A debate is a debate. While the question is phrased as a real-life personal issue. If not, the question could simply be raised as debate for the sake having one…
Whatever the Church teaches - keep it simple. I don’t know what that happens to be. I avoided marriage for the sake of everything to do with marriage on a normal daily schematic of things, let alone life-changing occurrences and tragedies. I salute anyone who can be brave enough to get wedded and put up with the difficulties. I replaced marriage with a coffee addiction and which suits me better.
The argument: One could argue that if a couple got married without prior knowledge of complications during pregnancy and which would be the probable reality, after which the couple is vowed to one-another, and love (eros) is to be shown during that marriage because otherwise you are really just good friends, rather than man and wife, then you have a right to engage in physical relations without the risk of death. You are married.
The difficulty: …However, a child can be born were you not to make it physically impossible. And so one would be replacing the death of a spouse with the death of a potential child. And one cannot kill.
I think the conclusion is to do with ‘killing indiscriminately’. You are not having an abortion; you are killing the possibility of naturally having a child and only because you kind of have to. And you are already married and neither of you suspected these particular trials.
‘Pastoral care’, a term I think sounds a bit fruity, might lead to such an answer as adopting as this way you honour the death of a potential child while being able to stay married. This answer, though, is not on behalf of the Church (necessarily, because I don’t know what the teaching is until grabbing a recap) but is rather a line of reasoning one could take up with the Church. God doesn’t do deals, while this would honour the concept of family.
You say the problems only began when you had your third child and so neither of you definitely knew of obstructions prior to marriage and therefore the above would seem appropriate. All the same, despite this rationale, changing yourselves, interiorly, is to be replaced with another less severe method to avoid (at all costs) a pregnancy - look up NFP.
And too, you should pray throughout so that any outcome is likely to be an answer. Only God can know for certain what will happen in any circumstance; by not bringing absolutes into your life and marriage it is a lived declaration that you intend to keep God as the one in charge.