Placenta and Placental Problems

Placenta is derived from both maternal and fetal tissue with approximately one fifth derived from fetal tissue at term. It comprises a large number of functional units called villi which are branched terminals of the fetal circulation allowing transfer of metabolic products.
At term, the normal placenta:

• Is blue-red in colour and discoid in shape.
• Is between 15-22cm in diameter.
• Is 2-4cm thick.
• Weighs 400-600g (15% normal neonatal weight).
• Has a maternal surface that is divided into lobules or cotyledons with irregular grooves or clefts.
• Has a smooth, shiny, translucent fetal surface covered in amniotic membrane.

The normal umbilical cord:

• Is 55-60 cm long and 2-2.5 cm in diameter.
• Should have abundant Wharton's jelly with no true knots.
• Contains 2 arteries and 1 vein.
• Can arise from any point on the fetal surface of the placenta.

Circumvallate

In approximately 1% of cases, there is a small central chorionic area inside a paler thick ring of membranes on the fetal side of the placenta. This is associated with an increased rate of antepartum bleeding, prematurity, abruption, multiparity and perinatal death.³

Succenturiate lobe

These are accessory lobes. Large torn vessels within the fetal membranes but beyond the edge of the delivered placenta are suggestive of an undelivered lobe and the uterus should be further explored for retrieval. Succenturiate lobes are associated with retained placenta and increased risk of postpartum infection and haemorrhage. They appear to be associated with increasing maternal age and are more common in women who have received IVF.⁴

Bipartite placenta

This is uncommon: the placenta appears as a bilobed structure joined by main vessels and membranes. If retained after birth, it can cause bleeding and septic complications. Again one should examine a small placenta for evidence of missing lobe and recover without delay.

Placenta membranacea

Failure of the chorion laeve to atrophy means that placental cotyledons form an envelope around the greater part of the uterine wall. This is associated with antepartum and postpartum haemorrhage as well as retained placenta.

Placenta in multiple pregnancy

Fraternal twins have either two distinct placentas or fused but there are always two distinct chorions and amnions. With identical twins the situation depends upon the timing of the division of the fertilised ovum: they can have two distinct placentas and sets of membranes or many different types of fusion with possible interchange of blood supply.

Placenta accreta/percreta/increta^2,5,6

These are conditions where the placenta is abnormally strongly attached to the uterine wall. Incidence is about 1/2,500 deliveries. All are associated with retained placenta requiring surgical removal and high risk of postpartum haemorrhage. It may be partial (accreta where there is diffuse penetration into the myometrium), more significant as the myometrium is deeply invaded (placenta increta) or even crossing the uterine wall and invading the peritoneum (placenta percreta).

Risk factors⁷

• Previous caesarean section
• Placenta praevia
• Advanced maternal age

The incidence of placenta accreta is thought to be increasing due to the rise in caesarean section deliveries.

Management

RCOG guidelines⁸ suggest:

• Where available, colour flow Doppler ultrasonography should be performed in women with placenta praevia to antenatally diagnose a morbidly adherent placenta. Where this is not available locally, they should be managed as if they have placenta accreta until proved otherwise.
• Where placenta accreta is thought likely, consultant anaesthetists and obstetricians should plan and manage the delivery. Crossmatched blood should be available. The risk of haemorrhage, transfusion and hysterectomy should be discussed with the patient as part of the consent process.

Repeated attempts to manually remove a placenta accreta can produce severe haemorrhage and the treatment in this circumstance is usually hysterectomy. Conservative management is sometimes applied where the preservation of fertility is paramount (leaving the placenta in place with or without therapeutic uterine artery embolisation or surgical internal iliac artery ligation or methotrexate therapy) but these may be complicated by delayed haemorrhage and the ultimate necessity of hysterectomy.

Placental abruption²

Separation of the placenta before delivery of the fetus occurs in approximately 1/77-89 deliveries causing bleeding from the placental bed of a normally situated placenta. A severe form, where >50% placenta is involved, occurs in 1/500-750 deliveries usually causing fetal death.
There are two main forms:

1. Concealed (20% of cases) - where haemorrhage is confined within the uterine cavity and is the more severe form.
2. Revealed (80%) - where blood drains through the cervix, usually with incomplete placental detachment and fewer associated problems.

Marginal haemorrhage occurs with a painless bleed and clot located along the margin of the placenta with no distortion of its shape. It is usually due to the rupture of a marginal sinus. Women should be admitted for observation and fetal monitoring.

Risk factors¹³

• Trauma (RTA or iatrogenic e.g. ECV)
• PET
• Multiparity
• Previous abruption
• Smoking
• Cocaine use
• Multiple pregnancy
• Thrombophilia
• Intrauterine infections
• Polyhydramnios

Presentation

• Usually with sudden abdominal pain and shock.
• Uterus feels hard, tender or 'woody'.
• Fetal parts are difficult to palpate.
• Where <1000 ml blood has been lost, the fetus is hypoxic and may show signs of fetal distress.
• Where >1500 ml blood has been lost, the woman is usually in shock and the fetus is dead.

Treatment is to restore blood volume and deliver baby immediately.

See separate article on Placenta Praevia.

Marginal insertion of cord (Battledore)

This occurs where the cord has a marginal rather than central insertion to the placenta. It is not of clinical significance.

Velamentous cord insertion and vasa praevia

Velamentous cord insertion is where the placenta has developed away from the attachment of the cord and the vessels divide in the membrane.
If the vessels cross the lower pole of the chorion, this is known as vasa praevia and there is high risk of fetal haemorrhage and death at rupture of membranes. Risk of vasa praevia is increased in:¹⁴

• IVF pregnancies
• Bilobate or succenturiate placenta
• Second-trimester placenta praevia

It can be diagnosed prenatally by ultrasound examination and good outcome depends on prenatal diagnosis and elective caesarean section prior to the rupture of membranes.⁵

Abnormal length of cord

• A long cord (>100 cm) is associated with increased risk of fetal entanglement, knots and prolapse of the cord.¹⁵
• A short cord (<40 cm) may be associated with a poorly active fetus, Down syndrome, cord rupture, breech position, prolonged second stage, uterine inversion and abruption. However, a short cord does not seem to impede vaginal delivery except where excessively short (<13 cm) in association with a fundal placenta.¹⁶
• A normal length cord may become relatively short because of multiple looping around the baby's neck.

Abnormal number of vessels

A single uterine artery is associated with increased risk of fetal anomalies, particularly trisomies,¹⁷ and cord compression.