Note that the sentence here does NOT use "je to", but only "je" (with an implied "ona").
If it said "Je to v sedmém měsíci" instead, it wouldn't refer to "her pregnancy" anymore, but to an event happening in the seventh month, although it's still strange. It would either mean "It's in July", in which case it would be much more common to say "Je to v červenci," or in the 7th month of some previously specified longer period, for instance of a year-long training program or something.
No, that would be "Je v práci sedmým měsícem" (instrumental case) or even simply with the accusative: "Je v práci sedmý měsíc".
If I wanted to say something like "in the seventh month of a year-long vacation", I would use "během", which means "during": "během sedmého měsíce roční dovolené".
"V sedmém měsíci" always means "in the seventh month of pregnancy".
If I really want to imagine another possible meaning, I have venture to the realm of sci-fi and imagine you can be inside a moon (a planet's satellite), instead of just on the surface of it ("na měsíci"). Then, take a planet that has at least 7 moons, like Saturn, assume they are numbered, and only then you can say that someone is in the 7th moon as "(on) je v sedmém měsíci." The preposition in/v used with a moon is the most problematic element here.