**Abstract**

Recurrent implantation failure (RIF) is one of the great challenges of current reproductive medicine. The term refers to the failure of repeated transfers of embryos of good morphological quality. Embryo implantation is a crucial moment in *in vitro* fertilization (IVF) treatments. A successful pregnancy depends on a synchronized interaction between a good quality embryo and a receptive endometrium. Its failure may be a consequence of embryo quality, anatomical or immunological factors. The anatomic causes constitute an important factor for RIF, although they are usually manageable. Fibroids, polyps and adhesions that develop after a surgical procedure or infection can hamper the embryo - endometrium attachment process. In addition, Mullerian abnormalities and hydrosalpinx can cause a negative impact on implantation rates and should also be taken into account in patients with RIF. In this chapter, we will address the main anatomical causes that may impact the implantation rates of patients undergoing IVF, as well as recommendations on management and its treatment.

**Keywords:** implantation failure, fibroids, endometrial polyps, adhesions, uterine septum, mullerian abnormalities, hydrosalpinx

## **1. Introduction**

Recurrent implantation failure (RIF) is one of the biggest challenges of the current reproductive medicine. Firstly, it is difficult to find its clinical standardized definition, despite the various articles on the topic. There is no agreement on issues, such as the number of embryo transfer failures, the embryo development stage, its morphology and aneuploidy, in order to define RIF [1]. There are also inconsistencies on the definition of implantation. Some authors consider it a failure when the gestational sac is not seen after the embryo transfer. Others claim that it happens when the β-hCG test is negative [1]. In 2014, some researchers proposed

the following definition: it is the transfer of at least four good morphologic quality embryos, with at least three fresh or frozen transfers to women below 40 years old. This is the most accepted definition up to date [2]. However, an international common understanding is necessary to standardize the definition in order to create more consistent scientific studies.

The embryo implantation is a key stage during *in vitro* fertilization (IVF) treatment. A successful pregnancy relies on a synchronized interaction between a good quality embryo and a receptive endometrium for implantation. Its failure can be a consequence of embryonic, anatomic or immunologic factors.

The anatomic causes constitute an important factor for RIF, although they are usually manageable. Fibroids and polyps can cause endometrial cavity distortion. Adhesions that form after surgery or infection can hinder the process of embryo implantation. Besides that, mullerian abnormalities such as septate or bicornuate uterus should be considered in patients with RIF.

According to the American Society of Reproductive Medicine (ASRM), the presence of hydrosalpinx can negatively affect implantation rates, either by alteration on the fluid nutrients or even by mechanically affecting embryo implantation.

In this chapter, we will address the main anatomic causes that can affect the implantation rates in patients undergoing to IVF as well as recommendations on the management and treatment.
