**3. Pathogenesis of niches**

Many studies focused on potential risk factors for developing niches after CS. The most promising considerations comprise problems in wound healing including a (reversible) retroflexio uteri, the number of previous CS and the location of the uterotomy. The latter is affected by the stage of labour and the dilation of the cervix when CS is done. For the surgeon the most important issue particularly might be the optimal technique for closure of the uterotomy.

#### **3.1 (Reversible) Retroflexio uteri**

In most women, the physiological direction of the uterus in the pelvic cavity is an anteflexion. A retroflexion is a non-pathologic alternative to the norm. In 2016, Ryo et al. reported that the uterus may change its flexion after delivery, shifting from ante- to retroflexion. Compared to vaginal delivery, a retroflexed uterus was observed significantly more frequently after CS, increasing with the number

of previous CS [23]. Other findings demonstrated a higher prevalence of niches in retroflexed uteri and notably large defects in cases of retroflexion [12, 13, 24]. Nowadays, it is widely accepted that there is a strong coincidence of niches and a retroflexion of the uterus [11–13, 25].

However, an important question remains: Does a retroflexion of the uterus facilitate the development of a niche or does a niche cause a (reversible) retroflexion? There are explanations for both hypotheses:

On the one hand, after CS, adhesions might cause mechanical tension on the anterior uterine wall leading to a retraction of the scar tissue with poor blood perfusion and resulting in an impaired wound healing. The retraction might be intensified if the uterus is already retroflexed [11]. This hypothesis is supported by the finding that the risk of developing a niche is more than twice higher, when the uterus is retroflexed [1].

On the other hand, the consideration of a niche causing a (reversible) retroflexion also seems plausible. Ryo et al. did not only examine the changes in uterine flexion after delivery but also provided a fairly logic objection to the above-mentioned pathogenesis: If a niche developed due to mechanical tension and retraction of the scar, the defect would be found at the outside of the anterior uterine wall [23]. In contrast, niches are generally found at the cavitary side of the myometrium. The uterine incision and the developing niche may compromise the contractility of the myometrium, leading to an imbalance between the anterior and posterior wall, causing traction backwards and resulting in a retroflexion.

Therefore, it seems more plausible that first a niche develops and second the uterus becomes retroflexed. Further studies are needed to better understand the role of a retroflexion in the pathogenesis of niches.

#### **3.2 Number of previous Cesarean sections**

Niches are almost solely diagnosed in women after CS. No studies report myometrial defects after vaginal birth. Osser et al. described a median myometrial thickness at the isthmus of 11.6 mm after vaginal delivery, compared to 8.3 mm/6.7 mm/4.7 mm after one/two/three or more CS [3]. Largely all studies report a positive correlation between the number of previous CS and increasing rates of niches [1, 3, 6, 23, 25]. The niche prevalence was found to be up to 63,1% after one, 76–81% after two and 88–100% after three CS, respectively [3, 7]. Not only the number of niches increase, but also the scar defect itself becomes larger the more CS found in the patient's history [3]. Total defects with no remaining RMT were more frequently found in women with multiple CS: 6%, 7% and 18% after one, two and ≥ three or more CS, respectively [1].

The possible explanation seems to be impaired wound healing: trauma to the uterine wall disrupts the physiological healing process due to a reduction of vascular perfusion [25].

Addionally, increasing CS rates correlate with higher prevalence of retroflexed uteri, underscoring the hypothesis about the association of retroflexed uteri with niches development [23].

Therefore, the careful evaluation of each CS in terms of its necessity seems to be the most promising step to reduce myometrial defects.

#### **3.3 Position of the uterotomy and timing of the CS**

The most common technique of uterotomy performance is a transversal incision of the isthmo-cervical region. During labour, the isthmo-cervical region undergoes continuous changes. First, the thinning and elongation, recognizable by a lifted

*Prevention of Cesarean Scar Defects: What Is Possible? DOI: http://dx.doi.org/10.5772/intechopen.97618*
