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The procedure of IVF involves preparing many eggs, their retrieval, insemination & finally embryo transfer. This sounds that once an embryo is prepared & kept in uterus, pregnancy should happen definitely, but unfortunately it is not true always. At every step, it is unpredictable, for example, success of IVF depends on the age of woman and the ovarian response. Success rate is lesser in older age woman as compared to young females.

The number of eggs retrieved also has influence on IVF success rates. Lower retrieval rates are expected in older woman, in woman with high FSH levels, in poor responders & in woman with ovaries difficult to approach during retrieval.

Number of follicles on sonography – usually we expect as many eggs as the number of follicles but many a times we get less number of eggs.

Number of mature eggs – are usually less than the number of eggs retrieved, because some of the egg may be immature, some degenerated & fractured. Only the healthy appearing mature eggs are expected to fertilize & develop into embryos.

Number of eggs fertilized- Once inseminated or injected by sperm, eggs are placed into an incubator & inspected after 18 hours for fertilization. Not all the mature eggs fertilize. Some remain unfertilized, some fertilize abnormally. On an average, about 60-70% of mature eggs become normally fertilized. Fertilization failure may happen because of poor sperm survival, poor sperm binding, thick zona or poor culture condition.

Number of embryos cleaved – Once fertilized, eggs develop into embryos. At this stage too, some embryo many not show any development or fail to cleave known as cleavage failure. Some may have a slow rate of cleavage indicative of poor quality embryo. Sometimes embryo may divide initially (e.g. two cell stage), but then stop dividing, known as embryonic arrests. 

Quality of embryo –  is also unpredictable; some embryos may have fragmentation,and unequal blastomeres (poor quality). Good quality embryo has no or very less fragmentation & equal blastomeres. Morphologically normal good quality embryo may not be productive of embryos potential. There may be other abnormalities that cannot be judged simply by morphology.

Transferring embryos at the blastocyst stage –  enhances our ability to assess embryos. Only 30 -40% of embryos reach the blastocyst stage. This enables us to select the embryo with the best viability. This means if a patient has 10 embryos, on an average only three to four are expected to reach blastocyst stage.

Cryopreservation of embryos – 2-3 good quality embryos are kept for immediate transfer. Remaining embryos are cryopreserved only if they are of good quality, so that they can survive freezing & thawing. There is approximately 70% recovery rate of frozen thawed embryos.

The Lining – The soil is important for the seed to grow. The lining of uterus should be good for embryos to implant and grow. Uterine lining may be damaged by infections like tuberculosis, repeated abortions and curettage, or by polyps or myomas. Hysteroscopy is done prior to IVF cycle if damage to uterine lining is suspected by history or on sonography. In case of severe damage , surrogacy may be the better option