Placenta and Cervix pathology

Placenta Praevia

Low lying and marginal placentas are identified  by measuring the distance from the edge of the placenta to the internal os. If the placenta covers the internal os it is called placenta praevia.

Placenta Praevia is defined as placental implantation that overlies or abuts the internal cervical os, and Low lying Placenta is defined as the lower placental edge measuring 20mm from the internal os on transvaginal imaging but not meeting the criteria to be classified as praevia.

Placenta Praevia

The placenta extends 30mm beyond the Int os from a posterior position.

This patient had vaginal bleeding and there is haemorrhage seen behind the placenta.

Vasa Praevia

Vasa Praevia is defined as foetal  vessels  running close to or across the internal os. They consist of  vessels which are within the membranes and not within the umbilical cord. This can be fatal as they are at risk of rupturing when the membranes rupture. Many women present with vaginal bleeding.

Risk factors include:

  1. low-lying placenta 
  2. placenta previa
  3. multiple pregnancies 
  4. multi-lobe placentas
  5. velamentous insertion 
  6. Placenta membranacea
  7. in-vitro fertilization 

Transvaginal imaging should be done to assess the vessels in B Mode and Colour Doppler.

This sagittal transvaginal image shows the vessels coursing across the internal os.

Close inspection of the cervix with colour doppler to identify maternal and fetal vessels can be the difference between life and death.

Placenta Accreta, Increta and Percreta

  • Placenta accreta, increta, or percreta are associated with major pregnancy complications, including
    life-threatening maternal haemorrhage, large-volume blood transfusion, and peripartum hysterectomy.
  • Risk factors for one of the three variants of abnormally invasive placentation include women who are multiparous, previous caesarean delivery, uterine surgery such as previous terminations , curettage, and manual removal of the placenta, and placenta praevia.
  • Antenatal diagnosis is associated with reduced levels of post partum haemorrhage.
  1. Placenta accreta is a process in which placental villi invade the surface of the myometrium.
  2. Placenta increta, is diagnosed if the placental villi extend into the myometrium;
  3. Placenta percreta, where the villi penetrate through the myometrium to the uterine serosa and may invade adjacent organs, such as the bladder. 

Signs include a reduction in the  myometrial thickness  < 1 mm (normal 4 ± 1 mm) or undetectable at the level of the inferior uterine segment, between the bladder wall and retroplacental vessels.

Placenta Accreta
  • Placenta accreta is a process in which placental villi invade the surface of the myometrium.

This is a normal myometrial placental interface show a clear distinction between the layers.

This is placenta accreta where the placenta is beginning to invade the myometrial wall.

Signs include loss of the retroplacental clear zone.

The normal hypoechoic retroplacental zone in the myometrium under the placental bed is not visible or irregular and there is increased subplacental vascularity. 

This image shows placenta accreta with increased vascularity in the retroplacental zone in multiple planes. 

Placenta Increta
  • Placenta increta, is diagnosed if the placental villi extend into the myometrium.

This case shows placenta praevia and a bulging placenta which are high risk factors for placental invasion into the myometrium. 

This patient had a previous caesarean and this is a precursor for placental invasion as the  scar stimulates implantation of the blastocyst at the scar site therefore there is abnormal adherence and as shown in the above image invasion of placental villi within the scar tissue. This was diagnosed in first trimester.

Placenta Percreta
  • Placenta percreta is where the villi penetrate through the myometrium to the uterine serosa and may invade adjacent organs, such as the bladder.

Signs include a reduction in the  myometrial thickness  < 1 mm (normal 4 ± 1 mm) or undetectable at the level of the inferior uterine segment, between the bladder wall and retroplacental vessels.

A sign of invasion includes intraplacental lacunae which are numerous, large, irregular sonolucent intraplacental spaces containing turbulent flow.

This case shows interruption of the bladder flap, with the bladder wall bulging and extending beyond uterine serosa.

Cervical Shortening

It is extremely important to assess the cervical length as a shortened cervix puts the mother at a high risk for for preterm delivery.

As the pregnancy progresses the length changes.

Saloman et.al (2009) developed a model for singleton pregnancies after assessing 6614 cervical length measurements across multiple gestational ages. These results are shown in the graph and table below.

Reference: Salomon, L.J., Diaz-Garcia, C., Bernard, J.P. and Ville, Y. (2009), Reference range for cervical length throughout pregnancy: non-parametric LMS-based model applied to a large sample. Ultrasound Obstet Gynecol, 33: 459-464. https://doi.org/10.1002/uog.6332

Close monitoring of the cervix is warranted in all pregnancies, however if there is a history of premature rupture of membranes (PROM) multiple cervical lengths measurement should be routinely done at least every 1-2 weeks.

The length should be measured transvaginally from the internal os to the external os.

Cervical Dilatation

The cervical length is shortened and funnelled.

This image shows the internal os is open and a funnelling effect is seen with the membranes bulging into the cervical canal.

Measurements should be done of the closed portion of the cervix, the open portion of the cervix and the width of the opening across the int. os.

These 2 images show the marked difference between a transabdominal and and a transvaginal image of an open cervix. The TV scan is more accurate and shows the dire situation more accurately.