Great question — answering as the UphillTech team:
If I remember the shots correctly (I watched this last week, but it's possible that I have conflated or misinterpreted things): Immediately before this scene, we see Lyudmilla tell Vasily (her husband) that they're going to have a baby, and we see her take his hand. When Prof. Khomyuk looks through the window, we cut to a shot (from Prof. Khomyuk's POV) showing that Lyudmilla has placed Vasily's hand on her womb. Prof. Khomyuk, seeing a young couple in such a pose, correctly interprets this gesture to mean that Lyudmilla is pregnant.
Full answer by Michael Seifert: https://movies.stackexchange.com/a/112762 (CC BY-SA 4.0).
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