The normal placenta
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 15-22 cm in diameter.
- Is 2-4 cm thick.
- Weighs 400-600 g (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.
Abnormalities of shape, size and surfaces
- 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.
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 in vitro fertilisation (IVF).
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.
- 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.
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).
- It is associated with preterm delivery - with 40% of women delivering before 38 weeks of gestation.
- 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.
Women who have had a previous Caesarean section who also have either placenta praevia or an anterior placenta underlying the old Caesarean section scar at 32 weeks of gestation, are at increased risk of placenta accreta and should be managed as if they have placenta accreta, with appropriate preparations for surgery made:
- Antenatal imaging techniques, eg grey scale/colour flow Doppler/3D ultrasonography that can help to raise the suspicion of a morbidly adherent placenta, should be considered in any situation where any part of the placenta lies under the previous Caesarean section scar, but the definitive diagnosis can be made only at surgery.
- In response to the last confidential enquiry and in recognition of the severe morbidity associated with the condition, a 'care bundle' for placenta praevia accreta has been devised:
- The consultant obstetrician has planned and is directly supervising the delivery.
- A consultant anaesthetist has planned and is directly supervising anaesthetic at delivery.
- Blood and blood products are available. Women who decline blood products should be transferred to a centre where cell salvage and interventional radiology are available.
- There has been multidisciplinary involvement in preoperative planning.
- There has been adequate discussion and consent includes possible interventions (such as hysterectomy, leaving the placenta in place, cell salvage and intervention radiology).
- Local availability of a level 2 critical care bed has been confirmed.
- Elective delivery by Caesarean section in asymptomatic women is not recommended before 36-37 weeks of gestation.
- Surgeons performing the Caesarean section should consider opening the uterus at a site distant from the placenta, and delivering the baby without disturbing the placenta. Going straight through the placenta to achieve delivery is associated with more bleeding and a high chance of hysterectomy and should be avoided.
Repeated attempts to remove a placenta accreta manually 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.
If the placenta is left in situ the woman should be warned of the risks of bleeding and infection postoperatively and prophylactic antibiotics may be helpful in the immediate postpartum period to reduce this risk.
See separate Retained Placenta article.
Separation of the placenta before delivery of the fetus. It is an important cause of perinatal mortality and morbidity (perinatal mortality rate is approximately 15%.) The maternal effect of abruption depends primarily on its severity, whereas its effect on the fetus is determined both by its severity and the gestational age at which it occurs.
It accounts for 30% of all cases of antepartum haemorrhage:
- The normal placenta separates from the uterus prematurely and blood collects between the placenta and the uterus.
- It is estimated to occur in 6.5 pregnancies per 1,000 births.
- 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.
- 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.
These include: 
- Trauma (road traffic accident or iatrogenic, eg external cephalic version).
- Previous abruption.
- Previous Caesarean section.
- Cocaine use.
- Multiple pregnancy.
- Intrauterine infections.
- May present with vaginal bleeding, abdominal pain, uterine contraction, shock or fetal distress.
- May not be demonstrable on ultrasound, as the blood clot is not easily distinguishable from the placenta.
- Moderate placental detachment and haemorrhage: at least one quarter of the placenta has become detached and less than 1,000 ml of blood lost. Abdominal pain and a tender uterus, the mother may be in shock, the fetus is hypoxic and may show abnormal heart rate patterns.
- Severe placental detachment and haemorrhage: at least 1,500 ml of blood lost, shock is usual, the uterus is firm-to-hard and very tender. The fetus is almost always dead. There is hypotension in one third of cases but may be normal in spite of shock. Coagulopathy is common.
- Restore blood loss, prevent coagulopathy, monitor urinary output. In moderate cases, give 1,500 ml of blood and, in severe cases, give 2,500 ml (the first 500 ml transfused rapidly). Ideally, measure central venous pressure (CVP) and adjust transfusion accordingly.
- Measure venous blood for coagulation 2-hourly; treat accordingly.
- Measure urine output 2-hourly. Oliguria may occur but, if sufficient blood has been given, then diuresis will follow birth.
- If the fetus is alive, perform either Caesarean section or artificial rupture of the amniotic membranes (restore blood volume first). 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.
See separate Placenta Praevia article.
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. Vasa praevia can be accurately diagnosed with transvaginal colour Doppler ultrasound. Risk of vasa praevia is increased in:
- IVF pregnancies.
- Bilobate or succenturiate placenta.
- Second-trimester placenta praevia.
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 cord
- A long cord (>100 cm) is associated with increased risk of fetal entanglement, knots and prolapse of the cord.
- 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 placenta.
- A normal length cord may become relatively short because of multiple looping around the baby's neck.
Abnormal number of vessels
Further reading & references
- Yetter JF 3rd; Examination of the placenta.; Am Fam Physician. 1998 Mar 1;57(5):1045-54.
- Suzuki S; Clinical significance of pregnancies with circumvallate placenta. J Obstet Gynaecol Res. 2008 Feb;34(1):51-4.
- Suzuki S, Igarashi M; Clinical significance of pregnancies with succenturiate lobes of placenta. Arch Gynecol Obstet. 2008 Apr;277(4):299-301. Epub 2007 Oct 16.
- Placenta Praevia, Placenta Praevia Accreta and Vasa Praevia: Diagnosis and Management; Royal College of Obstetricians and Gynaecologists (January 2011)
- Wu S, Kocherginsky M, Hibbard JU; Abnormal placentation: twenty-year analysis.; Am J Obstet Gynecol. 2005 May;192(5):1458-61.
- Saving Mothers' Lives. Reviewing maternal deaths to make motherhood safer: 2006-2008; Centre for Maternal and Child Enquiries (CMACE), BJOG. Mar 2011
- Weeks AD; The retained placenta. Best Pract Res Clin Obstet Gynaecol. 2008 Sep 13.
- Gaufberg SV; Abruptio placentae, eMedicine, Dec 2008
- El-Mowafi D; Bleeding in Late Pregnancy (Antepartum Haemorrhage), Geneva Foundation for Medical Education and Research, 2008
- Neilson JP; Interventions for treating placental abruption. Cochrane Database Syst Rev. 2003;(1):CD003247.
- Oyelese Y, Ananth CV; Placental abruption. Obstet Gynecol. 2006 Oct;108(4):1005-16.
- Yang Q, Wen SW, Oppenheimer L, et al; Association of caesarean delivery for first birth with placenta praevia and BJOG. 2007 May;114(5):609-13. Epub 2007 Mar 12.
- Papp Z; Massive obstetric hemorrhage. J Perinat Med. 2003;31(5):408-14.
- Baulies S, Maiz N, Munoz A, et al; Prenatal ultrasound diagnosis of vasa praevia and analysis of risk factors. Prenat Diagn. 2007 Jul;27(7):595-9.
- Oyelese Y, Smulian JC; Placenta previa, placenta accreta, and vasa previa.; Obstet Gynecol. 2006 Apr;107(4):927-41.
- Baergen RN, Malicki D, Behling C, et al; Morbidity, mortality, and placental pathology in excessively long umbilical cords: retrospective study. Pediatr Dev Pathol. 2001 Mar-Apr;4(2):144-53.
- LaMonica GE, Wilson ML, Fullilove AM, et al; Minimum cord length that allows spontaneous vaginal delivery. J Reprod Med. 2008 Mar;53(3):217-9.
- Lubusky M, Dhaifalah I, Prochazka M, et al; Single umbilical artery and its siding in the second trimester of pregnancy: relation to chromosomal defects. Prenat Diagn. 2007 Apr;27(4):327-31.
|Original Author: Dr Chloe Borton||Current Version: Dr Hayley Willacy|
|Last Checked: 18/03/2011||Document ID: 739 Version: 23||© EMIS|
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