Placental thickness

Placental thickness tends to gradually increase with gestational age in a linear fashion. Sonographically, this can be seen to be approximately 1 mm per week and the thickness of the placenta can be used to approximate gestational age:

• approximate gestational age (in weeks) = placental thickness +/- 10 mm

The maximum thickness of a normal placenta at any point during pregnancy is often considered to be 4 cm. Anterior placentas are ~0.7 cm thinner than posterior placentas and maximum thickness for an anterior placenta is ~3.3 cm ⁷.

Increased thickness

An abnormally increased placental thickness falls under the spectrum of placentomegaly. This can happen with a number of conditions and is associated with increased risk of placental insufficiency. Causes include:

• upper limit of normal variation
• fetal macrosomia
• fetal hydrops
• TORCH infections
• maternal medical conditions
  • maternal anemia
  • maternal diabetes

If the placenta is thickened and contains cysts, then other entities should be considered:

• partial molar pregnancy
• triploidy
• placental mesenchymal dysplasia
  • Beckwith-Wiedemann syndrome

Mimics

An important mimic for a thickened placenta is an isoechoic abruption. A transient myometrial contraction can also mimic a thickened placenta.

Decreased thickness

An abnormally decreased placental thickness can be seen with:

• pre-eclampsia
• intrauterine growth restriction (IUGR)
• placenta membranacea (extremely rare)