I grew up in a Catholic town. For birth control, most families loosely practiced the rhythm method, and as a result, most families were large. Recognizing that the method has limitations, natural family planning has become more popular, and it can be a lot more effective.
The rhythm method is also called the calendar method. You count the days in your cycle and avoiding intercourse on fertile days. If you have a very regular cycle (between 26 and 32 days), it is pretty simple. If you have no moral objections, combining the rhythm method with condoms on your fertile days can be highly effective.
If your cycle is always 28 days, the assumption is that you ovulate on day 14, so then your fertile days are between days 8-19 (6 days before, 6 days after, including the day of ovulation). Abstinence during those days should prevent pregnancy. If you are variable, the method becomes more difficult. You have to estimate when you expect your period and count back 20 days to the beginning of when you have to abstain. The abstinence period lasts for 12 days. That’s the flaw in the method—if you end up ovulating earlier, you risk pregnancy. Check out the “Conception Calendar Basics” article on the Conception website for a full explanation of the menstrual cycle days.
Why does the abstinence period start so early? Because sperm can live for 3-5 days inside the body, depending on what you read. If you ovulate on day 14, but you have been abstinent since day 8, it’s very unlikely any sperm inside you would still be alive. Since an egg only lives for 24 hours, if you abstain until day 20, an egg released on day 14 will definitely not be viable. Obviously, the method leaves a pretty wide margin on both sides.
The biggest advantage is that, since there are no drugs involved, there are no side effects, and it is free. The biggest disadvantage is that even one day of miscounting can result in pregnancy—so-called Vatican roulette. It's also a difficult method for those who do not have a regular cycle.
The Catholic Church and other conservative churches have endorsed natural family planning methods for many years. Natural family planning adds in a few other steps to the calendar method, including checking cervical position, cervical mucus and basal body temperature. When you use family planning methods, the efficacy goes up dramatically. It takes more work, though, to do the family planning methods. I have known several families who carefully practiced it, and it worked perfectly for them. It can also help if you are actually trying to conceive! Identifying your fertile days can help you in both the prevention and the conception situations.
The rhythm method is not a good choice during breast-feeding. Until you have re-established several months of regular menstruation, you could be ovulating. That old adage that you can’t get pregnant while breast-feeding is not true.
In choosing any method, you have to decide if failure of the method is acceptable to you. You also have to consider your moral stand on contraception, your health (sometimes options like oral contraceptives are contraindicated), and your relationship.