta - chorionic villi penetrate/perforate the myometrium
  • Sonographic Findings:
    • Depends on type of pathology
    • Loss of normal hypoechoic retroplacental vascular complex
    • Focal basal plate thinning (accreta)
    • Increased myometrial thickness and echogenicity (increta)
    • Focal myometrial bulge (percreta)
    • Sonographic diagnosis is difficult
    • Color/power Doppler may be helpful

Umbilical Cord

  • The umbilical cord contains two arteries and one vein, surrounded by Wharton's jelly and enclosed in a layer of amnion.
  • The umbilical arteries are longer than the vein and are twisted around the vein, giving the cord its "braided" appearance.
  • The cord allows transport of blood between the fetus and the fetal portion of the placenta.
  • At term, the cord length is 50-60 cm (range 22-130 cm).
  • Insertion of umbilical cord is usually central.

Abnormalities of the Umbilical Cord

  • Single umbilical artery (SUA)/Bivascular cord:
    • SUA is the most commonly encountered umbilical cord abnormality (1% of pregnancies).
    • It may be caused by primary agenesis of umbilical artery or atrophy of a normally formed umbilical artery.
    • By itself, SUA does not put the fetus at risk. However, SUA may be associated with other fetal abnormalities.
    • A thorough fetal anatomic evaluation should be conducted in the presence of SUA.
    • Associated abnormalities include:
      • GU anomalies
      • Trisomies 18 and 13
      • Cardiovascular anomalies
      • CNS anomalies
      • Omphalocele
    • Sonographic Findings:
      • Absence of an umbilical artery (UA)
      • Single UA may be as large as umbilical vein
      • Two vessel cord in transverse section
      • Fetal anatomy should be thoroughly evaluated

Nuchal Cord

  • The wrapping of the umbilical cord around the fetal neck.
  • Present in one-fifth (20%) of all deliveries and rarely associated with fetal complications.
  • Color Doppler is useful to demonstrate nuchal cord.
  • Care must be taken to discern between cord draping over the neck and cord wrapped around the neck

Cord Prolapse

  • Umbilical cord protruding through cervix or adjacent to presenting fetal part (cervix open).
  • This is an emergent situation.

Vasa Previa

  • Cord crossing the cervical os, passing between the cervix and presenting fetal part with the membranes intact.
  • Associated with velamentous insertion.

Amniotic Fluid

  • Physiology and Production:
    • Amniotic fluid is produced by the fetal kidneys, tissues, skin and fetal membranes.
    • Amniotic fluid is removed from fetus by GI tract (to GU tract), lungs, membranes and cord.
  • The functions of amniotic fluid include:
    • Acting as a protective cushion
    • Equalization of pressure/temperature
    • Prevents adherence to membranes
    • Reservoir for fetal metabolites
    • Essential for development of fetal lungs
  • Sonographically amniotic fluid is anechoic, but may have echogenic foci floating (vernix caseosa, cellular debris).

Amniotic Fluid Index (AFI)

  • Uterus is divided into four quadrants, and vertical (AP) measurement of fluid in each quadrant is obtained.
  • Quadrants are added to obtain AFI.
  • A progressive increase in AFI is noted until approximately 28 weeks, after which time the AFI slowly decreases.
  • Charts for AFI values dependent on gestational age may be consulted, but as a guideline:
    • NORMAL AFI range 5 - 22 cm

Oligohydramnios

  • Oligohydramnios is an abnormally decreased amount of amniotic fluid.
  • Oligohydramnios is associated with:
    • Demise
    • Renal abnormalities
    • IUGR
    • Post dates
    • PROM
    • (Pre-eclampsia/eclampsia)
  • Anhydramnios is absence of any amniotic fluid.
  • oligohydramnios secondary to PROM

Polyhydramnios

  • Polyhydramnios (hydramnios) is an abnormally increased amount of amniotic fluid.
  • Polyhydramnios may be acute or chronic.
  • In addition to the AFI, single vertical pocket measurements may be used to evaluate polyhydramnios:
    • mild polyhydramnios - vertical pocket 8-12 cm
    • moderate - vertical pocket 12-16 cm
    • severe - vertical pocket > 16 cm
  • Causes of polyhydramnios:
    • maternal causes
      • gestational DM
      • Rh incompatibility
    • fetal causes
      • CNS anomalies
      • GI anomalies
      • facial clefts/masses
      • fetal hydrops
      • twin-to-twin transfusion
      • sacrococcygeal teratoma
      • skeletal dysplasia

Amniocentesis

  • Amniocentesis provides a method of retrieval of amniotic fluid for laboratory analysis.
  • The rate of complications for amniocentesis (i.e., pregnancy loss, infection) is 0.5%.
  • In the late second and third trimesters, amniocentesis can be performed to assess fetal lung maturity through the following tests:
    • Lecithin/ sphingomyelin ratio (LS ratio)- ratio greater than 2:1 indicates respiratory distress syndrome will be unlikely. This is the most accurate method to assess lung maturity.
    • Phosphatidylglycerol (PG) - appears at about the time of lung maturity (35 weeks), so presence of PG is associated with lung maturity.