## 1 Introduction  
Damage of the anal sphincter can lead to fecal incontinence, a condition seriously impairing the quality of life and interfering with activities in multiple spheres of life [1]. The most common cause of fecal incontinence is anal sphincter injury occurring during natural delivery. The incidence of obstetric anal sphincter injuries (OASIs) reported in the literature varies between 3% and 20% of women after natural births [2; 3; 4; 5; 6; 7], and is probably strongly underestimated. Often, the only diagnostic method available is a rectal physical examination, and the diagnosis of OASIs is based on the experience of the examining physician. Meanwhile, ultrasound evidence of OASIs was found in 27-35% of women giving birth for the first time, and in 3-12% at subsequent births [8; 9; 10]. Endoanal ultrasound remains the gold standard for OASI diagnostics. The images obtained via its 360-degree endoanal probe allow the assessment of external and internal anal sphincter morphology. These images are able to show a break in muscle continuity, and thinning or fibrosis. The examination is characterized by high sensitivity, reaching 100% [11]. Nowadays, 3D ultrasound allows determination of the OASI degree during the immediate postpartum period [12; 13; 14]. The grade in OASI classification is assessed based on the type of injured tissue and the extent of the injury diagnosed with endoanal ultrasound, as shown in Table 1. A mark of 1 or 2 on the OASI classification means that there was no injury to the anal sphincter muscles, and any result greater than 2 means that anal sphincter injury is present. However, endoanal ultrasound requires specialized equipment (rarely available in maternity wards) and highly skilled medical staff with extensive knowledge and experience in detecting injuries arising during delivery. Both significantly limit the use of this diagnostic tool in clinical practice. Because of these limitations, many women leave maternity wards with undetected anal sphincter injuries, which can cause problems with gas or stool incontinence. Similar limits are applicable for the other diagnostic test, anorectal manometry, which evaluates the function of the sphincter by measuring pressure in the rectum and anal canal. Based on the analysis of the results, it is possible to assess muscle function, and probable cause of the disorder [18]. A disadvantage of manometry is lack of unification for conducting the examination and reporting its results [19], which further limits its diagnostic value. This unification is only now beginning to be determined [20]. Another device that fits to this category is Fecobionics, which imitates defecation; however, further studies are needed to present its clinical performance [21]. Some studies indicate that electromyography (EMG) signals gathered from the sphincter muscles can also assess their functionality [22; 23; 24]. However, needle electromyography is an invasive technique and is hence not widely used in clinical practice. Other types of this examination, such as high-density EMG, are promising techniques for studying electrophysiological mechanisms but require further investigation [23]. Moreover, this approach does not examine the anatomy of the muscles, only their physiology.