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We cannot conclusively label it as a cyst with 18 mm size at present unless a follow-up scan shows no increase in size. This may be a developing follicle so that next follicular study would be significant. It is always best to wait till 20 to 22 mm size of the follicle to initiate an hCG injection to allow follicles to mature entirely and then subject them to rupture.
They may rupture earlier before administration, then you may not need an hCG trigger at all, but the chances of conception with a possible immature egg which ruptured early in the cycle would be low, and one would have to accept that fact. Another important factor is the endometrial thickness at present. It has to increase to allow implantation if any, and pregnancy to be facilitated in this cycle if the left ovarian follicle ruptures naturally in the next two to three days.
So my advice would be to practice timed intercourse, depending on the result of your day 11 transvaginal scan, after reaffirming the status of the developing follicles in both ovaries. Please start Progynova from today.