A/N:This came to me as I was reading a fanfic about an alternate universe where everyone's gender as switched, and how horrible it would be for the Marines to attack the Straw Hats when they were all on their periods. I nearly pissed myself. 'It's That time' by elvenarchress. Go read it.

Very minor spoilers, obviously. Also an established relationship.

With two women on the boat, one of them with a notoriously short fuse and the other a former assassin, it was a wonder that the men of the Thousand Sunny survived each month. About half of the men on board actually noticed the symptoms (and Chopper, despite being a Reindeer, was a doctor). They knew to steer clear of the two demons during this time. That is, besides the more oblivious members and Sanji, who if anything smothered them even more in his attempt to make them feel better.

Luffy, it seemed, was not of the more informed half of the crew.

So that morning when Nami groaned in frustration, the pain in her abdomen making her want to throw up, and Luffy smiled at her from his spot next to her on the bed, she did what came natural to her: she punched him.

Mugiwara was unceremoniously kicked out of his navigator's room pretty early in the morning, earlier than he usually would have left to go bug Sanji for breakfast. And he had no idea why. Thus, he took it personally and dragged himself towards the kitchen for an early breakfast, all the while sulking like a kicked puppy.

Both Sanji (naturally, because he had to beat Luffy to the kitchen) and Franky were up. When Luffy walked in, shoulders slumped and eyes downcast, the shipwright looked over at his captain with raised eyebrows. "Oi, senchou, you're not looking so super today."

This caught Sanji's attention (and the fact that Luffy had snuck in so quietly) and he glanced over his shoulder with a rather smug expression on his face, having guessed that the rubber man had gotten in his Nami-swan's bad graces. He was in the metaphorical doghouse.

"Nami hit me, and I don't even know what I did," the black-haired pirate mumbled into the table.

Franky laughed at the younger man's troubles, "She gave you the cold shoulder?"

Luffy growled at him, before a sudden loud noise distracted the two of them. The cook had deigned it necessary to put down whatever had been in his hands with more force than necessary and scrambled over to the little calendar on the wall. With an over the top gasp of surprise the blonde ran back into the kitchen and dove head-first into the fridge, obviously looking for something in the back where it was less likely to be nabbed by hungry fingers.

Another laugh erupted from Franky's mouth, who had also seen the significance of that particular day.

"What's going on?" Luffy asked the both of them.

"Looks like it's nee-chan's time of the month."

"I don't get it," Luffy said with genuine confusion.

"What?!" both men in the vicinity shouted at him, causing the captain to clutch his ears from the vicious onslaught on his eardrums.

"I really don't understand, why is today so important?" As far as Luffy knew, today wasn't anyone's birthday and there were no holidays for a while. He was also confused as to what this had to do with Nami.

"Mugiwara, are you trying to say that you're the only one here currently getting any and you have no idea what a woman's 'time' is?" Franky asked the captain with an expression that spoke the question: how the hell did that happen?

"No idea," Luffy repeated. He was a little annoyed that they were still beating around the bush and not explaining to him what this 'time' meant. From behind them, both could hear the now angry muttering of Sanji. Things like 'shitty-captain', 'shitty-rubber-brat' and 'why him?' drifted towards them.

Franky, still grinning, flipped his sunglasses up onto his forehead before leaning forward to stare right at Luffy. "You see, once a month a woman has this special time. It's usually lasts around a week and is at different times for every woman. They become demons and will rip out the entrails of anyone who pisses them off. They get angrier, grumpier and like to extract revenge on people they get annoyed at."

Luffy paled, eyes widening in horror, "And that's happening to Nami?"

Franky nodded seriously, "This'll probably go right over your head, and I'm no expert, so if you need details ask Chopper, but their hormones go outa wack because their cycle is just ending."

"So it's a mystery feeling!"

"Pretty much," the cyborg shrugged. It was all their captain needed to know at any rate. Franky dreaded what Nami would do to him if he told Luffy that she was bleeding and in pain for the entire week. He probably would have gone ballistic and do something to completely embarrass them both and the rest of the crew. He need not mention the wrath Nami would unleash upon them later.

"So you need to treat Nami-san extra nicely for the next week, or I'll use you as bait for our next meal," Sanji growled.

Having agreed to be careful, the morning proceeded normally. Breakfast was an interesting affair, like always, but many of them noticed that Luffy seemed to be making an effort to stay out of Nami's way. Something they found odd.

And then, on their way out of the Kitchen, they all glanced one by one at the calendar, where, for the next week, every day was circled in red and a drawn in stick person with X's for eyes was splayed along the margin.

They all assumed that Luffy had finally learned what was going on. Zoro thought it was hilarious and continued to keep a close eye on the pair.

Luffy avoided Nami like the plague, but when she talked to him he would try to not say something stupid. When he thought that he did say something to set her off, his sentence would end in something along the lines of 'I mean – sorry!' The skeptical look that she shot him in return was one of the funniest things that Zoro had ever seen.

What really took the cake was when three days later Nami got fed up with Luffy hiding from her and literally dragged him by the ankles into her quarters (Franky, to be prepared, had made more than one bedroom on the ship, and when Luffy and Nami had hooked up Robin had taken one of the others). The expression on Luffy's face was of a man about to be brutally murdered by their significant other, which Zoro thought pretty accurate. Luffy was a man who feared few things, and yet he could still be scared shitless by the evoked wrath of their navigator.

Sanji was satisfied, Robin was amused, Usopp, Brook and Franky laughed while Chopper wondered why everyone was making such a big fuss over the reproductive cycle.

"Luffy, why are you avoiding me?" Nami snapped at the boy, who was currently across the room and cowering behind her laundry basket.

"Franky told me that you become a demon once a month!" he squeaked out.

"Oh, did he now?" Nami's eyebrow twitched in annoyance. She then sighed, knowing exactly how to reassure him that she wasn't going to grow horns and a tail before ripping his intestines out. "Luffy, I'm not a demon. I'm just in a little pain and it makes me grumpy. It's not your fault so you don't have to hide from me."

Luffy's attention snagged on the word 'pain' and everything after that went right over his head. "You're in pain?!" he exclaimed in shock.

Nami nodded and rolled her eyes, "Yes, but nothing too bad. Every woman gets it and it isn't something that you can't fix, so don't try."

He seemed to deflate and instead pouted, "If you say so."

She smiled at how utterly adorable he looked when pouting like that and walked across the room to kneel in front of him. "I'm fine, alright?"

A cloud-splitting smile spread across his face, "Right!"

With a mischievous grin, she slunk over and slid into his lap, gripping his shoulders and watching his eyes widen ever so slightly. He gave a smirk to match her own, and they were making out with such heat that it would have made Sanji spurt a nosebleed (then he would have tried to kill Luffy after). This lasted at least five minutes until both of them needed to get their heart-rates in check.

"You know," Nami started, resting her head in the crook of his neck, "Once the bleeding stops, I'll make it up to you."

"You're bleeding?!"