Don was already in his lab when the others awoke and came down that morning, with the intent, focused look of a scientist grappling with a puzzle. As Leo went down for breakfast, he saw Don removing a length of rubber tubing from where it had been tied on his upper arm, and withdrawing a hypodermic needle from his inner elbow. A little later, he was seen swabbing his own mouth, and dropping the cotton-tipped stick into a beaker full of blue fluid.

Mikey brought him breakfast — a box of cereal and bowl of milk — and found him glued to his microscope, scowling down at a small petri dish as if it had offended him. "Did you even go to bed last night, Don?" he asked.

"Hmm? Yes, I did," Don said, looking up. "But I wanted to get started on these tests."

"Tests for what?"

"To find out how I could have impregnated April."

"I thought it was pretty obvious how that happened," Mikey said with a grin. "Master Splinter told us all about it a few years ago. When a Turtle and a human love each other very much…"

Don gave him a sour look, but accepted the cereal and milk. After quickly devouring his breakfast, he went back to studying samples under the microscope, wrapped up in his scientific inquiries.

April came down after the rest of them, sleepy-eyed and yawning, since she wasn't used to waking in complete darkness. The first thing she saw was Don in his lab, which was usually her cue to join him in whatever he was doing. She wrapped her bathrobe more tightly around herself, and came over to glance at what he was studying — a length of paper scrawled with chemical markings and sketches of D.N.A.

Not wanting to disturb him, she slipped off into the kitchen, where his brothers were enthusiastically — and rather messily — eating their own breakfasts. Leo's face brightened as soon as he saw her, and he quickly rose to his feet. "April," he said, gesturing at the empty bench. "Can I get you something?"

"Hi, Leo. Maybe some toast. I'm feeling a little queasy this morning," April said, sliding into the table.

Leo popped two slices of bread into the toaster, and brought the results back to April a few minutes later. She couldn't help but wonder why he was being so solicitous of her — surely he didn't think that pregnant women needed to be waited on hand and foot, especially in the early months when nothing was showing. Then again, she reflected, their entire knowledge of pregnancy probably came from a few TV shows, and nowhere else. They would need to study up on what was needed during a pregnancy.

She spent the rest of the morning watching Raph, Leo and Mikey going through their katas with Splinter, their loud grunts echoing through the lair. But her mind was on Don, feverishly working to uncover how a mutant turtle could have fathered a child. It seemed to almost be an obsession with him — a way to make sense of something that completely blindsided him and dashed his theories about his own body. Clearly he wasn't going to have any peace until he figured it out.

Don only emerged from the lab halfway through the afternoon, loudly announcing, "I found it." He spent the next ten minutes going from person to person, rousing them from whatever they were doing — practicing with swords, watching movies, having tea — and demanding that they come with him to hear what he had found.

He looked tired but triumphant when April, Splinter and his three brothers were all assembled in his lab, waiting expectantly for the answers. "I think I found how I impregnated April," he said, gesturing at his microscope. "I've been testing different fluid samples from myself all afternoon, and found the same thing in most of them — infinitesimal amounts of the mutagen that made us what we are today."

"Is that normal?" Leo asked.

"For us, it is. Our cells have absorbed the mutagen, and it can be found in our blood very easily. It was a little harder to nail down some of the other samples, but I suspect that our ejaculate, containing cells intended for one purpose, also contains trace amounts of mutagen." Don held up a petri dish to the light. "So the mutagen acted as a catalyst, causing cellular bonding between my gametes and April's. Normally sperm from a mutant turtle would never penetrate a human ovum, but the presence of the mutagen causes… unforeseen changes to happen. It took two things that were never meant to go together, and caused them to combine and grow."

April put a hand on her stomach, wondering what those changes were. Her scientific expertise lay more in technical and computer areas than in biological sciences, but some of the things Don was saying were starting to make her feel nervous.

"So now that you know how the conception happened, doesn't that mean everything's fine?" Leo said, a hint of hope in his voice.

Don's face twisted slightly, as if Leo had reminded him of something that caused him anguish. "Not by a long shot. I've only encountered mutagen as the catalyst for creatures that were already fully formed, and never more than one species. There's no precedent for this, and… I have no idea what kind of results a mutagen-based conception might produce. There could be problems, like…"

"Like what?" Mikey asked.

Don looked at April, misery swirling in the depths of his eyes. "Like birth defects and deformities. Non-viability."

Silence fell over the lair. April suddenly felt cold, and both of her hands came up to touch her stomach. She knew her baby was barely the size of a pea now, but she had a desperate desire to know what it looked like — to make sure it was okay, that somehow her "miracle" had been conceived without anything wrong with it.

And then even worse thoughts floated into her head — what if it didn't survive? She was only a month and a half into her pregnancy; there was still plenty of time for an unworkable genetic structure to unravel what had been made, for that tiny life to blink out. And despite her anxiety the day before, April found that she desperately did not want that to happen. She wanted her baby whole and healthy, whatever kind of creature a human and a mutant turtle had produced.

"Let us not seek trouble," Master Splinter said, moving closer to Don. "You told us that you had no idea what such a conception might involve, Donatello. That means you do not know if such problems will arise, or even if they are likely?"

Don's shoulders slumped. "No, I don't know. I don't know anything about this."

"Then we should not seek out fear over things we cannot control," the rat master said firmly. "You will only give yourself and others sorrow and grief, and it may be that the child will not suffer these problems. The mutagen is a powerful substance, and if it has the ability to turn four ordinary turtles into my strong and skilled sons, surely it has the ability to conceive a child without terrible consequences."

Don looked into April's eyes, and she could see that the turmoil in him was still there — he was still afraid of what might happen to their baby. She moved closer to him and gently put an arm around his shoulders, drawing him into a loose, brief hug that bumped his abdomen against her belly. One of his hands came up to lightly touch it, almost as if it frightened him.

"I have one more question," Leo's voice punched through the silence. "If I understand correctly, this means that… this could happen to any one of us."

"With a female human partner, yes," Don said raising his head. "I — when I previously tried to determine our fertility, I was basing it entirely on our genetic changes and our uniqueness. It never occurred to me to test for mutagen. And since the four of us are very similar physically, it means that the three of you also have ejaculate laced with mutagen. It means that… we're fertile with humans."

"That's — something very important to know," Leo said slowly, crossing his arms in front of his chest. "It's information we need to have about ourselves. For the future."

"And it's all thanks to Donnie's potent and fertile loins," Mikey announced.

"Please, never talk about my loins again," Don groaned.

"But dude, they're the reason you're in this situation. And they got April pregnant — they're awesome!"

Raph smacked Mikey across the head, eliciting a pained yelp of "ow!"

"Well, with no idea of what's ahead, and no way of affecting it," Leo said at last, "we should probably get back to preparing for the baby. There's a lot to think about, like medical care and various things that babies need, which we'll need before it's born."

"I don't think I can go to a regular doctor about the baby," April said quietly. "They'll know that something is strange about it before too long."

"I'll start building an ultrasound machine," Don said, determination coming into his eyes.

"And probably April should start eatin' healthy," Raph said unexpectedly, resting his elbows on his knees. "Babies need extra food, don't they? So they grow right?"

"We'll start drawing up a diet plan for pregnancy, if April's all right with that," Leo said, resting his hand on his chin. "And we'll have to adjust it for turtle physiology — more calcium, for one thing."

April began to reply, but stopped as they heard a familiar voice booming from the elevator door to the surface. Normally it wouldn't have caused her much distress, but right now all she could think of was how she was going to explain what was going on.

"Hey, guys! It's Casey! Where are all of you?"