**1. Introduction**

#### **1.1 Definition of uterine scar defects**

With the increasing rate of CS worldwide a meaningful long-term complication has been observed within the last two decades: the development of defects of the uterine scar. Previously, these defects were referred to as "pouch", "uterine/cesarean scar defect", "uterine diverticulum" or "sacculation" [1]. Nowadays, the terms "isthmocele" or "niche" are best established [2].

As niches occur almost exclusively after CS [3, 4], they are located within the uterine isthmus at the site of the former uterotomy [5]. There is still no international consensus about the exact definition of a niche or a standardized classification. Therefore, prevalence rates are of wide range [1]. Due to ongoing research, initial gaps in knowledge start to close [6]:

Most commonly, a niche is defined as any indentation of the myometrium at the location of the uterotomy with a depth of at least 2 mm [2]. The depth of a large niche either ranges between 50 and 80% of the myometrial thickness, or leaves a remaining myometrial thickness (RMT) thinner than 2.2 mm (measured by transvaginal ultrasound -TVUS-) or 2.5 mm (measured by contrast enhanced sonohysterography –SHG-), respectively [1]. Concerning the shape of the niche, the following subclassification was suggested: A simple niche (without any further branch), a simple niche with one branch, and a complex niche (with more than one branch) [2] (**Figures 1**–**3**). In most cases, the appearance of a niche is triangular in shape, but it can also be round [3]. A scar dehiscence is defined as a complete defect of the myometrium (**Figure 4**) [3].

**Figure 1.** *Simple niches.*

**Figure 2.** *Simple niches with branches.*

**Figure 3.** *Complex niches.*

**Figure 4.** *Complete defect of the myometrium.*

Several studies examined the prevalence of niches, resulting in a range from 22 to 84% of patients after CS [6, 7]. This rather wide range results from the different modes of assessment (TVUS or SHG) and the timing of the examination (several weeks up to 12 months after CS).
