Transfer of frozen or vitrified embryos

The transfer is performed in the operating theatre adjacent to the embryology laboratory to minimise the time the embryos spend outside the most suitable conditions, from the culture dish to the uterus. It is performed under abdominal ultrasound guidance, with the aim of placing the embryos in the centre of the uterine cavity to achieve the best pregnancy rate.

The patient should arrive at the centre at the agreed time, having drunk enough water to ensure that the urinary bladder allows for proper ultrasound visualisation of the uterus. The patient is placed in the gynaecological position, the cervix is sterilised, the cervical mucus is aspirated, and the transfer cannula containing the embryos is inserted and deposited into the endometrial cavity.

On the laboratory side, preparation for transfer consists of selecting the embryos to be transferred and maintaining them in the appropriate conditions before transferring them to the uterus. With all preparations complete in the operating room, the embryologist loads the cannula with the embryos and then the gynaecologist handles uterine insertion and embryo placement. Subsequently, back in the laboratory, the cannula is examined under a microscope to ensure that no embryos have remained inside.

Embryos generated in an IVF cycle are kept in culture for two or three days until the time of transfer. During these days, they are observed and classified. In our unit, we follow the morphological criteria established by ASEBIR (Association for the Study of Reproductive Biology), a pioneer in standardising criteria applicable to all centres in Spain.

The embryos that meet the highest quality criteria are selected for transfer.

The pertinent Spanish law in force permits the transfer of a maximum of three embryos. However, to reduce multiple pregnancies, the most common practice is to transfer two embryos. The decision on the number of embryos to be transferred must be personalised for each couple, and the factors to be taken into account are the woman's age, the quality of the embryos and any previous reproductive history.

The procedure itself is very short, lasting between ten and twenty minutes from the time the patient enters the operating theatre until she leaves.

The procedure is the same as that used for a gynaecological examination. It may be somewhat uncomfortable but is not usually painful. However, in cases where difficulties are anticipated in passing through the cervix, the transfer may be performed under anaesthesia.

Apart from taking the medication prescribed by your doctor, it is generally recommended that you rest for a couple of days and avoid exerting yourself until you know the result of the pregnancy test.

The fate of embryos that are not transferred depends on their quality. Thus, embryos that meet minimum quality standards are vitrified, while poor-quality embryos are discarded.

The vitrification of embryos left over from the transfer can be carried out at two different times: if the embryos are of good quality, they are frozen on the same day as the transfer; if the embryos are of medium or low quality, they are kept in culture for two or three more days to assess their development, which, if correct, means that the embryo is viable and, therefore, vitrified.