In a normal cycle once ovulation has occurred the corpus luteum (the leftover follicle the egg was in that is now collapsed due to it’s absence) produces progesterone. The progesterone level it produces is necessary to sustain the remainder of the luteal phase (later half of the cycle from ovulation to your period) and also sustains a pregnancy through the majority of the first trimester should one get pregnant in that cycle. If a pregnancy doesn’t occur then the progesterone levels drop and estrogen rises indicating to the body to flush the uterine/endometrial lining (hence the bleeding of your period) to start a new cycle and start the process all over again.
Okay so with infertility progesterone is notoriously low in the luteal phase. There can be several reasons for this (one is forcing maturation of these follicles with stims or forcing ovulation with HCG injection, another is damaging the corpus letuem’s during egg retrieval in the case of an IVF and so on…), but if your progesterone levels are too low then they cannot sustain the luteal phase and/or a pregnancy should it occur and clearly that is counteractive to the desired results of a successful pregnancy so…long answer shorter–they supplement with the injection to ensure that you have enough progesterone and don’t flush out the lining prematurely before the embryo has a chance to fully implant and to ensure the embryo doesn’t miscarry due to the lack of progesterone supporting it’s growth until the placenta is strong enough to take over this production closer to the second trimester.
In essence they want to give a pregnancy the best possible chance and reduce the possibility of miscarriage or chemical pregnancy. Also, the reason they don’t just test you and only supplement those that are low is because the level can rise and drop at any time so it isn’t a good indicator of who needs supplementation AND often by the time the low level does show up it is too late to reverse the issue with supplementation so this is a situation of needing to be proactive and ensure the proper levels are in place following ovulation and continuing until a pregnancy is established enough to take over it’s own production.
Hope this helps. :flower: