Biggest concern in IVF (in vitro-fertilization) is Implantation failure. Why the best of embryo fails to implant, to find a single reason is difficult but interaction at maternal interface is very complex and has a definite role. There can be problem at different levels such as abnormal sperms and eggs, mainly because of chromosomal abnormalities simply as a result of aging or some genetic alterations. As a result of strong natural selection abnormal embryos are discarded which otherwise may have resulted in abnormal babies. Immunological interaction when embryo tries to attach to mother .A series of complex immunological activities may help the embryo acceptance or may cause rejection. Abruption of growth of conceptus due to sudden withdrawal of support as a result of hormonal imbalance or any infection may gain access. Chronic maternal diseases like diabetes, high blood pressure, obesity, immunological pathologies like SLE, RA may itself create a hostile atmosphere for embryo growth. Endometrial lining plays a special role in acceptance and chronic infections like genital tuberculosis leaves adhesions or scarring in uterus and may not leave enough blood supply for embryos to attach. Further every embryo is a natures one of its kind building block and any abnormality of growth may stop its development may result in miscarriage which may even get unnoticed if occurs before the date of expected menses, in that case menses may arrive at normal time or may be little delayed.
These factors and many more unknown factors are the reason of averagely 33% implantation rate in humans.