Dajeil stroked Byr’s arm again. ‘Well, however it turns out, we’ll see. But you see this way we can have . . . whatever time in between Ren being born and our other child.’ She looked at Byr, smiling warmly. ‘We haven’t decided on the other name yet. Anyway,’ she went on, ‘doing it this way will give me time to recover and get the two of us used to coping with a baby, before Byr has his . . . well, hers,’ she said laughing, and put her arm round her partner’s shoulder.

‘Yes,’ Byr said, glancing at her. ‘We can practise on yours and then get it right with mine.’

‘Oh, you!’ Dajeil said, squeezing Byr’s arm. The other woman smiled briefly.

The term used for what Dajeil and Byr were doing was Mutualling. It was one of the things you could do when you were able - as virtually every human in the Culture had been able to do for many millennia - to change sex. It took anything up to a year to alter yourself from a female to a male, or vice-versa. The process was painless and set in action simply by thinking about it; you went into the sort of trance-like state Dajeil had accessed earlier that evening when she had looked within herself to check on the state of her fetus. If you looked in the right place in your mind, there was an image of yourself as you were now. A little thought would make the image change from your present gender to the opposite sex. You came out of the trance, and that was it. Your body would already be starting to change, glands sending out the relevant viral and hormonal signals which would start the gradual process of conversion.

Within a year a woman who had been capable of carrying a child - who, indeed, might have been a mother - would be a man fully capable of fathering a child. Most people in the Culture changed sex at some point in their lives, though not all had children while they were female. Generally people eventually changed back to their congenital sex, but not always, and some people cycled back and forth between male and female all their lives, while some settled for an androgynous in-between state, finding there a comfortable equanimity.