Cannon minions (egg cells) spawn (ovulate) every other wave (cycle). Minion waves build up the game state (endometrium) by pushing (sustained by progesterone from the corpus luteum).
Every so often, a top laner will come around with S7 Banner of Command (sperm cell) and promote (fertilize) one of the cannon minions. Combining a Banner of Command with a cannon minion creates a promoted minion (zygote) which can then push down the lane (fallopian tubes) and reach the base (uterus) to hit the nexus (implant in the uterine lining and create a pregnancy).
When the minion wave (endometrium) becomes too big, the enemy team has to respond to stop your push (involution of the corpus luteum after 14 days). So the Level 16 Sivir runs it down mid and presses W and clears the entire wave, stopping the entire push, and resetting the lane (menstruation). Watching the Sivir clear the whole wave with one button is painful and makes you feel like shit, even though your top laner will never understand what it feels like. This cycle repeats every month.