Cesarean Scar Niche
A caesarean scar niche is a pouch or indentation in the wall of the uterus at the site of a previous caesarean section, where the incision has healed with a defect rather than sealing completely. It is extremely common—found in well over half of women who have had a caesarean when specifically looked for—and most cause no problems at all. A minority produce a recognisable set of symptoms, most characteristically bleeding or brown discharge that continues for days after a period has otherwise finished, caused by old blood collecting in the pouch and draining slowly. The condition is often missed because the symptoms are vague and because it is only visible on imaging when someone is looking for it.
What is it?
During a caesarean section, an incision is made through the lower part of the uterus and then closed with sutures. In most women this heals well, but in many the healing is incomplete, leaving a wedge-shaped or pouch-like gap in the inner surface of the uterine wall at the scar. This is a caesarean scar niche, also called an isthmocele, a uterine scar defect, or a caesarean scar defect—the multiplicity of names reflects how recently the condition has been consistently recognised.
The niche is a pocket that opens into the uterine cavity. The muscle behind it is thinner than the surrounding uterine wall, and this remaining thickness—the residual myometrial thickness—is the measurement that most influences management decisions, since it determines both the strength of the scar and which surgical approaches are feasible.
The mechanism producing symptoms is straightforward once the anatomy is understood. During a period, menstrual blood passes through the cavity on its way out, and some collects in the pouch. Because the niche is a blind recess and the surrounding tissue does not contract to expel it, this trapped blood drains slowly over the following days. The result is bleeding or brown discharge continuing after the period itself has finished—the characteristic symptom of this condition, and one that many women describe as periods that simply never quite stop.
That same accumulation of old blood and fluid explains the proposed effect on fertility. Persistent fluid in the lower uterus may impair sperm transport, alter the environment in which an embryo would implant, and interfere with implantation directly. The association with reduced fertility is reasonably well described, though establishing causation is more difficult, since women with niches have by definition had a caesarean and may have other reasons for secondary infertility.
Several factors influence whether a niche forms and how large it is.
The number of previous caesareans is the strongest, with prevalence and size both increasing with each operation.
The circumstances of the caesarean matter considerably. An operation performed during advanced labour, when the lower segment is thinned and stretched and the cervix is substantially dilated, tends to produce an incision placed lower than intended—sometimes into cervical rather than uterine tissue, which heals differently and less reliably.
Surgical technique has been examined extensively. Closure of the uterine incision in two layers rather than one is associated with better healing and thicker residual muscle in most studies, though evidence is not entirely uniform and practice varies. Whether the full thickness of the muscle is included in the closure, the suture material used, and whether the incision edges are properly aligned all appear to contribute.
A retroverted uterus—one that tilts backward—is associated with larger niches, thought to result from differences in tension on the healing scar.
Factors affecting wound healing generally, including diabetes, obesity, smoking, and infection after surgery, are also implicated.
Most niches cause no symptoms whatsoever, and this is worth establishing clearly, because the condition is common enough that finding one explains nothing on its own.
Where symptoms occur, postmenstrual spotting is the most characteristic and the most useful diagnostically. Bleeding or brown discharge continuing for days after the period has finished, in a woman who has had a caesarean, is a pattern that should raise the possibility directly. It is frequently attributed to hormonal irregularity, contraception, or stress for extended periods before anyone considers the scar.
Chronic pelvic pain and painful periods occur in a proportion, thought to relate to the retained blood and to associated inflammation or adenomyosis in the surrounding tissue.
Pain during intercourse is reported by some women.
Difficulty conceiving, particularly secondary infertility after a caesarean with no other identified cause, is a recognised presentation, and persistent fluid visible in the uterine cavity on scanning in such a woman should prompt assessment of the scar.
Two pregnancy-related considerations deserve particular mention.
Caesarean scar pregnancy occurs when an early pregnancy implants directly into the niche rather than in the normal cavity. It is uncommon but serious, carrying a substantial risk of severe bleeding and uterine rupture as the pregnancy grows, and a high likelihood of progressing to placenta accreta spectrum if it continues. It is diagnosed on early ultrasound and requires prompt specialist management. Any woman with a previous caesarean should have an early scan in a subsequent pregnancy to confirm the pregnancy is correctly located.
Scar strength in a subsequent labour is a question that arises frequently, and the honest answer is that it is less settled than women are often told. A very thin residual muscle layer is intuitively concerning, and thickness measurements have been studied as a predictor of rupture during attempted vaginal birth after caesarean. However, no universally agreed threshold exists, the measurement varies between operators and across pregnancy, and uterine rupture is rare enough that predicting it reliably from a single measurement has proven difficult. Decisions about mode of delivery are made on the overall clinical picture rather than on a scar measurement alone.
Diagnosis depends on someone looking for it.
Transvaginal ultrasound is the primary test. The niche appears as a dark, fluid-filled, wedge- or pouch-shaped indentation in the anterior wall of the lower uterus at the scar. The examination measures its depth and width and, most importantly, the thickness of muscle remaining over it.
Saline infusion sonohysterography substantially improves detection. Instilling a small amount of fluid into the cavity distends it and outlines the defect clearly, revealing niches that standard scanning misses and allowing more accurate measurement. Where the diagnosis is being specifically sought—in a woman with characteristic symptoms or unexplained secondary infertility—this is the preferred technique.
Three-dimensional ultrasound adds further detail, and MRI gives comprehensive assessment and is used when planning surgical repair.
Hysteroscopy visualises the defect directly from inside the cavity and is both diagnostic and, for many women, the means of treatment.
A practical point about the timing of scanning: assessment is often most informative shortly after a period, when any retained blood is visible within the niche and the defect is easiest to appreciate.
The main differential considerations for the symptoms are endometrial polyps, adenomyosis, fibroids, hormonal causes of irregular bleeding, infection, and in older women endometrial pathology, which should be excluded on their own merits rather than assuming the scar is responsible.
Important to Know
The most important point is that finding a niche does not mean anything needs to be done about it. Niches are present in the majority of women who have had a caesarean, and the great majority cause no problems. Treatment is for symptoms, not for the finding. Care is typically coordinated by gynaecologists, with reproductive medicine involvement where fertility is a concern.
A woman told she has a caesarean scar niche on a scan, without symptoms, needs explanation rather than intervention, and no follow-up imaging is required.
Where postmenstrual spotting is troublesome, treatment is reasonable and effective.
Hormonal treatment is a sensible first step. Combined hormonal contraception used continuously, or a levonorgestrel-releasing intrauterine system, reduces or eliminates bleeding and avoids surgery. Many women find this sufficient. It does not alter the niche itself, so symptoms return if treatment stops, but for a woman not currently trying to conceive this is often an acceptable trade.
Hysteroscopic resection is the most commonly performed procedure and is effective for postmenstrual spotting. The operation trims the lower rim of the niche so that blood drains freely rather than pooling, and may also treat the surface tissue within the pouch. It is performed through the cervix without any incision, as a day case, with rapid recovery, and reported success rates for resolving bleeding are high. It requires adequate residual muscle thickness—commonly cited thresholds are around 2.5 to 3 millimetres—because the resection removes tissue and a very thin wall risks perforation.
Repair procedures that rebuild the uterine wall, performed laparoscopically or vaginally, are used where the residual muscle is thin, particularly in women planning further pregnancies. These excise the scar tissue and close the defect in layers, restoring thickness. They are more substantial operations than hysteroscopic resection, and their advantage is restoring the integrity of the uterine wall rather than simply improving drainage. Evidence on whether repair improves fertility outcomes is encouraging but not definitive, and this is an area still developing.
Hysterectomy resolves symptoms definitively and is a reasonable option for women with significant symptoms who have completed their families and prefer not to pursue conservative approaches.
For women with secondary infertility and a large niche with persistent uterine fluid, repair or resection is sometimes recommended before fertility treatment, on the reasoning that the fluid may impair implantation. Evidence supports this in observational work but randomised data are limited, and it is worth discussing the strength of the rationale rather than assuming benefit.
An honest overview of the evidence is appropriate. This condition has only been consistently recognised and defined over the past two decades, standardised definitions and measurement methods were agreed relatively recently, and much of the treatment literature consists of observational series rather than randomised trials. Treatments for bleeding have reasonably consistent results; the case for treating niches to improve fertility or pregnancy outcomes is less firmly established. This does not mean treatment is unjustified, but it does mean expectations should be realistic and that a woman should feel entitled to ask what the evidence actually shows for her particular situation.
Prevention is largely a matter of surgical technique and is relevant to any woman facing a caesarean. Double-layer closure of the uterine incision is associated with better healing in most studies. Avoiding a caesarean performed very late in labour where possible, and careful placement of the incision, also contribute. These are decisions for the surgical team rather than the woman, but they are reasonable things to understand.
For subsequent pregnancies, the practical implications are that an early scan should confirm the pregnancy is in the cavity rather than in the scar, and that the obstetric team should know the full caesarean history. The presence of a niche does not by itself determine mode of delivery.
Care is coordinated by gynaecology, with reproductive medicine and specialist minimally invasive surgical services as needed. Imaging findings are interpreted alongside symptoms and reproductive plans rather than in isolation, and the decision to treat should follow from symptoms rather than from measurements.
Patient education plays an important role. Understanding that niches are common and usually harmless, that postmenstrual spotting is the characteristic symptom and is treatable, that hormonal options avoid surgery, that treatment choice depends on remaining muscle thickness and on whether further pregnancy is planned, and that a niche found incidentally needs nothing all contribute to appropriate care.
Red flag symptoms include heavy or prolonged vaginal bleeding causing dizziness, breathlessness, or a racing heart; bleeding after menopause; severe pelvic pain; a positive pregnancy test with pain or bleeding in a woman with a previous caesarean, which requires urgent early ultrasound to exclude a caesarean scar pregnancy; severe abdominal pain in pregnancy; and persistent abnormal bleeding that has not been investigated, which warrants assessment to exclude other causes rather than being attributed to the scar without confirmation. These warrant prompt medical evaluation.