Before knowing the reasons why an egg might not attach to the uterine endometrium, one should know the causes and factors behind implantation.
All the eggs produced in a woman's lifetime are stored in her ovaries since birth and are in their peak condition during her teenage years. With increasing age, and attainment of puberty (beginning of the menstrual cycle) the eggs start getting released from the ovaries, monthly. Hence women do not keep producing eggs. This is in stark contrast from men, who continuously make more sperm.
The process of monthly release of an egg is called ovulation. The egg then enters the nearby fallopian tube that leads to the uterus. Now, when the male and female partners have unprotected sexual intercourse, sperm that is ejaculated from the man's penis may reach the egg in the fallopian tube. If one of the sperm cells penetrates the egg, the egg is fertilized and begins developing.
The egg takes several days to travel down the fallopian tube into the uterus. On entering the uterus succesfuly, a fertilized egg usually attaches to (implants in) the uterine endometrium (inner lining of the uterine wal). But not all fertilized eggs can successfully implant themselves. If the egg is not fertilized or does not implant, the woman's body sheds the egg and the endometrium. This shedding causes the bleeding in a woman's menstrual period.
When a fertilized egg does implant, a hormone called human chorionic gonadotropin (hCG) begins to be produced in the uterus. This is the hormone that a pregnancy test measures. It prevents the uterine lining from being shed, so the woman does not have a period. Other signs such as breast changes and nausea occur in a woman's body, further indicating that pregnancy has begun.
Coming to the fate of the fertilised egg, researchers have conducted newer studies which might explain the failure of eggs to implant. In the study, researchers concluded that one important determinant in the implantation of embryos was trypsin. They found that human embryos typically produce an enzyme called trypsin, which signals the womb to prepare its lining for implantation. But due to significant genetic abnormalities in some embryos, the trypsin cannot signal the womb properly, and it is due to such altered signals that a stress response is produced in the womb that could make implantation difficult.
If it so happens that the endometrium – the inner lining of the uterus - fails to sense the otherwise normally generated chemical signals from the fertilized egg, it in turn, silences many of the genes involved in allowing it to embed in the uterus. This is comparable to an ‘entry on the basis of an examination’ model where if trypsin is not generated in sufficient amounts, the embryo ‘fails’ – it is not accepted and is left to disintegrate, resetting the cycle. One in six couples will experience some sort of infertility, which can be both frustrating and daunting, and many will turn to IVF