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The principal modes are growth within the uterine wall, towards the cavity, and towards the outer surface: intramural uterine fibroids, submucosal uterine fibroids and subserosal uterine fibroids. “Fibrous tumour” is the historical label; the usual modern lesion is a leiomyoma, a benign growth of smooth muscle with a variable connective-tissue component, rather than a mass of collagen alone.^[ref]

An intramural uterine fibroid enlarges within the myometrium. It can remain enclosed by uterine muscle, push the cavity aside, or expand both inner and outer contours. Several nodules can give an irregularly enlarged uterus. Expansion compresses surrounding tissue into a cleavage plane or pseudocapsule; this differs from the destructive infiltrative growth typical of a malignant lesion.

A submucosal uterine fibroid grows towards the endometrium, projecting into and distorting the uterine cavity. It may remain broad-based or acquire a stalk as the surrounding wall is displaced. A pedunculated uterine fibroid can be pushed through the cervical canal and appear in the vagina as a uterine polyp. A polyp formed in this way is a mode of presentation of the same smooth-muscle tumour, not necessarily a different histological species. Its relation to the cavity helps explain why a comparatively small lesion may be associated with substantial bleeding.

A subserosal uterine fibroid projects outwards beneath the serosal covering. It may have a broad attachment or a long pedicle and become a large extrauterine-looking mass. Lateral outward growth can extend between the layers of the broad ligament; cervical growth can displace neighbouring pelvic structures. Rarely, a detached pedunculated lesion gains a blood supply from adjacent tissues and persists as a parasitic growth; this is not ordinary distant metastasis.^[ref]

The three directions are not mutually exclusive: a large nodule may affect both the uterine cavity and its external surface. Modern location classifications describe its relation to endometrium, myometrium and serosa more precisely, but those later type numbers are unnecessary to explain the anatomical modes requested here.

Growth is also distinct from secondary change. A poorly supplied area can undergo hyaline or cystic degeneration, haemorrhagic change or calcification; these changes alter consistency without establishing a new malignant mode of growth. Uterine fibroids often respond to the reproductive hormonal environment and commonly shrink after menopause, but neither every lesion's course nor its rate of growth is identical.^[ref] Location and attachment describe the mode of expansion; tissue examination establishes the nature of the tumour.

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.

Abnormal adherence of the placenta to, or invasion into, the myometrium, associated with deficiency of the normal intervening decidua. Attempted forced separation may cause severe haemorrhage.