Implantation After Embryo Transfer

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Key Points

  • Receptivity window happens during the secretory phase of the menstrual cycle.
  • It is the time when implantation is possible.
  • There are 3 stages of implantation: apposition, adhesion and invasion.

Embryo Transfer

Embryo transfer in IVF consists in transferring a day 3 or day 5 embryo into the uterus. When transferring a day 3 embryo, it will continue to develop inside the uterus until it becomes a blastocyst (day 5) after which the implantation process will start. The blastocyst (day 5) will hatch from the protective 'shell' which has surrounded the embryo through its early development. This is called the Zona Pellucida. It is this mass of hatched cells which, once free from its shell, will implant into the lining of the uterus. Implantation happens around 1-2 days after a day 5 blastocyst transfer, or 3-4 days after a day 3 embryo transfer.

What is Implantation?

Implantation is defined as the process by which the embryo attaches to the endometrial surface of the uterus and invades the epithelium and then the maternal circulation to form the placenta. Before the initiation of implantation, however, both embryo and endometrium should embark on an elaborated process of cross-talk.

From the clinical point of view, implantation is considered to be successful when gestational sac is diagnosed by ultrasound.

Receptivity Window

Endometrial receptivity is a complex process that provides the embryo with the opportunity to attach, invade, and develop.

The receptivity window (window of implantation) extends 3–6 days within the secretory phase in most women.

Stages of Implantation

There are three stages of implantation:

1. Apposition: the embryo seeks its position on the endometrial tissue and remains immobile while it is oriented, so that its internal cell mass points towards the endometrium to allow the proper formation of the placenta later on.

2. Adhesion of the embryo to the maternal wall.

3. Invasion of the placental cells, making contact with the maternal blood. This process is controlled by cytokines, which are molecules that modulate the dialogue between embryo and endometrium.

The Small Book of Fertility Hormones

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