Luteal phase defect essentially is when the second phase of a cycle after ovulation is not supported well. This second phase depends on the corpus luteum, from where the egg has been released, to start secreting progesterone and it is this progesterone that supports the endometrium (lining of the womb).
There are cases in which a few days after ovulation there is a slight bleeding or spotting of blood that tends to occur, and this can slowly result in a period. To a certain extent this signifies a defect in the luteal phase. It is in fact extremely difficult to diagnose since its diagnosis requires a histopathological, a biopsy, evidence which is very difficult to obtain. Thus to a large extent it is what we observe from the evidence from how the treatment is carried out.
Luteal phase defect can be endometrial which means the defect lies in the progesterone activity in the endometrium or the defect could lie in the corpus luteum where the secretion is inadequate which may reflect on the quality of egg.
Both tend to mean very much the same. Often it is the quality of the egg that is released which may also determine what is left behind and thus supporting the endometrium, but sometimes the secretion of hormone is adequate and the progesterone effect on the lining of the womb, the endometrium, is not adequate.
There are different ways to treat luteal phase defects, one of the simplest is to add a small amount of HCG, the pregnancy hormone, a few days after ovulation, which helps to sustain and prolong the life of the corpus luteum. This is what a pregnancy tends to do. Pregnancy generally prolongs the life of the corpus luteum and thus allows for the corpus luteum to continue to work. By mimicking this, you can often extend the life of the corpus luteum and in a large number of cases prevent the spotting. Another treatment option is to add progesterone, a high dose of progesterone given after ovulation may also enable the support of the endometrium.
Ultimately, there is no fixed treatment and to a large extent the decision is made on a case by case basis depending on how the response is.