Placenta problems

What is the placenta?^{1 2}

The placenta provides for exchange of nutrients, oxygen and waste products between the mother and fetus, as well as being an endocrine organ for the synthesis of hormones and neurotransmitters and a barrier to toxins and infection. It 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 about 22 cm in diameter.

• Is 2.5 cm thick in the centre.

• Weighs, on average, 450 g but weight will be affected by when and where the cord was clamped.

• Has a maternal surface that is divided into lobules or cotyledons with irregular grooves or clefts.

• Has a smooth, shiny, translucent fetal surface, the chorionic plate, covered in amniotic membrane.

• Has a basal plate which is the maternal surface and is created at the separation of the placenta from the uterine wall at delivery.

The normal umbilical cord³ :

• Is 51-60 cm long and 2-2.5 cm in diameter.

• Should have abundant Wharton's jelly with no true knots.

• Contains two umbilical arteries and one umbilical vein.

• Can arise from any point on the fetal surface of the placenta; it usually arises in the centre or just off-centre.

• Has a length not associated with length, weight or gender of the baby.

COVID-19

There has been a case report of a woman with COVID-19 in the second trimester of pregnancy, who had severe hypertension, coagulopathy, and pre-eclampsia⁴ . This case demonstrated clear SARS-CoV-2 invasion of the placenta, with associated placental inflammation distinct from typical pre-eclampsia and highlighting the potential for severe morbidity among pregnant women with COVID-19.

Placenta abnormalities - shape, size, surfaces and function⁵

Circumvallate⁶

• 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.

Succenturiate lobe⁶

• These are accessory lobes that develop in the membrane some distance from the periphery of the main placenta.

• Occurs in 1.7% of pregnancies, two thirds of which also have velamentous cord insertion (see under 'Abnormalities of the cord', below).

• 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 in vitro fertilisation (IVF).

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.

Placenta membranacea⁷

• Failure of the chorion laeve to atrophy during the development of the placenta 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 a fused placenta 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.

Pathological lesions⁵

• Lesions of the placenta that are consistent with maternal vascular underperfusion are consistently associated with stillbirth and neonatal mortality. In surviving children there is a significant association with neurological impairment.

• Obstructive vascular lesions (fetal thrombotic vasculopathy) are strongly associated with neonatal morbidity, including necrotising enterocolitis and fetal cardiac abnormalities.

• Ascending uterine infection (amniotic infection syndrome) is also strongly associated with neonatal morbidity.

• It is recommended that paediatricians access the results of the histological examination of the placenta, as this may help explain poor neonatal outcomes and may have implications for treatment.

Placental accreta spectrum (PAS)⁸

The attachment abnormalities placenta praevia and retained placenta are dealt with in detail in their separate articles Placenta Praevia and Retained Placenta.

PAS includes conditions where the placenta is morbidly attached to the uterine wall to an increasing degree:

• Placenta accreta: chorionic villi penetrate the decidua basalis to attach to the myometrium.

• Placenta increta; the villi penetrate deeply into the myometrium.

• Placenta percreta: the villi breech the myometrium into the peritoneum.

They are all associated with retained placenta requiring surgical management and have high risk of massive postpartum haemorrhage. They may be partial with some parts of the placenta unaffected.

Epidemiology⁹

• Incidence is about 1/2,500 deliveries.

• They are associated with preterm delivery: 40% of women deliver before 38 weeks of gestation; caesarean should be planned for 36-37 weeks.

Risk factors for placenta accreta spectrum

• Previous caesarean section:

  • 0.24% risk if no previous caesarean section.

  • 0.31% risk if one previous caesarean section, rising to 6.74% if five previous caesarean sections.

• Placenta praevia.

• Advanced maternal age.

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

Placenta accreta spectrum treatment⁸
Women who are at increased risk of placenta accreta should have ultrasound imaging to make a diagnosis. Women with a history of previous caesarean section seen to have an anterior low-lying placenta or placenta praevia at the routine fetal anomaly scan should be specifically screened for placenta accreta spectrum. Additionally those with suggestive features on routine scanning should have further scanning by someone with expertise in this area. Women with a diagnosis should be referred to a specialist unit for management.

Delivery for women diagnosed with placenta accreta spectrum should take place in a specialist centre with logistic support for immediate access to blood products, adult intensive care unit and neonatal intensive care unit by a multidisciplinary team with expertise in complex pelvic surgery.

NB: repeated attempts to remove a placenta accreta manually produce massive haemorrhage requiring emergency hysterectomy in 100% of cases. It is left in place and either conservative management is followed or immediate hysterectomy is undertaken.

Conservative management

• The placenta is left in place with or without therapeutic uterine artery embolisation, surgical internal iliac artery ligation or methotrexate therapy.

• If followed by elective hysterectomy, this is associated with less blood loss than hysterectomy at the time of caesarean section.

• May be applied where the preservation of fertility is paramount.

• May be complicated by delayed haemorrhage, sepsis and the ultimate necessity for hysterectomy.

Placental abruption

Abruption is the premature separation of a normally placed placenta before delivery of the fetus, with blood collecting between the placenta and the uterus. Oxygen supply to the fetus is compromised and maternal blood loss may be significant in affected women.

The aetiology is unknown but is possibly part of a wider placental syndrome caused by underlying vascular pathology associated with defective deep placentation. It is one of the two most important causes of antepartum haemorrhage (the other being placenta praevia), and it affects 0.3-1% of pregnancies¹⁰ .

• It is an important cause of perinatal mortality: in a study of 7.5 million singleton births in the USA, perinatal mortality rate was 11.9% with abruption compared with 0.8% among all other births¹¹ .

• It is estimated to occur in 6.5 pregnancies per 1,000 births.

• Its effect on the fetus depends on its severity and the gestational age at which it occurs

• Its effect on the mother is dependent on its severity.

• The cause of placental abruption is unknown.

There are two main forms:

• Concealed (20% of cases) - where haemorrhage is confined within the uterine cavity and is the more severe form. The amount of blood lost is easily underestimated.

• 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.

Three deaths due to placental abruption were reported to the UK and Ireland Confidential Enquiry into Maternal Deaths between 2016-2018¹² . Both died of catastrophic haemorrhage. NB: women who have had a placental abruption are also at increased risk of postpartum haemorrhage.

Risk factors for placental abruption^{10 13}

There are recognised factors that increase the risk - these include:

• Previous abruption carries the highest risk of abruption in current pregnancy.

• Multiple pregnancy: twice as common with a twin pregnancy than with a singleton.

• Trauma:

  • Road traffic accident¹⁴ .

  • Domestic violence.

  • Iatrogenic - eg, external cephalic version.

• Pre-eclampsia.

• Hypertension.

• Multiparity.

• Previous caesarean section¹⁵ .

• Non-vertex presentations.

• Cocaine or amfetamine use during pregnancy.

• Thrombophilia.

• Intrauterine infections.

• Polyhydramnios.

Abruption is a sudden unexpected obstetric emergency that usually occurs in pregnancies without any risk factors and so cannot be predicted in most cases.

Placental abruption symptoms and presentation

May present with vaginal bleeding, abdominal pain (usually continuous), uterine contractions, shock or fetal distress¹⁶ .

Diagnosis¹⁷

Abruption is a clinical diagnosis with no available sensitive or reliable diagnostic tests.

• A tense, tender uterus with a 'woody' feel on abdominal examination suggests a significant abruption.

• Ultrasound is not reliable in identifying abruption, as the blood clot is not easily distinguishable from the placenta. However, if ultrasound suggests abruption, the likelihood of an abruption is high.

• Fetal hypoxia due to an abruption will lead to heart rate abnormalities seen on cardiotocograph (CTG).

• Platelet count, if low, may suggest significant abruption. Coagulation screen should be checked, as coagulopathy is common and should be anticipated.

• Depending on the degree of detachment and the amount of blood loss, the mother may be collapsed and the fetus hypoxic or already dead.

• NB: blood pressure may be normal, even with massive haemorrhage, as fit healthy women can tolerate significant loss prior to showing signs of decompensation¹⁸ .

Placental abruption treatment^{17 13}

See also the separate article Antepartum Haemorrhage.

The mother should be resuscitated and stabilised before any decision is made regarding delivery of the baby, regardless of the gestation. Surprisingly, a Cochrane review has found no trials to inform management¹⁶ .

Guidance from the Royal College of Obstetricians and Gynaecologists for moderate or severe placental abruption is to follow ABCD of resuscitation:

• Assess Airway and Breathing: high-flow oxygen.

• Evaluate Circulation:

  • Intravenous access, FBC, coagulation screen, U&E, Kleihauer test, crossmatch four units.

  • Position in the left lateral position tilted and keep the woman warm.

  • Until blood is available, infuse up to 2 litres of warmed crystalloid Hartmann's solution and/or 1-2 litres of colloid as rapidly as required.

  • With continuing massive haemorrhage and whilst awaiting coagulation studies and haematology advice, up to 4 units of fresh frozen plasma (FFP) and 10 units of cryoprecipitate may be given empirically.

  • Ideally, measure central venous pressure (CVP) and adjust transfusion accordingly.

• Assess fetus and Decide on Delivery:

  • If the fetus is alive, perform either caesarean section or artificial rupture of the amniotic membranes. Monitor the fetus and switch to caesarean if fetal distress develops.

  • Vaginal delivery is the treatment of choice in the presence of a dead fetus, although if the abruption is massive, caesarean may occasionally be indicated to control haemorrhage¹⁸ .

  • If bleeding has settled and delivery is not imminent, maternal steroids may be indicated in order to promote fetal lung development and reduce the risk of respiratory distress syndrome and intraventricular haemorrhage¹⁹ .

Complications

Abruption has been found to be associated with elevated risk of caesarean delivery, postpartum haemorrhage and transfusion, preterm birth, intrauterine growth restriction or low birth weight, perinatal mortality, and cerebral palsy^{20 21} .

Prevention

A 2020 meta-analysis showed that aspirin at a daily dose of ≥100 mg (for prevention of pre-eclampsia) that is initiated at ≤16 weeks of gestation, rather than >16 weeks, may decrease the risk of placental abruption or antepartum haemorrhage²² .

Abnormalities of the umbilical cord²

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 the term for 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. Vasa praevia is uncommon in the general population with a prevalence ranging between 1 in 1,200 and 1 in 5,000 pregnancies, although the condition may have been under-reported²³ .

If suspected, vasa praevia can be accurately diagnosed with transvaginal colour Doppler ultrasound. Risk of vasa praevia is increased in²⁴ :

• Placenta praevia

• Velamentous cord insertion

• Bi-lobed placenta

• Succenturiate lobe

• Assisted reproduction

• Multiple pregnancy

In the presence of bleeding vasa praevia, delivery should be achieved by emergency caesarean section. Elective caesarean section prior to the rupture of membranes should be performed when vasa praevia is diagnosed before labour.

Abnormal length of umbilical cord

• A long cord (>100 cm) is associated with increased risk of fetal entanglement, knots and prolapse of the cord, which in turn are associated with poor fetal outcome and an increased risk of intrauterine death²⁵ .

• A short cord (<40 cm) may be associated with a poorly active fetus, Down's 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 placenta²⁶ .

• 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 and cord compression²⁷ .