Echoing some points from earlier, I've experienced it both ways: I've used ecredits and succesfully extended their expiration once a new ticket was issued, but I've also lost value after cancelling a ticket after an ecredit (which I forgot I had partially used for the ticket - thankfully only ~$50) expired. At least for Delta, I haven't been able to figure out exactly what determines it for each scenario, but the takeaway for me is that there is always a chance you could lose the value. I'd suggest calling a CSR and if they say the credit would be extended, make sure it ends up in the notes on your account. Best of luck, and hoping for no complications in the pregnancy so this is all moot anyway