**Figure 11.**

*Uterine incision of a former CS within the cervical channel.*

urinary bladder or plica vesicouterina. Consecutively, also the inner cervical os with its mucous glands and only few muscular tissue moves upwards. Hence, if the surgeon incides the uterus at the same height in late labour as in an earlier stage of labour, the incision is placed either lower or even beneath the inner cervical os (**Figure 11**). The most likely explanation for distinct wound healing effects is found in the morphological difference between the region around the inner cervical os and the myometrium above. The mucus may dilate the sutured rims of the uterotomy or cause retention cysts, both leading to impaired wound healing [11, 26]. Furthermore, as a result of poor contraction in this area, wound edge adaptation may be insufficiently.

Therefore, CS at advanced stage of labor may provoke a lower incision including cervical tissue and resulting in more scar defects. On the other hand, contractions or rupture of membranes have already occurred in advanced stage of labor with a crucial and beneficial influence on the postoperative healing process of the uterine scar.

The following chapters discusses the effects of both of these opposite forces on scar healing:

On the one hand, CS performed before the onset of labour resulted in a thinner uterine wall in subsequent pregnancies than CS performed during labour [27, 28]. Jastrow et al. [29] showed significantly decreased LUS measurements depending on stage of labour prior to the CS (full LUS after CS in latent phase: 2.8 mm, in active phase: 3.1 mm and in CS prior to labour 2.4 mm, p < 0.01). Importantly, Park et al., were able to demonstrate no increased risk to develop a niche when CS was performed in situations with >8 cm dilated cervix [6].

A CS performed intrapartum reduces the probability to develop a placenta praevia in a following pregnancy [30] as well as the risk of uterine rupture in cases of vaginal birth after a CS [31], underlining the importance of contractions on wound healing. A case control study of 307 women showed a threefold increased risk to develop a morbidly adherent placenta after an elective CS compared to an emergency CS. In this study, the increasing severity of a morbidly adherent placenta (placenta accreta versus increta versus percreta) was associated with a

higher probability of a prior elective CS, respectively [32]. Hence, we hypothesize that uterine contractions and their immonological triggers may contribute to the postpartum tissue repair. To elucidate possible mechanisms, we shorthly discuss this topic on the end of this chapter.

On the other hand, several studies demonstrated, that the incidence of defect wound healing increased significantly when CS was performed in advanced labour [1, 6, 33].

It was shown that a prolonged labour beyond 5 hours or a CS performed during active labour with the cervix is dilated >5 cm, is associated with an increased risk to develop a niche (5–9 hours OR 13.0 (2.2–76.6), > 10 hours OR 33.1 (6.6–166.9); p < 0.001) [21].

Also, a higher percentage of RMT < 3 mm was observed in cases with advanced cervical dilatation at CS [7]. Park et al. showed a higher risk of niches in cases with a CS at a cervical dilatation of 5–7 cm compared to cases with CS at closed cervix [6].

But how can these conflicting results be explained?

The hypothesis to answer this question is that in cases of CS at advanced stage of labor, the uterotomy position is closer to the internal cervical os and wound healing is compromised in this area. This hypothesis can be confirmed by the following observations:

As shown by Vikhareva Osser, the uterotomy position was exactly positioned at the internal os in 97% of cases with a cervical dilatation >5 cm compared to 55% in cervical dilatation <5 cm [21].

Hanacek et al. demonstrated that if the CS is performed at full cervical dilatation, the resulting scars were closer to the external os and the RMT was markedly thinner [34]. These findings might be explained by the localization of the uterine incision, which is often overly caudal, due to cervical incorporation into the lower uterine part.

A very recently published prospective cohort study showed a higher prevalence of scar defects when the uterotomy was placed cranially of the internal os compared to a uterotomy placed caudally of the internal os in patients with a first CS [35]. Subgroup analysis, however, showed that in cases of a CS before the onset of labor, the uterotomy position was mostly cranial of the internal os whereas in cases with advanced labor, the position was rather caudal. Therefore, the beneficial effect of advanced labor was undermined by the more caudally position of the uterotomy.

In order to confirm the negative effects of a low uterine incision, a prospective randomized study published in 2019 compared the incidence of niches in patients with low and high incision (2 cm below vs. 2 cm above the plica vesicouterina) 6–9 months after an elective CS. Large scar defects occurred significantly more often in the low-incision group (41% vs. 7%) [33]. This leads to the conclusion, that the position of the uterotomy is one of the most important factors in the pathogenesis of niches.

In summary, CS performed under contractions in active stages of labour may elicit mechanisms that improve scar healing subsequently resulting in thicker RMT and in reduced development of niches. However, this benefit inverts when labour or cervical dilatation is too advanced and the uterotomy is localized overly caudal. In consequence, every obstetrician needs to consider the stage of labour and the cervical dilation for choosing the optimal position of uterine incision.

#### **3.4 Closure of the uterotomy**

Closure of the uterotomy is a crucial and very controversially discussed topic. Several studies addressed this important step in order to optimise scar integrity, endometrium rehabilitation, postoperative recovery and the reduction of risks in subsequent pregnancies. The following section illustrates partially contrary strategies to close a uterotomy in order to prevent a niche.
