Caesarean birth is a common operation in obstetrics, accounting for 41% of births in Australia.⁷ The caesarean scar niche (CSN), also known as an isthmocele or caesarean scar defect, is an indentation in the myometrium at the site of a previous hysterotomy. It is often identified on transvaginal ultrasound after caesarean delivery, and healing of the uterine incision can result in a niche of varying depth.

When symptomatic, a niche can present with abnormal uterine bleeding, secondary infertility, or pelvic pain, which can have profound impact on a woman’s quality of life. A recent Canadian clinical consensus statement recommends that practitioners become familiar with the diagnostic criteria for caesarean scar disorder and consider this diagnosis in symptomatic patients following caesarean birth, once a niche is confirmed on ultrasound.⁴

CSN is worth considering whenever a patient presents with abnormal uterine bleeding, pelvic pain, or infertility following a caesarean birth.

What Is a Caesarean Scar Niche?

A CSN is diagnosed when transvaginal ultrasound shows a hypoechoic indentation at least 2mm deep at the prior hysterotomy site, assessed no sooner than three months after delivery.¹ Reported prevalence varies widely depending on the imaging modality and diagnostic definition, ranging from approximately 20% to over 80%, with higher detection rates using saline contrast sonography.¹

Niches become more common with each subsequent caesarean: a recent Egyptian cohort found niches in 16.3% of women after one caesarean birth, rising to 24.4% after two, and 31.2% after more than two.⁶ An earlier Australian study reviewed pelvic ultrasound images from 318 women referred for gynaecological scanning by their GP or specialist, most commonly for menstrual symptoms or pain; of the 71 with a caesarean history, 40% had a visible defect, nearly doubling the odds of prolonged periods or postmenstrual spotting.⁵

Recognised risk factors include uterine retroflexion, and a low or single-layer hysterotomy closure.^1,6 The majority of niches are asymptomatic and require no treatment. Niche formation is multifactorial, and a prognostic model built from established patient, labour, and surgical risk factors was unable to reliably predict which women would go on to develop a niche, reflecting how nuanced its development remains.¹

Surgical Technique and Niche Formation

No single technique eliminates the risk of niche formation.¹ Several studies have nonetheless explored technique-related associations at the time of caesarean birth. A hysterotomy placed at least 2cm above the vesicouterine fold, particularly during advanced labour, is associated with reduced niche formation compared with a lower incision.⁴ A two-layer, unlocked closure that avoids incorporating the endometrium into the first layer, sometimes called the endometrium-free technique, has been associated with a reduction in niche rates from between 20% and 34% down to just 4% to 6% in trial data, with a correspondingly thicker residual myometrium.⁴

Adjunctive azithromycin prophylaxis, in addition to standard cephalosporin cover, has also been associated with a reduced risk of defect formation in trial data, an area warranting further study.¹ Interestingly, while a two-layer closure produces a thicker residual myometrium on ultrasound than a single-layer closure, this has not consistently translated into a lower niche prevalence across studies, a discrepancy likely reflecting inconsistent CSN definitions between trials rather than an absence of true benefit.¹ These findings may be useful considerations at the time of primary caesarean.

From Niche to Disorder

An international Delphi consensus of experts from three continents proposed the term caesarean scar disorder (CSDi) to describe a niche associated with clinically significant symptoms.² Diagnosis requires sonographic evidence of a niche together with at least one primary symptom, such as postmenstrual spotting or pain during bleeding, or two secondary symptoms, such as dyspareunia or chronic pelvic pain, once other causes, including cervical dysplasia, infection, and ovulatory dysfunction, have been excluded (Table 1).²

Table 1. Diagnostic Criteria for Caesarean Scar Disorder.

CSDi gives clinicians a shared language for a condition that has, until recently, been described using more than a dozen different terms across the literature, from isthmocele to uterine diverticulum to sacculation.¹ This lack of a consistent definition has made it difficult to compare studies directly, and standardising terminology is expected to improve the quality of future research.¹

Abnormal Uterine Bleeding: A Long-Recognised Association

Abnormal uterine bleeding was among the first symptoms linked to CSN, and the two remain closely associated. A systematic review and meta-analysis found that women with a confirmed defect were three to five times more likely to experience abnormal uterine bleeding than those without.¹ The bleeding pattern is often distinctive: affected women report a mean of 14 days of prolonged menstruation and a mean of seven days of early-cycle bleeding, a pattern thought to reflect inflammation, fibrosis, and necrosis within the defect itself.¹

An Australian study of 71 women with a caesarean history similarly found that a visible defect nearly doubled the odds of prolonged periods or postmenstrual spotting.⁵ Given how well established this association is, CSN is worth including in the assessment of any woman presenting with abnormal uterine bleeding after a caesarean history, once other causes have been considered.

Pain: An Under-Recognised Symptom

Pain is a related but distinct symptom that deserves equal attention. A 2025 systematic review and meta-analysis of 64 studies found that women with a confirmed niche were more than twice as likely to report dysmenorrhea, dyspareunia, or chronic pelvic pain than those without, with the prevalence of these symptoms ranging from 15% to 38%.³

Pain severity has also been quantified: affected women report a mean pain score of 4.7 out of 10 on a visual analogue scale, indicating moderate pain, with a measurable impact on quality of life using validated Short-Form 36 scoring.³ Larger defects were associated with greater pain, and, encouragingly, both medical and surgical treatment reduced the odds of dysmenorrhea by 87%.³

Population screening data tell a more reassuring story: in the Egyptian cohort above, only around one in four women with a confirmed niche reported any symptoms at all, most commonly dyspareunia or postmenstrual spotting, suggesting that referral-based cohorts overrepresent more severe disease.⁶

Secondary infertility is increasingly recognised, likely related to inflammation from retained blood affecting cervical mucus, sperm transport, and embryo implantation, and a review of more than 10,000 embryo transfer cycles found that niches were associated with lower live birth rates in women undergoing IVF, though evidence on whether surgical repair improves these outcomes remains limited and continues to evolve.⁴

Making the Diagnosis

Transvaginal ultrasound, with or without saline or gel infusion, remains the preferred first-line investigation.⁴ Diagnosis is frequently missed because the defect is not specifically sought during routine pelvic ultrasound. Saline or gel contrast improves detection by outlining the defect with fluid, and imaging in the mid-follicular phase, when intrauterine fluid is more likely to be present, may further assist visualisation.¹

Clinicians should document niche depth, length, width, residual myometrial thickness (RMT), and adjacent myometrial thickness, as these measurements guide counselling and treatment planning.⁴ Magnetic resonance imaging and hysteroscopy have a role in select cases but are not first-line tools.¹

Treatment: Medical First, Surgery When Needed

For women not seeking pregnancy, hormonal suppression is recommended first-line. The levonorgestrel intrauterine device has the strongest evidence, improving both bleeding and pain over six months of use.⁴ Combined hormonal contraception is a reasonable alternative: prospective cohorts have reported a reduction in menstrual duration of around four days after three months of use, with one small study noting resolution of abnormal bleeding in 10 of 11 patients.¹ When symptoms persist despite medical therapy, or fertility is desired, surgical repair is considered.

Hysteroscopic remodelling is preferred when RMT is 3mm or greater, while laparoscopic or vaginal repair suits thinner defects and can also increase RMT in women planning pregnancy.⁴ The hysteroscopic technique typically involves resecting the distal and proximal edges of the defect and fulgurating its base, remodelling the niche to prevent menstrual blood pooling and to facilitate drainage.¹

A systematic review comparing these approaches found improvement in abnormal uterine bleeding in 85% of patients treated hysteroscopically, 93% treated laparoscopically or with hysteroscopic-guided laparoscopy, and 83% treated vaginally, with a network meta-analysis further suggesting the combined laparoscopic-hysteroscopic approach achieved the greatest reduction in menstrual bleeding and defect depth.¹ Hysteroscopic repair is also associated with a shorter hospital stay and less intraoperative blood loss than the vaginal or laparoscopic approaches, although no comparative data on pain reduction between techniques were reported.¹

In one Australian case, a woman with post-caesarean pain and bleeding underwent transvaginal excision and resuturing of her scar defect, with a normal uterus and regular cycles confirmed on ultrasound at four-month follow-up.⁸ Patients should understand an important caveat: current evidence does not show that niche repair reduces the risk of obstetric complications, such as caesarean scar pregnancy or uterine rupture, in a future pregnancy.⁴

Why This Matters for Practice

With caesarean rates remaining high, CSN and its associated symptoms are likely to be encountered across O&G practice. CSN, and the broader entity of CSDi, is worth considering in any patient presenting with abnormal uterine bleeding, pelvic pain, or secondary infertility following caesarean, once other causes have been excluded.

Awareness of the diagnostic criteria and management options allows these women to be assessed for an eminently treatable cause, rather than dismissed or referred on without a clear diagnostic pathway.

Further research into standardised diagnosis, pain phenotyping, and long-term outcomes remains a priority, and international collaboration between the groups working in this space, including the Canadian, European, and Australian data drawn on here, will be key to progressing this agenda.