Because Kushina didn't loose her first child, and suddenly the world was a very different place. A "what-if" of a happier place.
The day Minato found out was the day after Kushina had to be admitted to the hospital.
The war was still ongoing, brutal and bloody and destroying everything in its wake. Children were killing and, even worse, it was starting to become the norm. The war was forcing people's ideals and morals to change, and what was once unacceptable was now excused for the sake of the greater good of the village.
Minato was deployed whenever he could, with very few time in between missions. Normally, shinobi had a certain quota of missions to reach before they were forcibly removed from service in order to recuperate properly. Murder was an emotionally taxing job.
Minato was pretty sure he had fulfilled that quota about 10 times over since the war began, and he didn't see it ending any time soon.
The exhaustion, both mental and physical, as well as the stress of fulfilling his missions and keeping his kids alive (Dear god, they were all so young and they had to see so much. Endure so much.) was taking it's toll on his personal life, and as much as he loathed to admit it, his skills. Both social and shinobi.
That's probably why he didn't realise his (secret) wife had missed her cycle (They had to keep a very strict schedule to prepare for when Kushina went ape-shit. Crazier than normal, anyway. And the results were usually disastrous for all parties involved). He didn't realise her suspicious aversion to certain foods, her sudden cravings for random food, and most significantly, the limited intake of her favourite non - nutritional ramen (out of everything, that should've been Kushina's most incriminating clue).
There were loads of little clues that should've added up his mind (and probably, at the back of his mind, the clues were accumulating to a bigger picture, genius as he was), but quite honestly, Minato was far too taxed to think in the few hours he got of non-kill-or-be-killed.
So when he got back from a mission one day and found out that his Uzumaki, vitality-full, long-living wife of 3 months had collapsed in the hospital due to exhaustion, Minato didn't know what to think. He could only consider the worst. Enemy attack, chakra sickness, cancer. Anything.
But Minato felt one overwhelming feeling above all the panic and confusion he felt as he rushed out of the building, abandoning his team with the confused ninja who had been the one to tell Minato the news. Regret.
Regret at the fact that he didn't know. At the fact that he couldn't know, with all the killing and surviving he had to do. Regret that he couldn't be a better man. A worthier man. A man who didn't have to kill, who didn't have to go to war.
A man who could look after the woman he loved without having to go away to war for a few months mere hours later.
He could've been so much better. He SHOULD be so much better.
But right now, he should be there, when she needed him most.
He needed to see Kushina.
...
"Good morning"
Kushina woke up to the feeling of warmth and comfort. Of a gentle touch making its was through her hair, as a familiar scent and presence enveloped her in its security.
"Mornin', 'Nato" she whispered softly, as if scared that a louder noise would disturb the peace and get rid of presence she had so craved in the 2 months of his absence. Kushina missed that little smile he got whenever she called him by the nickname he allowed only for her, of the food he hated so much.
His hand didn't stop its motion, although his other hand did tighten its hold around her waist slightly, from where he laid beside her on the hospital bed. His words were just as soft, maybe even softer, looking at her both in awe and fear. "How have you been?"
He thought she was the most powerful woman in the world, but he feared so much that she could break so easily as soon as he turned his head away.
Her answering smile was so bright, it blinded him far more than the early morning sun filtering through the curtains. "I was just a little tired. And stressed. You've been gone so long and there were rumours of an ambush and I just... I panicked, because I never got to tell you, and I'm sorry and you never would've known that you would be-"
"Kushina, Kushina, stop. It's fine. You're fine. I... I'm the one who should be apologising. I'm sorry I wasn't there for you and compromised your health like that, especially now. When... Kushina. Fucking hell" he ran a shaking hand through the mess of blonde on his head. She had only heard him swear twice before. "I just... I'm so happy... How... What... I-"
Kushina placed a gentle hand on top of his shaking ones, fingers curling around his to placate the trembles. "Minato. I'm fine. We're fine. She- she's great"
Minato's eyes shone a brighter blue at this new piece of information, his eyes like that of the sky on a clear summer's day. Kushina could only hope that she inherited that from him, if nothing else. "She's a girl?"
Kushina's answering laugh was like that of the wind chimes which tinkered on that clear summer's day. "We're having a girl!" She squealed, their laughter blending together and echoing with the happiness only a mother and a father could have.
They were going to have a child.
A light in the darkness of war.