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The embryo generated from the egg and sperm during IVF needs to embed (implant) into the lining of a woman’s womb for pregnancy to take place. The chance of an embryo successfully implanting is only approximately 30%, meaning the chance of an embryo failing to implant is approximately 70%.
The chance of pregnancy increases slightly with increasing numbers of transferred embryos such that the chance of two embryos failing to implant is 49%. Although this may sound low, most women will have successfully achieved a pregnancy by their third cycle of IVF.
Recurrent implantation failure
Should this not be the case, there may be an underlying problem causing the lack of pregnancy. Recurrent implantation failure (RIF) is therefore generally defined as the absence of pregnancy (embryo implantation) following three transfers of good quality embryos in IVF cycles.
Causes of implantation failure
Many factors determine whether pregnancy occurs in an IVF cycle including:
Quality of the embryo: one important variable is clearly the quality of the embryo as a poor quality embryo is less likely to implant and lead to pregnancy. A good-quality embryo is one that has the correct number of normal cells corresponding to the day of its development.
There is no widespread agreement on the number of embryos transferred that qualify for the definition of RIF, but most specialists would agree to apply the definition after three unsuccessful embryo implantation procedures.
The age of the woman is probably the most important determinant of the chance of pregnancy following an IVF cycle. Embryo quality is closely related to maternal age.
Inherited or acquired abnormalities of the womb can contribute to RIF including:
- Uterine fibroids (the closer the fibroid is to the lining of the womb the greater its impact)
- Polyps in the lining of the womb
- Inflammation of the lining of the womb
- Abnormal shape of the lining of the womb (Septate uterus)
- Adhesions in the lining of the womb
- Uterine endometriosis (adenomyosis)
Reduced endometrial receptivity such as:
- Thin endometrium
- Altered expression of adhesive molecules
- Immunological factors affecting the endometrium