Christopher West has a pretty good introductory-type article on these issues here:
christopherwest.com/article4.htm
Here’s probably the most germane section:
"So what constitutes a “serious reason” for avoiding a child? Here’s where the discussion typically gets heated. Correct thinking (ortho-doxy) on the issue of responsible parenthood, like all issues, is a matter of maintaining important distinctions and carefully balancing various truths. Failure to do so leads to errors on both extremes.
An example of one such error is the “hyper-pious” notion that if couples really trusted in providence, they would never seek to avoid a child. This simply is not the teaching of the Church. As Karol Wojtyla (John Paul II’s pre-papal name) observed, in some cases “increase in the size of the family would be incompatible with parental duty.”[10] Therefore, as he also affirmed, avoiding children “in certain circumstances may be permissible or even obligatory.”[11]
We are certainly to trust in God’s providence. But this important truth must be balanced with another important truth if we are to avoid the error of a certain “providentialism.” When the devil tempted Christ to jump from the temple, he was correct to say that God would provide for him. The devil was even quoting Scripture! But Christ responded with another truth from Scripture: “You shall not put the Lord your God to the test” (see Lk 4:9-12).
A couple struggling to provide for their existing children should likewise not put God to the test. Today, knowledge of the fertility cycle is part of God’s providence. Thus, couples who make responsible use of that knowledge to avoid pregnancy are trusting in God’s providence. They, no less than a couple “who prudently and generously decide to have a large family,”[12] are practicing responsible parenthood."