This makes sense, if you think about it. Doctors mostly agree that it's good for a woman's body to get a chance to recover from pregnancy before getting pregnant again. And yet many cultures throughout history have used no birth control at all, while still having healthy children. For instance, the !Kung tribe of Africa and the Gainj tribe of New Guinea have children spaced about four years apart, despite their lack of birth control, and they don't (as some cultures do) practice abstinence while breastfeeding.
One reason for such varying results is that amenorrhea (the absence of cycles) depends on the level of prolactin, the milk-producing hormone, in the mother. And the level of prolactin depends mainly on how often she nurses her baby, as well as how much time she spends close to and touching her baby. So a mother who pumps milk and feeds in a bottle, four times a day, is in a very different boat from the mother who holds her baby in a sling where he can nurse as often as he likes.
My source for all this information is the book The Seven Standards of Ecological Breastfeeding by Sheila Kippley. (Mrs. Kippley is a very nice lady who taught my online NFP course, and was very understanding of the fact that I was only taking it because my marriage prep required it -- you can find her and her organization here. (She also cofounded the Couple to Couple League, but is no longer affiliated with them.)) Two helpful online sources are Wikipedia and this page from Kellymom.
Mrs. Kippley describes seven rules that will encourage the mother to remain in lactational amenorrhea. As an added bonus, they also are good parenting tips. The seven standards are:
1. Breastfeed exclusively for the first six months of life; don't use other liquids or solids, not even water. Solids should be introduced gradually after six months so that they don't take the place of nursing.
2. Pacify or comfort your baby at your breasts. In other words, don't wait till you believe the baby is hungry; nurse him whenever he wants, even if you know it's just for comfort. Babies nurse when they're hurt, scared, lonely, or bored as well as just hungry. You can't always tell which is which, but nursing is a safe bet!
3. Don't use bottles and don't use pacifiers. Mothers are told they must pump milk for an occasional bottle, "just to keep the baby used to it" or "just to give dad a turn." That's not really necessary, and will reduce the mother's prolactin. Pacifiers get babies to nurse less frequently by fulfilling his need to suck. My baby so preferred a pacifier he was losing weight at four months old. Very scary -- much better to avoid where possible.
4. Sleep with your baby for night feedings. Being in close contact with your baby increases prolactin levels dramatically, even if he's not nursing much at night. If he is, it's an added bonus. Some mothers will remain in amenorrhea without this rule; some won't.
5. Sleep with your baby for a daily-nap feeding. This one is the hardest for many mothers, who wait for naptime to get things done. But taking a nap with the baby, which I've done a lot recently, really helps to make up for all the nighttime sleep you miss and all the energy baby takes up. It's also really cozy. You don't actually have to sleep for this one, but you should rest for at least half an hour, Mrs. Kippley says.
6. Nurse frequently day and night, and avoid schedules. So the once-every-three-hours or once-every-four-hours schedule is just not going to do the trick. Preferably you should be nursing every two hours or less, but it does depend on what the baby wants. Nursing sessions don't have to be long at all; traditional peoples studied nursed every half hour, with nursing sessions of a minute or so long!
7. Avoid any practice that restricts nursing or separates you from your baby. Working moms do not usually have a very long period of amenorrhea. Neither do moms who are in a hurry to leave their baby with a babysitter at a young age. Obviously, if you're not giving a bottle, you won't be leaving your baby for long anyway. Now that Marko's nine months old, I leave him for short periods pretty readily, but if he starts calling for Mama or nurse, I hurry straight back.
People are different, and so some people will experience a long period of amenorrhea with only some of the seven standards. Others have to follow them slavishly to get the same result. Even with strict adherence, some will have 9-12 months of infertility and some will have two years, three years, or even more. A few will have to wean completely before they are able to achieve pregnancy again.
Because of this uncertainty, I'd stop short of calling breastfeeding a "form of birth control." Because control is the one thing you haven't got. You might have nine months between pregnancies, or you might have two years. You might be surprised to get pregnant before you feel ready, and you might be impatient for your next child and be unable to conceive for awhile. It's pretty much out of your hands!
A baby who is very needy will usually nurse a lot, providing a longer period of infertility than an "easy" baby. When the baby starts sleeping through the night, fertility is encouraged to return. So the spacing will turn out to be what the baby needs, and he'll get your undivided attention for the amount of time he needs it.
Adding to the lack of control you have is the fact that you may not know fertility is returning before conception happens. This happens in about 6% of cases. If you really need to know, some method of natural family planning or fertility awareness should be used.
I'm not doing that, though, because I like not being in control. I believe that deciding when babies come is God's job, not mine, and unless some catastrophe were to make childbearing truly imprudent, I want to allow my children to be born in the timing planned by God and nature.
Another advantage of this method is that a period of amenorrhea is beneficial to a woman's body. This is why breastfeeding reduces the risk, not only of breast cancer, but of uterine, cervical, and other cancers. (source) Not to mention the infertile period is a time to bounce back from birth and renew vitamin and mineral stores. Clearly a pause in fertility is just what nature intended.
So, when I tell people I am not using any form of birth control to prevent pregnancy, I'm not being irresponsible, nor am I likely to have 19 children like Michelle Duggar. (Though she can have 19 if she wants to -- no skin off my nose!) Instead, I'm following the natural course of human reproduction that my body is designed for. I trust that it will return to normal fertility when it's recovered and ready to support another child. Yes, it's an exercise in trust because I have no idea when that will be -- but that's okay. I like it better this way.
Note: The methods outlined above are useful for increasing prolactin for any reason, not just child spacing -- so mothers who need to increase milk supply, for instance, should try some of the seven standards.