The only tried and true way to prevent pregnancy is to use birth control methods. The pill, condoms, implants, IUDs, the Depo shot... there are lots of ways to prevent pregnancy, and some of them are low-cost. Smoking causes fertility problems, but not enough to make it a reliable birth control method.
Smoking can reduce fertility, so if you're trying for a baby both you and your partner will want to stop. Smoking during pregnancy can actually harm the fetus, so if you do get pregnant you should stop.
Here's a list of effects that smoking has on pregnancy, both before and during:
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