it takes only one time to have sex to get pregnant. so your really just going to eye ball the question of is she fertile or isnt she and not use any protection??
look at the family The Duggers, dont think they eye balled if his wife was fertile or not and the now have 18kids.
i dont think that would work.
that is your catholic belief, but not everyone feels this way. so many people whine about teen pregnancies, but hey if your preaching this, dont complain if every teen acts out this way of 'birth control'.
if a family has 2 or 3 kids or more, and have been together for like ever, and are deeply in love with one another and dont want anymore kids they are entitled to do what they please with there bodies. be it getting a vescetomy or tubes tied..they have had the children they want, and now want to enjoy each other. it would not be using either spouse because clearly there going to talk about it first.
this natural family planning of knowing when your spouse is fertile sounds alot like the 'pull out method', not a very good one..
that may work for you, but it only takes once to get pregnant.