**3.5 Single versus double layer**

Single vs. double layer sutures of the myometrium were discussed in a large number of studies and reviews. It is currently postulated that double layer is superior to single layer as it is associated with a thicker RMT (4.6 vs. 5.2 mm) [34, 36–38] and lower niche prevalence (4.2% vs. 1.3%, p < 0.001, RR 0.32, [28]).

These observations were confirmed in a large review including 20 randomised controlled trials or prospective cohort studies and more than 15000 women. Here, a double layer suture resulted in a thicker RMT (+ 1.26 mm double layer vs. single layer), a better healing ratio (=anterior wall thickness/anterior wall thickness + height of the wedge-shaped defect) and less dysmenorrhoea than single layer suture [39]. A 50% reduced risk of uterine rupture during subsequent pregnancy was also assumed following a double layer suture compared to a single layer [40]. Finally, Vachon-Marceau et al. demonstrated a significantly higher rate on scar dehiscence in the single- compared to double-layer group [28].

Of note, a number of studies pointed out that a proper single layer (unlocked) suture might not be inferior to double layer [36]. The RMT is commonly lower compared to double-layer suture, but interestingly the risk of uterine rupture in a subsequent pregnancy is influenced only marginally [38, 41, 42]. However, since the frequency of uterine rupture is very low in general, the sample size of most studies was too small to reach reliable data.

One large review including 9 randomised controlled trials (3969 patients) demonstrated a thinner RMT in the single layer suture group (mean difference − 2.19 mm), but no statistically significant differences regarding uterine scar defects, uterine dehiscence or uterine rupture. The authors acknowledged that even if uterine scar defects are associated with lower RMT, it remains questionable whether RMT alone is a proper marker for prospective uterine ruptures [38]. Jastrow et al. calculated that a cut-off value for myometrial layer tickness in third trimester below 1.4–2 mm and complete lower LUS of less than 2–3.5 mm correlates with a higher risk of niche incidence and therefore uterine rupture. Unfortunatly, the retrospective study design and small sample size limit the scope of these results [29].

A proper single layer suture was suggested to be helpful when the CS is performed during advanced labour and myometrial layers cannot be correctly identified [28]. One trial demonstrated that double layer suture results in a higher RMT than single layer suture only in elective CS but not in CS at advanced labor [43].

In summary, although there is no evidence for a higher uterine rupture rate following a single layer suture compared to a double-layer suture, RMT and therefore probably the integrity of the lower uterine segment is improved by the double-layer suture of the uterus, at least in cases with an elective CS.

#### **3.6 Locked versus unlocked suture**

Locking a suture was used for long time to reduce bleeding. However, there is some evidence that a locked suture may provoke defect scar healing.

One trial showed that only unlocked double-layer, but not locked double-layer suture was superior to locked single-layer in either RMT (3.8 ± 1.6 mm vs. 6.1 ± 2.2 mm) and healing ratio [36]. Higher rates of scar separation were described when a continuous suture was locked (OR 5.4, 95% CI 3.17–9.20, p < 0.001, [41]).

Stegwee et al. reviewed data from three randomized controlled trials and two prospective cohort studies to compare locked and unlocked suture. RMT decreased significantly and niche prevalence was non-significantly higher when a locked suture was performed (RR 1.23, 95% CI 0.93–1.61, p = 0.14). Also one study including 48 women reported the healing ratio, which was lower in locked vs. unlocked sutures [39].

In summary: Locking a suture decreases healing due to the reduction in blood flow and consequently in oxygen supply to the scar, which is required for the healing process [44].
