 First of all, let's figure out what happened to the estrogen. What happens to estrogen at this stage in the game? I can do this. There's my pink. Estrogen, we don't have the follicle anymore, so the estrogen levels are going to drop, just like we would expect. However, who's this guy again? Progesterone, and look at what happens. I'm changing progesterone's color. I started with it as green because I was showing you that the granulosa cells started to produce progesterone toward ovulation. Guess who takes over the job and takes it over with gusto? The corpus luteum. The corpus luteum suddenly starts producing mad progesterone. Now, let's talk about progesterone. Everything that estrogen was, fertile, slippery cervical mucus, soft open cervix, nice feelings like happy, cuddly, squishy, uterus. Life is good with estrogen. Progesterone is like back up out of there, dog pound. Cervical mucus becomes like a sticky, mucky, like good luck getting through there, you punk sperm. You're not getting in here. Cervix closes up. There's a plug, like there's that actual like block that gets formed in the opening of the cervix so bacteria can't get in, keep those punks out. Sperm can't get in, like, dude, just get out of here. Why are we creating an environment so now hostile to everyone around us? Huh, this is an interesting time of the cycle, isn't it, folks? Well, we become quite hostile because if we were successful at getting some sperm in during this time, we don't need more. There are only, like, what, 200 to 500 million sperm in every single ejaculation? So, dude, one time is plenty. We only have one secondary oocyte capable of being fertilized. No need to bring anybody else into the mix. All right, that's cool. Oblation has already happened. The egg is only going to live for 24 hours maximum. So corpus luteum starts producing progesterone. Now, here's the fact. Corpus luteum lives for 12 days. That's it. After 12 days, the corpus luteum dies, which means what are you going to expect to happen to the progesterone levels? You are going to expect them to drop back down to their original levels, and in fact, it happens. In fact, if you knew the day you ovulated, you could guarantee that you will start your period 12 days later. Unless there's only one way to prevent that from happening. 12 days later, there's one person who is capable of keeping the corpus luteum alive. Stay alive. Who do you think that could possibly be? Let me first of all make sure that I told you all the horrible things that progesterone does. Yes, I did. All right. The only thing that can keep the corpus luteum alive is if one of those sperm that you actually welcomed in when estrogen was in charge, if one of them gets lucky. Let's talk about how that works.