The Diamond Brothers in that NFP video states how breastfeeding could be used as a contraception (which Planned Parenthood promotes as natural birth control) if it's used with that mindset of spacing kids out and not for feeding your baby. However, would it be contraceptive if you plan on breastfeeding for more than a year or two, and of course, there are other benefits to it?

To answer your question --

A woman has to breastfeed exclusively (no formula or other supplemental food) to suppress fertility. At least that's what some older women told us years ago. And guess what? Babies start needing solid food around 9-12 months old. They just can't get enough nourishment from just breast milk at that point. You can do extended or "ecological" breastfeeding, yes, but the baby ceases to use breast milk as the PRIMARY source of nourishment after a certain point. Past a certain point, it's more for emotional/psychological benefit.

Another interesting tidbit -- when do babies' teeth come in? Hmmm....

Long story short, you get some spacing, but don't expect miracles. (A miracle would be "I'm getting married at 20, but I'd really only like 2 kids...")
Also, to make matters even more complicated, it varies by woman.

Ladislaus, here is the Deferrari translation of the passage in question:

Contra the translation you are using, the one I have produced says that if the intrinsic nature of the act is preserved, then the ends are duly ordered. So, which translation is correct?

   I guess it's time for another logic and grammar lesson.

Uhm, I'm using the traslation on the vatican.va site.  And, guess what ... BOTH are actually correct (with different emphases).

In the passage there is both a principle and a practical application thereof to the situation cited, and both are embedded in the same sentence.

So, in context, Pope Pius XI is teaching that it is not forbidden for a couple to make use of their matrimonial rights during times of interfility (on infertile days, when they're older and no longer fertile, or if the couple were just never fertile to begin with, etc.) if they do not obstruct the intrinsic nature of the act, and SO THEREBY (ideoque) do not subordinate the primary ends to the secondary.

So the ultimate guiding principle is that the secondary ends may not be subordinated to the primary.  That would be forbidden.  In periods of INVOLUNTARY infertility, as in the scenarios given by Pope Pius XI, simply not frustrating the intrinsic nature of the act suffices to prevent a subordination of the primary to the secondary end of marriage rights.  But the actual KEY MORAL PRINCIPLE here is that the primary ends of marriage rights cannot be subordinated to the secondary ... IN THESE PARTICULAR CASES OF INVOLUNTARY FERTILITY being satisfied simply by not frustrating the intrinsic nature of the act.

Thus the vatican.va correctly lists FIRST the PRINCIPLE, and then second the requirement in the situation of involuntary infertility.  So IMO it is the better translation, as the ideoque clearly indicates a subordination of the concrete application (not frustrating the intrinsic nature of the act), the first being a concrete means to the end fo non-subordination.  This is completely lost by the Deferrari rendering, although the latter is not incorrect.  This is in fact the force of the Latin ideoque.

Consider the following analogous construct: 
It is not forbidden to shoot at targets in the woods with guns so long as one stops shooting when people enter the range, so as to avoid killing someone. (analogous to Deferrari translation)

What's the key moral principle indicated?  Obviously it's to avoid killing someone, and the stopping of shooting is subordinate to that principle, and carried out precisely in order to abide by the principle.  And the ideoque has the very same force as the "so as to..." portion of my English sentence above.

If I were to rearrage the sentence as follows: 
If is not forbidden to shoot at targets in the woods with guns so long as one avoids killing someone and stops shooting when people enter the range.
(analogous to vatican.va translation)

As you can see, the two sentences in BOLD above mean the exact same thing, with a different emphasis.  So, as indicated at the very beginning of my post here, BOTH are actually "correct" and BOTH mean exactly the same thing.  And the fact that both means exactly the same thing actually clarifies that the formal motive or formal moral principle is to avoid kiling someone (and, in the case of NFP, to avoid subordinating the primary end of marriage rights to the secondary).

Now let's go ahead and lop off the last part of each sentece:
It is not forbidden to shoot at targets in the woods with guns so long as one stops shooting when people enter the range.
If is not forbidden to shoot at targets in the woods with guns so long as one avoids killing someone.

#1 expresses the practical application (with the principle implied?)
#2 expresses the principle (the practical application of which can vary depending on the scenario)

Both effectively mean the same thing, where one is expressed with emphasis on the guiding moral principle, whereas the other expressed in terms of the practical/concrete action in this scenario.

Now, if you look at the first (practical) expression, it would be technically permitted to shoot with guns, so long as you stop shooting the guns when people enter the range.  But when people enter the range, the individual stops shooting, but instead picks up a bow and starts firing away at the targets with arrows, resulting in someone's death.  Technically speaking, he didn't do anything than "forbidden" as expressed in the first concrete/practical expression of the same thing.  Whereas with the second, principled expression, that second person did violate the moral expression.

So clearly GUIDING MORAL PRINCIPLE of the liceity of exercising marital rights has to do with not subordinating the primary end of marital rights to the secondary (or even lesser ordered) ends.

So with that ignorant and sophistic nonsense of yours out of the way, I've said this before and say it again now --

IF ANYONE CAN EXPLAIN HOW ATTEMPTING TO ATTAIN THE SECONDARY ENDS OF MARITAL RIGHTS WHILE DELIBERATELY ATTEMPTING AND INTENDING TO EXCLUDE THE PRIMARY END DOES NOT COSTITUTE A SUBORDINATION OF THE PRIMARY END TO THE SECONDARY, I WILL ENTERTAIN THE NOTION THAT NFP IS LICIT.

Until such a time -- and thus far no one has met the challenge -- NFP is clearly grave sin.

But your absurd sophistic argument implies that it is PERMITTED to subordinate the primary end of marriage to the secondary (which, uhm, seems to render absurd that we even bother to enumerate them as such), so long as the intrinsic nature of the act is not violated.

As per my previous post, the INTENT of artificial (unnatural) birth control is ABSOLUTELY IDENTICAL to the INTENT of using natural "family planning" (aka birth control).  Both actions are morally identical from the aspect of formal INTENT, which is what determines the morality or lack thereof (i.e. sinfulness) of human acts.

Take note of the fact that they DELIBERATELY EUPHEMIZE the natural means as "family planning" while demonizing the artificial as "birth control".  Meh.  We have artificial birth control and we have natural birth control.

We should begin calling it NBC (Natural Birth Control) to call out this sleight of hand deceit.  In both cases it's BIRTH CONTROL, as the FORMAL INTENT is, in both case, to CONTROL BIRTH.

Are we utilitarians? Is 'the end result' what makes something moral? Is there no moral difference between keeping a thief locked up in your basement and handing him off to the authorities? Imprisonment is the end result in both cases.

  Indeed, it's the "ends" of an action that determine its morality, i.e. the formal end of the intent.  But then you simply conflate "end" with the "net practical effect", but they're not the same thing.

Your objection to the passage cites is beginnig to reveal your motivations for defending Natural Birth Contorl tooth and nail.

Reminds me of a joke I heard years ago...

"What do you call a woman who uses the Rhythm Method for birth control?"

"Mom"

That one priest who left CMRI over this matter (and another theological dispute), his name escapes me at the moment, produced an extremely well written letter from a US Bishop banning the promotion of "Rhythm Method" from his diocese for all the right reasons.

Stats show that ABC fails as often as NBC -- "NFP" proponents cite these stats to promote their nonsense -- but in neither case does the fact that the methods are not foolproof justify the immoral action behind the INTENT and ATTEMPT to prevent birth while enjoying marital rights.

I will look into it. It sounds like the most coherent argument I've heard so far.

Still, we're keeping in mind that the Magisterium is protected and the intention can only clarify something if it is unclear, I'll have a look at the Latin.

It's nothig but sophistry that he's promoting in order to justify Natural Birth Control.  See my post explaining how both translations are correct and why, the force of the Latin ideoque, and the fact that despite Myth's hell-inspired bluster, it remains that the chief governing moral principle of the liceity of marital relations has to do with the primary and the secondary ends of marriage.