Can optimal serum levels of progesterone in in vitro fertilization and frozen embryo transfer cycles be defined?

Progesterone is essential for endometrial receptivity, implantation and the maintenance of an early pregnancy. In spontaneous conception cycles, progesterone is produced by the corpus luteum; however, after hormonal stimulation for preparation for in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) the luteal phase is characterized by a variably severe luteal phase insufficiency. The necessity for a luteal phase support is indisputable and has to be selected according to the preceding treatment. So far, "minimal" and "optimal" ranges of progesterone levels to achieve and maintain a pregnancy are not clearly defined, neither for transfer cycles with fresh embryos nor for transfer cycles with frozen embryos.