There is only one way for a person to get pregnant without actually making an effort. During vaginal intercourse, at a time of the month when a woman is fertile:

1. The man (probably) has to ejaculate*
2. Sperm has to make it’s way through the cervix to find an egg
3. The sperm and egg have to engage in the ultimate bonding experience – known as fertilization
4. The fertilized egg has to implant in the uterus
5. The body has to respond to the implantation by making the hormones necessary to sustain a pregnancy

… and all of it has to go perfectly. If you think about it, it’s absolutely amazing that 85% of women who are having unprotected sex will get pregnant within a year.

When you have vaginal intercourse, there’s no way to be sure of the outcome. You could get pregnant, you could get an STD, or everything could be just fine. It doesn’t matter if you’re a virgin. It doesn’t matter if you’re on your period. It doesn’t matter if you stand on your head or do jumping jacks until you fall over. If you have vaginal intercourse, pregnancy is a risk.

Fortunately, there are ways to adjust the odds in your favor. If you don’t want to get an STD, reduce your risk by using a condom. If you don’t want to get pregnant, use a reliable form of contraception.

Abstaining from vaginal intercourse is the only 100 percent effective way to avoid getting pregnant, but other forms of contraception can be reasonably effective as well. The chart below is stolen from The Guttmacher Institute.

Need an explanation of the columns in the table?

• Perfect use refers to a contraceptive method’s effectiveness in people who use the method exactly as it is supposed to be used every single time they have sex.
• Typical use refers the method’s effectiveness the way most people use it – missed or late pills, forgotten spermicide**, and all.

Take that as the warning for which it is intended. Unless you’re absolutely certain you’re going to be really reliable about using a method, base your expectations on the typical use column.

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*There is a small amount of sperm in pre-ejaculatory fluid, but recent research suggests that there may not be enough to present a serious pregnancy risk. Still… I know that I wouldn’t risk getting pregnant on my partner’s ability to pull out in a timely manner or on the hopes that there wasn’t sufficient sperm in his pre-come. Condoms or other forms of contraception, in addition to withdrawal before ejaculation, are going to keep you much safer than withdrawal alone.

** Personally I think that nonoxynol-9 containing spermicide is nasty stuff. You won’t find me letting it anywhere near my sex life. There’s some evidence that it can increase the risk of STDs, and in my humble opinion it’s not such a good contraceptive that the trade-off is worth it.