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In a "flashback" that Louise has, we learn that
Louise told Ian that she knows their daughter will die young.
Why would she tell him that, given that
she has already had this "flashback", so she should know that telling him this information will lead to his disillusionment and their divorce?
This quote from the novella the movie is based on ("Story of Your Life" by Ted Chiang) explains the nature of the heptapod's language and determinism:
For the heptapods, all language was performative. Instead of using language to inform, they used language to actualize. Sure, heptapods already knew what would be said in any conversation; but in order for their knowledge to be true, the conversation would have to take place.
Freedom isn't an illusion; it's perfectly real in the context of sequential consciousness. Within the context of simultaneous consciousness, freedom is not meaningful, but neither is coercion [...] Similarly, knowledge of the future was incompatible with free will. What made it possible for me to exercise freedom of choice also made it impossible for me to know the future.
– Andres F.
Feb 27 '17 at 2:13
Conversely, now that I know the future, I would never act contrary to that future, including telling others what I know: those who know the future don't talk about it. Those who see the future don't make choices, but "play" their part in enacting said future, always.
– Andres F.
Feb 27 '17 at 2:15
Ian knows that Louise can see the future due to having learned the Heptapod language. (If not at first then surely by the time she publishes her book on the subject.) Therefore:
When Hannah is eventually diagnosed and then dies as a result of this rare disease, Ian would be able to deduce that Louise knew it would happen and didn't tell him. He most likely would have divorced her at this point anyway.
Knowing that he could figure this out by himself, she decided to tell him beforehand. One way or another, the result would have been the same.
Also, learning the Heptapod language and gaining the ability to perceive time in this way apparently brings with it a compulsion to steward in the future rather than trying to change it. Perhaps a better understanding of Time changes your opinion of the idea of changing it. The Heptapods surely could avoid their crisis by themselves with a 3000 year head start - if nothing else they could simply evacuate the affected planet(s) long before disaster strikes. Instead, they contact humanity to ask for our help. Likewise:
Louise could simply not have a child with Ian. Or she could make a point of being abstinent for a few months around Hannah's conception date in order to ensure a different ovum - and therefore a different child - who would (hopefully) not have the same rare disease. (If she knows Hannah's birth date via "future memory" she can calculate the approximate time of conception.) But instead, Louise decides to go ahead and have the child she foresaw having, despite the tragic circumstances that go with it, and the inevitable divorce with Ian.
