90% of the time that doctors say this, it's nonsense. I've known people who were told this who went on to have several more children. They do it to cover their butts.
In the event that it's true, it's reason to abstain and not have any more children, but not to "space them out". Lethal is lethal, right? So what does that have to do with spacing them out?
This is just an excuse people use to justify NFP. Either you can have more children or you can't. There's no such thing as having to have fewer children.
Doctors have proven that they do not know if this is true or not - God knows if it's true, only God knows, no one else knows. Yes, Doctors cover their butts, and unlike a broken clock, the doctor might actually be correct 50% of the time, but when it comes to this, God said to multiply - period.
The couple cannot abstain based on this opinion of the doctor without sinning. If they ever abstain from the act, it can only be for reasons that promote the greater glory of God - this is not one of those reasons. Using that reasoning, why would not contraception/abortion be an acceptable option if saving the life of the mother is what matters?
Preeclampsia or severe hemorrhaging during birth does not even equate to a certain death sentence to the mother or child and in fact many women experience this during birth and pull through just fine. Even were the doctor to say "your next child means certain death for the mother or child", that is still not sufficient reason to abstain, least ways not without permission from the Church.
Somehow, the husband and his needs are entirely forgotten in all of this.