don't do it on her fertile days
go and read up on female fertility. There are only certain days she can get pregnant. A woman can learn to predict her ovulation days. It takes time to learn it but it greatly reduces the chances of getting pregnant.
Here is some info about female ovulation, but ALWAYS use a condom anyways!
Ovulation, occurs 14 days before her next period starts. Exactly when a woman ovulates isn't altogether clear. It all depends on the length of her menstrual cycle . A woman typically ovulates 14 days before her next period -- not mid-cycle, as is commonly held. If you have a 28-day cycle, then she would, in fact, ovulate halfway through her cycle on day 14. But if she has a 35-day cycle, she would ovulate around day 21, not day 17.
While many women find it difficult to predict when they ovulate, some do get clues from their body. Other subtle signs of ovulation include breast tenderness, slight feelings of discomfort in her middle abdomen, and increased vaginal discharge.
As the day of ovulation approaches, her cervical discharge takes on the consistency of egg white. The texture of her cervical mucus is a fairly accurate predictor of ovulation. When she is fertile, her body releases fluid from her cervix that looks and feels like raw egg white: clear and stretchy.
One way to tell when she is ovulating is by taking and charting her basal body temperature. Basal body temperature is the temperature when she first wakes up in the morning before she even gets out of bed. To take her basal body temperature, she'll need a basal thermometer, available at chemists, which shows the minute incremental degree changes that an ordinary one can't. Her temperature ranges from 97.0 to about 97.5 degrees before ovulation. When she ovulates, hormonal changes trigger a rise of between 0.5 and 1.6 degrees, which lasts at least until her next period. She'll probably notices her temperature spiking on other days, but unless it stays that way, she is probably not ovulating. She are most fertile the day of, and the few days preceding, the spike. If she becomes pregnant, her temperature will stay elevated.
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