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Extracted from Survival Guide to Midwifery, 2nd Edition (2012) Diane M. Fraser and Margaret A. Cooper, Oxford; Churchill Livingstone: 2012. Courtesy Elsevier
The placenta is the flattened circular organ in the uterus of pregnant women (and indeed other mammals) which links closely to the mother's circulation to carry out functions that the fetus is unable to perform for itself during intrauterine life. It provides oxygen and nutrients to the fetus, and removes waste products such as carbon dioxide, returning them to the maternal circulation for elimination. The placenta also produces hormones that aid the growth and development of the fetus, and provides some protection against bacterial infection. At term the placenta is about 20 cm in diameter, and 2.5 cm thick. The maternal surface is arranged in about 20 cotyledons (lobes) made up of lobules, each of which contains a single villus with its branches. The fetal surface is covered in amnion, which gives it a white, shiny appearance. Branches of the umbilical vein and arteries are visible, spreading out from the insertion of the umbilical cord.
Delivery of the placenta
During the third stage of labour, separation and expulsion of the placenta and membranes occurs. The placenta may sheer off during the final expulsive contractions accompanying the birth of the baby, or remain adherent for some considerable time. The third stage usually lasts between 5 and 15 minutes, but any period up to 1 hour may be considered within normal limits. On delivery of the placenta and membranes (products of conception), they should be examined as soon as possible so that, if there is any doubt about their completeness, further action can be taken. Retained products of conception are one of the main causes of postpartum haemorrhage and infection.
A diagnosis is reached when the placenta remains undelivered after a specified period of time (up to 1 hour following the baby's birth). Conventional treatment is to separate the placenta from the uterine wall digitally, effecting a manual removal.
Placenta accreta: abnormally adherent placenta into the muscle layer of the uterusPlacenta increta: abnormally adherent placenta into the perimetrium of the uterusPlacenta percreta: abnormally adherent placenta through the muscle layer of the uterusPlacenta praevia: A condition in which some or all of the placenta is attached in the lower segment of the uterusPlacental abruption (Abruptio placentae): Premature separation of a normally situated placenta. The term is normally used from viability (24 weeks). Any woman with a history suggestive of placental abruption needs urgent medical attention.
Royal College of Midwives (2012). Evidence Based Guidelines for Midwifery-Led Care in Labour: Third stage of labour. https://www.rcm.org.uk/sites/default/files/Third%20Stage%20of%20Labour.pdf
NICE (2014) Intrapartum care for healthy women and babies. Clinical guideline 190. https://www.nice.org.uk/guidance/cg190
Royal College of Obstetricians & Gynaecologists (2011). Placenta praevia, placenta praevia accrets and vasa praevia: diagnosis and management. Green-top guideline No 27. https://www.rcog.org.uk/globalassets/documents/guidelines/gtg_27.pdf