Is the womb lining important and how is the lining prepared for receiving the embryo?

The womb lining is very important for IVF success because the embryo needs to embed in the lining by a process called implantation for pregnancy to occur. Implantation will only be successful if the embryo is of good quality and the lining is suitably prepared (or receptive) for the embryo. Professor Homer wrote the chapter on Implantation in one of the world’s most popular textbooks of Gynaecology (Gynaecology 4th Edition; Chapter 19).

During ovarian stimulation, in addition to monitoring follicle development, it is therefore very important to evaluate the womb lining for its appearance as well as thickness. A lining that is developing well during simulation will show three clear “lines” (or a trilaminar appearance) due to the effect of oestrogen and will usually increase in thickness to around 7mm or more.

To make the womb lining receptive to the embryo requires the hormone, progesterone. The follicles that have had their eggs removed at egg pickup will produce some progesterone but this will not be enough. Following EPU, it is therefore very important to start using progesterone medication. Progesterone is most often inserted into the vagina in the form of a gel or pessary. Depending on the particular product, it may need to be inserted once, twice or three times per day. Occasionally, progesterone is given in the form of injections into the muscle. It is important that progesterone is continued at least until the time of pregnancy test.

Occasionally, pregnancy fails to occur despite the repeated transfer of very good quality embryos; this is called recurrent implantation failure. There are a number of possible reasons that have been suggested to account for recurrent implantation failure, for instance, an imbalance in immune factors, a tendency to form clots etc. A recently developed test called the Endometrial Receptivity Assay (ERA) may be helpful in this situation. ERA is used to analyse a sample of the lining that is taken at a stage in the cycle when implantation is expected to occur. It measures the expression levels of a panel of genes that are important for receptivity and implantation and can give advice on the most appropriate time for performing the embryo transfer.