Probably after Tsukasa is made mortal again. Reminder it hasn't even been said to be four months in-comic as of this stage of the story, either, so even if she was pregnant as early as their first night (which they never got anywhere near friendly enough then), it'd still be about five more months.
But I currently suspect that her immortal state makes her effectively sterile, thus why I say probably after she's mortal again. What need for an immortal avatar of extraterrestrial conquest with reproductive abilities? And even if she did have them, how would that even work for the conceived children? Would they be instantly locked into the state at the moment of conception, thus remaining ungrowing zygotes for eternity? Would it not even pass on to them? There's too many unknowns relating to her reproductive organs and the effects of the hourai elixir that haven't been made clear at this stage.
My current thinking on the matter suggests Tsukasa's entire reproductive system is essentially in stasis- all eggs as of when she imbibed the elixir have remained where they are and inaccessible to natural insemination. Her lower organs might even be eternally preserved in the state they were, as well, meaning she will have an eternally-regenerating hymen that will, if one persists, constantly break and bleed during the act of procreation, likely causing excruciating, constant pain (something I imagine Nasa would dread and avoid putting his dear wife through). Worst-case scenario is that the reconstruction process of the hymen might destroy any matter in the way, such as the male's organ. Can you say... emasculation?
There's a lot of scary thoughts associated with them trying to have a child while she's still 'cursed' with immortality.