The main objective of the study is to evaluate whether there is any association between the location into the uterine cavity, where the air bubbles containing the pre-embryos were placed, and the implantation rate. Therefore, the images generated during pre-embryos transfer are available in our database and studied retrospectively.

The success of a reproductive treatment is determined by three variables: the pre-embryo quality, endometrial receptiveness and pre-embryo transfer. Several previous studies show a lack of agreement concerning the greatest measurement. For instance, some of them refer to the distance between the catheter and the uterine fundus, whereas some others take as a reference the air-bubble where the pre-embryos were placed.

It is for this reason that in the present study, we aim to evaluate the air bubble location ratio into the uterine cavity with reproductive success, taking into account the different variables to reach success. That is, embryo characteristics (quality, development day) endometrium preparation (cycle type, hormone levels on day 5) and even the transfer procedure (type of catheter, blood, difficulty channeling).

It is a study that aims to collect data from January 2013 to october 2019. To this end, the images generated during pre-embryos transfer are analyzed retrospectively. Therefore, it is evaluated the existing differences on implantation rate according to air-bubble placement into uterine cavity during the transfer procedure.

Inclusion Criteria:

• Patients with normal uterine anatomy, who underwent an ultrasound guided embryo transfer.
• BMI< 30kg/m2.
• Ultrasound guided embryo transfer made by the same researcher.

Exclusion Criteria:

• Presence of uterine fibroid in the anterior uterine body that make difficult the transfer visualization.
• Asherman Syndrome.
• Adenomyosis that reduces the uterine cavity visibility.
• Difficult transfers where the use of a semi-rigid cannula sheath is needed to entry into the uterine cavity.
• Previous uterine surgery that alters its anatomy.
• Istmocele

IVIVigo patients with a normal uterine anatomy and a single good quality embryo to transfer at blastocyst stage, performed by the same gynaecologist using the "three-drop technique" were included. All transfer were atraumatic, without blood presence, using soft catheter and with controlled ultrasound transfer. The distance between the fundus surface and the tip of catheter, the distance between the fundus and the air bubble, the endometrial thickness and the implantation rate were recorded.