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A trained technique for placental delivery using traction on the umbilical cord with counterpressure supporting the uterus, under appropriate third-stage conditions.

The amnion, allantois and yolk sac are distinct extraembryonic structures whose relations change as the embryo folds and the umbilical cord forms. The historical umbilical vesicle is the yolk sac; its duct must not be identified with the allantoic channel.

Early in development the embryonic disc lies between the amniotic cavity dorsally and the yolk sac ventrally. The amnion has an epithelial lining facing its fluid and an outer supporting mesodermal component. The yolk sac has an endodermal lining and an external splanchnic mesodermal covering. Outside them is the chorionic cavity, interrupted by the connecting stalk linking the embryonic region to the chorion. The chorion consists of trophoblastic tissues and extraembryonic somatic mesoderm; it is not simply another name for the amnion or an outgrowth of yolk-sac endoderm.

The human allantois appears as a small endodermal diverticulum from the caudal yolk-sac region and extends into the connecting stalk. With incorporation of the caudal region into the embryo it becomes related to the cloaca and developing urinary bladder. Its intraembryonic remnant is associated with the urachus, later represented by the median umbilical ligament. Unlike the large allantoic sac of a bird, the human allantois is not an expanding external urine reservoir or an independent aerial respiratory organ.^[ref]

Embryonic folding converts part of the original endodermal roof of the yolk sac into the gut tube. The remaining sac lies outside the embryo and communicates with the midgut through the narrowing vitelline duct. Vitelline vessels accompany its early vascular apparatus. The sac supports early nutritional and blood-forming functions even though it contains no large yolk store in humans. Its connection normally regresses; persistence of a vitelline duct is a different anomaly from persistence of the urachus. The presence of a duct alone does not make the two structures interchangeable.^[ref]

As the amnion expands, it surrounds the embryo more completely and encloses the roots of the connecting stalk and yolk-sac connection at the umbilical region. Those structures are brought together within an amniotic covering to form the early umbilical cord. The cord initially includes the stalk with allantoic remnants and developing umbilical vessels, together with the vitelline connection and associated structures. The mature cord chiefly contains two umbilical arteries and one umbilical vein embedded in Wharton's jelly, beneath an amniotic surface. The vessels arise within vascular mesodermal tissues; it would be inaccurate to say that the endodermal allantoic tube simply turns into all three blood vessels.

The amnion subsequently comes into apposition and fuses with the chorion, greatly reducing the intervening chorionic cavity. Its surface continues along the cord towards the fetal placental surface. The yolk sac remains outside the amniotic cavity rather than becoming a normal free object floating in amniotic fluid. The mature amnion is avascular, whereas the placental chorion and the cord have crucial fetal vascular connections.^[ref]

These relations can be understood by comparing developmental stages in section: first a disc between two cavities, then a folded embryo with narrowed ventral connections, and finally an expanded amniotic covering around the cord. A section through a later cord cannot by itself reveal every early channel that has regressed. The same membrane names are used across vertebrates, but their relative sizes and functions are not identical in a human fetus and a yolk-rich bird egg.

After the fetus is born, contraction and retraction of the myometrium reduce the area of the placental bed. The placenta cannot shrink with it and normally separates through the maternal decidua. Continued contraction compresses the maternal vessels crossing the myometrium, providing an important mechanism of haemostasis. A fragment left attached can both expose bleeding vessels and interfere with effective contraction.

Three mechanisms organise the causes:

Consequences include immediate or delayed postpartum haemorrhage, hypovolaemic shock, anaemia, endometritis and sepsis. Retained fragments may delay normal shrinking of the uterus. Forceful pulling on the umbilical cord adds the risks of cord rupture and uterine inversion. Conversely, indiscriminate delay while bleeding continues can be fatal.

An 1867 answer would assess bleeding, the size and contraction of the uterus, whether separation had occurred and whether the outlet was obstructed. Emptying the urinary bladder, encouraging effective contraction and supporting the uterus were rational preliminary principles. If the placenta remained attached or trapped and the circumstances demanded intervention, manual removal with external support of the uterus was the principal operative resource. The historical difficulty was to distinguish a removable attachment from a dangerously adherent one. A September 1867 report describes an apparent “hour-glass” retention actually relieved by loosening a tight abdominal binder; it is useful evidence for examining the mechanism rather than trusting the diagnostic label.^[ref] The case is an observation, not proof that all retention has an external cause.

Current care monitors blood loss and maternal condition, secures intravenous access and obtains skilled obstetric help. With active third-stage management, failure to complete the third stage of labour by 30 minutes is prolonged; with physiological management, the cited NICE threshold is 60 minutes. These thresholds do not justify waiting through active haemorrhage or instability. A trained clinician may use controlled cord traction with uterine counterpressure under appropriate conditions; unopposed pulling is not the same technique.^[ref]

When removal is required, examination and manual removal take place with appropriate anaesthesia, aseptic technique, external support and readiness to manage haemorrhage. The placenta and membranes are checked for completeness; persistent bleeding prompts assessment for retained fragments, uterine atony, trauma and disordered clotting. Antibiotic prophylaxis accompanies manual removal. Give uterotonics for associated postpartum haemorrhage and to maintain contraction after removal, rather than repeatedly giving them as a substitute for removing a retained organ in a non-bleeding patient. WHO's 2025 recommendations advise preparing for manual removal after 30 minutes if the placenta remains undelivered and distinguish this decision from treatment of established postpartum haemorrhage.^[ref]

If no safe separation plane exists and placenta accreta spectrum is suspected, repeated forceful attempts to peel off the placenta can precipitate catastrophic haemorrhage. Escalate to an experienced multidisciplinary team and a haemorrhage plan. Definitive surgery, including hysterectomy, may be necessary; selected conservative management leaving the placenta in place requires specialist selection and follow-up. Current guidance explicitly advises avoiding attempted separation in women with a high probability of placenta accreta spectrum.^[ref] Thus the word “retained” identifies an urgent clinical problem, but the cause determines whether ordinary manual separation is safe.