Prologue
Tobirama presents as an Omega at 14. Fortunately, when he experienced his first heat, his father was away on a mission. Butsuma would only have a few servants in the house at a time and does not permit anyone from the clan to enter without his permission because he had become paranoid ever since he lost his wife. Tobirama was born albino and was constantly ill, so they thought little of his absence. Hashirama became concerned. He went to check on his brother, and Tobirama begged not to tell their father that he had presented as an Omega.
It was a fucking nightmare concealing his Omega status in a compound full of Shinobi. Fortunately, Hashirama had been here to help him. Hashirama had presented as an Omega three years prior, at 13. Tobirama has a hard time dealing with Omega's instincts and needs. He can't keep a nest in his room permanently. When he builds a nest, he has ensured it is not visible from the doorway. Tobirama can’t leave the nest, so he takes it apart afterward. Although, Hashirama was helpful because he could join Hashirama in his nest anytime he needed it when he couldn't make his own. In the case their father asks, the answer is straightforward: Hashirama simply wanted his family close to him since Butsuma refuses to join, which causes the instinct to be triggered more often than normal. The reason Butsuma always refuses is that he has not been in a nest since the death of his wife.
They restricted Omegas of the Senju clan from fighting unless they are the clan heir or clan head. He had never told his clan that he presented, nor had he ever planned to. Because Senju law prohibits him from the battlefield just because he is an Omega. If the clan finds out, since Tobirama is Hashirama's heir, he will still continue to train until Hashirama has children of his own. If Butsuma died before Hashirama had children, so Tobirama became the clan's heir. Once he is heir, he may join the battlefield and undertake missions. Since Hashirama is Butsuma's heir and will be the next clan head, he does not have to worry about being barred from the battlefield, unlike Tobirama.
Butsuma wants Hashirama to have a female Alpha. The rationale with this is that if Hashirama has a female alpha, she can assist him with his heat and carry a child instead of Hashirama, so stop the clan from losing their future clan head from battles or missions while he's pregnant. Unfortunately for Butsuma, Hashirama refuses all the female alphas in the Senju clan and firmly states that he intends to carry any children that he has. As a consequence, it prohibited Hashirama from getting married or having kids if he intends to carry them himself, as long as their father is the clan's head.
The Senju elders didn't want to spare Hashirama from the battlefield or missions for the duration of pregnancy and breastfeeding, but after Butsuma died and Hashirama became the clan's head, there wasn't much they could do. Even though they attempted to introduce him to female alphas from their allied clans who wanted an Omega but also wanted to carry the pregnancy. However, all their efforts failed and fell on deaf ears. Tobirama would be permanently banned from the battlefield as soon as Hashirama gives birth to his first baby. Even after the death of Butsuma, Tobirama refused to reveal to the clan that he had presented as an Omega. As a result, very few people were aware of his secondary gender.
Only Hashirama and, more shockingly, Izuna Uchiha are aware of his secondary gender.
Now, Izuna is a recent discovery about his secondary gender that could have something to do with a suicide mission they both found themselves on, that might or might not have required them to cooperate to survive, and yet even despite that, they escaped only by the skin of their teeth. This may have resulted in them being trapped in a cave during a lightning storm, where they had some 'oh-gods-we're-not-dead' sex a week before Tobirama’s heat was due.
Tobirama's heat cycle was irregular, so when it was overdue, he paid it no mind. Tobirama had vomited up after the battle yesterday, where he wounded Izuna with newly unveiled Hiraishin, and he had felt a little queasy the morning after the battle. He was also more tired than usual. He thought he was coming down with something, so he went to the infirmary.
The head med-nin did a diagnostic jutsu to find out what was going on, but froze in shock.
"What's wrong?" Tobirama asked.
The head Med-nin responds, "You're an Omega! Why didn't you tell us the moment you presented?"
Tobirama froze before asking, "What's wrong with me?"
"You're pregnant," the Head Med-nin replies. She blurts out as she exits the room, "I'm going to report your secondary gender to the council and inform them you are pregnant."
Tobirama panics and goes to find Hashirama. Hashirama was in his garden.
"Tobirama." Hashirama grinned when he saw him, then frowned. "What's wrong? You look panicked."
“Recall the mission I went on a month and a half ago? I met Izuna on the same mission, which ended up being a suicide mission. It forced us to work together. I ran out of the scent blocker halfway through the mission, Izuna found out, and then we got trapped in a cave near the end of the mission, and we had sex. I didn't start feeling nauseous until yesterday. I thought I was coming down with something, so I went to the infirmary and the head med-nin found out that I'm an Omega and also told me I'm pregnant. Before I had time to process, the med-nin left to go inform the Senju Clan Council that I'm an Omega and pregnant. Soon everyone in the Senju Clan will know I'm an Omega." Tobirama exclaims in a panic.
"What do you mean, you're pregnant?" Hashirama exclaimed in shock. "And what do you mean, you had sex with Izuna?"
"Yes, I had sex with Izuna, but it was consensual. I didn't find out I was pregnant until just a couple of minutes ago. What should I do? What are we going to do? The elders are probably discussing what's going on right now, and the whole Senju Clan is going to know I'm an Omega! What are we supposed to do? Senju Clan law states that if a pregnancy occurs between Alpha and Omega, that is consensual, they are required to marry by clan law. What am I supposed to do? Izuna's wound will be fatal because of how lacking the Uchiha of skilled Med-nin is." Tobirama says, still panicking.
Hashirama pulls Tobirama into a hug and releases calming omega pheromones.
Having waited for Tobirama to calm down, Hashirama says, "I should send a scroll to Madara explaining what's happening, offering to heal Izuna, and offering peace. After sending the scroll, we can deal with the council."
The Senju Clan elders discuss the newfound discovery of tobirama secondary gender while Tomirama takes a minute to respond and then follows his brother to his office to write a Scroll, so he has at least an hour before they come for them.