River's first thought is that she isn't keeping it.
Her second and third are that she's never going to tell him and that she should probably check in case she really did just eat something that disagreed with her. Every morning for the past three days. River knows her biology, knows that this sort of thing shouldn't happen and never lasts when it does. Let nature take its unkind course and she won't have anything to worry about.
River also knows that Time Lords aren't like other people.
She keeps putting it off and she isn't sure why. She doesn't want children, certainly not at 26 with a career to think of and several partners who have expressed similar thoughts on matters of reproduction. She knows, just knows, that it's his, but that still leaves some uncertainty. They're all the same, they're all different. She runs into Suit & Sideburns when she isn't even looking. She wonders if it's his. He takes her to an amusement park in the 20th century and on four occasions the words falter in her throat. She can never tell this one, even if she's changing her mind about keeping her secret forever. This one would fall to his knees and beg her, so needy and so utterly selfish.
She loves him anyway, and would never hurt him the way she could. So she says nothing, and talks nonsense as they eat candy floss on the pier.
It doesn't mean anything that the one in the Amnesty t-shirt doesn't know why she slaps him and calls him a useless bastard. She's known the Doctor long enough to know that his future and hers are capable of changing each other. He doesn't know yet, if yet is a concept that can be applied to the pecularities of time-travel and out-of-sequence meetings. He might find out in her unknown future, his malleable past. Time is being rewritten inside her.
She's putting on weight when she sees him again, in the tweed and the bow-tie. Could be worse, could be better, but she knows she's been found out when she steps into the TARDIS and that mechanical hum rises in pitch to something like disapproval. She waits, and waits, and after two endless days he opens the door of her bedroom and says "Sorry."
"She told you," says River, and closes the book she's been pretending to read. "Now what?"
"I don't know," he says, and in the half-light he looks terrified and miserable and overjoyed all at once. Trick of the shadows, she tells herself.
At last she asks "How many children have you lost?"
"One's too many."
"Then more than too many."
She moves to sit on the edge of the bed and beckons him into her room. "I haven't decided yet," she answers before he can ask. "I don't know what I want anymore."
"It'll age, it'll die, it'll leave me more alone than I was to start with."
"This is the worst idea either of us has ever had."
"But you always get what you want, don't you?"
"I'm not going to make the choice for you. I could never... I could. I know I could. I'd never forgive myself, but I could. Look into your eyes, touch your skin, you'd have no more worries."
"Don't you dare."
"I won't. I was just... being honest for once." He ducks his head. "I'm not very good at that."
"Oh, I know." She tries for levity. "Heads we win, tails we lose?"
She can't pretend she's shocked when he produces a sixpence.