DOI: 10.6084/m9.figshare.25878365
title: The impact of caesarean scar niche on fertility - a systematic review

description: The trend of increasing caesarean section (CS) rates brings up questions related to subfertility. Research regarding the influence of CS on assisted reproduction techniques (ART) is conflicting. A potential mechanism behind CS-induced subfertility is intra uterine fluid resulting from a caesarean scar defect or niche. The vaginal microbiome has been repeatedly connected to negative ART outcomes, but it is unknown if the microbiome is changed in relation to a niche. This systematic review describes literature investigating the effect of a niche on live birth rates after assisted reproduction. Furthermore, studies investigating a difference in microbial composition in subfertile persons with a niche compared to no niche are evaluated. Pubmed, Embase and Web of Science were searched on March 2023 for comparative studies on both study questions. Inclusion criteria were i.e., English language, human-only studies, availability of the full article and presence of comparative pregnancy data on a niche. The quality of the included studies and their risk of bias were assessed using the Newcastle-Ottawa scale for cohort studies. The results were graphically displayed in a forest plot. Six retrospective cohort studies could be included on fertility outcomes, with a total of 1083 persons with a niche and 3987 without a niche. The overall direction of effect shows a negative impact of a niche on the live birth rate (pooled aOR 0.58, 95% CI 0.48-0.69) with low-grade evidence. Three studies comparing the microbiome between persons with and without a CS could be identified. There is low-grade evidence to conclude that the presence of a niche reduces live birth rates when compared to persons without a niche. The theory that a caesarean has a negative impact on pregnancy outcomes because of dysbiosis promoted by the niche is interesting, but there is no sufficient literature about this. The increasing number of caesarean deliveries has raised concerns about how it might affect a woman’s ability to get pregnant afterwards. Some studies suggest that having a caesarean section (CS) could make it harder to conceive, particularly through in vitro fertilisation (IVF). The reason could be the scar or niche from a previous caesarean. This niche can cause fluid inside the uterus. We also know that the mix of bacteria in the vagina, called the vaginal microbiome, can affect a woman’s chances of getting pregnant, especially with treatments like IVF. But we are not sure if having a caesarean affects the vaginal microbiome. To understand this better, van den Tweel’s team looked at studies on whether having a niche from a caesarean affects a woman’s chance of having a baby through IVF. They also looked at studies comparing the bacteria in the vagina of women who have had a caesarean with those who have not. They found that having a caesarean niche makes it harder for a woman to have a baby through IVF. However, the evidence from these studies is not very strong. We still do not know enough about whether having a caesarean niche affects the bacteria in the vagina.