Or use a Condom. Here's some statistics:
(Periodic Abstinence is the Sympto-thermal Method which uses body temp to determine if you have ovulated)
Actual vs. Theoretical Failure Rates for Various Birth Control Methods
Method Actual Failure Theoretical Failure Rate
Chance 85% 85%
Spermicides only 21% 3%
Periodic Abstinence 20% 2 - 9%
Withdrawal 18% 4%
(pulling out)
Cervical Cap 18% 6%
Diaphragm 18% 6%
Condom 12% 2%
IUD 3% 0.8 - 2%
Oral Contraceptive Pill 3% 0.1%
Injected Depo-Provera 0.3% 0.3%
Female Sterilization 0.4% 0.2%
(Tubal Ligation)
Male Sterilization 0.2% 0.1%
(Vasectomy)
Levonorgestrel Implants 0.04% 0.04%
(Norplant)
Method Actual Failure Theoretical Failure Rate
Taken from
http://www.ecofuture.org/pk/pkbc01.html