All he wanted was tea. A simple thing to ask for and to get, all he had to do was pop to the kitchen and get some. It wasn't even the tea that was the problem, it was what caused him to spill it.
A baby.
In his console room, on the jump seat in one of those baby-carrier things and all, was a tiny little infant. The thing looked at him with a small high pitched giggle like it recognized him and reached it's chubby little hands towards him. The Doctor was sure that it wasn't there when he'd gone to get tea.
He stared at it, frozen to the spot and it reached out for something. He looked behind himself to see if there was something it might be reaching for and realized the thing wanted him. He swallowed, taking a hesitant step towards it. When the nothing blew up or tried to kill him, he took another step until he was right in front of it.
Both of them stared at each other until the baby giggled something, causing the Doctor to flail his arms.
"Did you just call me dad?!" The baby reached for him again and this time he lifted it, holding it arms length away from him. "I'm not your daddy." He said sternly. The baby babled again. "Oi! That's rude. And stop calling me dad. I'm not your dad." It's face started to scrunch up as if it were about to cry. The Doctor panicked, holding the child close to his chest and shushing it as it whimpered. It nuzzled into his chest, giving him an uneasy feeling.
He had no idea how to deal with this child He didn't know where it belonged or how it even got into his TARDIS. He needed help and there was only one person who he trusted to help him. He began to pilot his ship to the 51st century, flipping leavers franticly.
The baby began to cry. As soon as he landed, he raced out of the door to find River. this would definitely be her in her later years as a professor, based off of the rows of flats he materialized near. "RIVAH!" He banged his hand on the front door of a blue flat. "RIVAAAH!" He yelled.
River grunted at the noise, awakening from her peaceful sleep. She pulled herself off the couch she was napping on, padded through her living room, and looked at her husband through the peephole in the door. "What is it, Doctor?" She squinted at the light as she opened the door. Her house seemed to be a mess as well as her hair. She looked quite tired, but the Doctor didn't notice.
He thrust the baby towards her. "HOW DO I CARE FOR THE BABY THAT SOMEBODY PUT IN MY TARDIS!?"
She blinked at him. "Care for her like you have been for the past few months."
"What is that supposed to mean? I've never seen this baby before in my lives!"
River took the child from him, rolling her eyes. "I knew this would happened eventually." She gestured for him to follow her inside, the baby calming under her touch.
He nervously walked in behind her, stepping over towels and baby toys. "How are we supposed to find it's parents? What if we can't, how am I supposed to take care of this thing? I don't know how to change diapers and what do I do about feeding?" He didn't have a mammary glands the last time he checked.
He watched as River rocked the child and stroked her hand along it's back. She looked like she knew exactly what she was doing, which surprised the Doctor. He hadn't thought she had much experience with children, let alone little infant-y whine-y ones. "It's fine, Doctor. I've got her." She murmured, sitting back down on her couch.
"What just like that? I don't even know how it got on my TARDIS or how to put it back." He ran a hand through his hair. "What am I going to do?"
"I put her there." She looked up at him through tired eyes.
"You what?" He was stunned into sitting.
"I put her on the TARDIS, dear." River repeated.
"Well why would you do that? What does it have anything to do with me? Or you."
"She," River emphasized this time, not a huge fan of the Doctor calling the baby an it. "is our daughter."
"Our what?" His jaw hung open. River simply gestured around the room. It finally clicked, for the Doctor. All of the toys and little bottles suddenly had a reason that they were there. River's tired appearance and expert ways handling of the child made sense now. "…A baby. You and I… we… baby…?"
"Yes, to put it one way." She closed her eyes. "It's your turn to watch her for a while so I could get some rest, but I must've given her to a you that didn't know about Donna yet." When she opened her eyes again, the Doctor was sitting next to her on the couch. He was staring at his daughter.
"We named her Donna?" He swallowed.
River nodded. "It was your idea, actually. Now I see why."
"…Baby." He sputtered again. "We have a baby. We made that… when?"
"Spoilers." She muttered. She didn't seem to worried at the fact that he'd found out about their daughter before she'd even been conceived (in his time.) That, or she was too tired to care. "In good time but by the looks of it," She glanced at her husband. "Soon, for you."
Donna stirred, blinking up with large blue-green eyes at her father. "No, you cannot put her in a fez." River said.
"I didn't even say anything!" He defended.
"No but you were thinking it. Here." She handed Donna to him. "I'm exhausted and now that you won't completely panic, you take her. I need sleep." With that she went up stairs to her bedroom. When she woke that evening, she found the the Doctor had dressed their daughter in a mini fez and bowtie.