Women are generally fertile for about 5- 6 days out of the month.

More Info: Fertility is calculated based on a woman's monthly cycle. In optimal conditions and with good swimming sperm, a woman may get pregnant almost 3 days before ovulation begins and for almost 3 days after ovulation has begun. Sperm can live for up to 72 hours inside the body but most of them give up after 48 hours.

How to Calculate Fertility 

It is easy to calculate fertility days once a woman knows the length of her cycle. For instance, women with a 28 day cycle generally begin ovulation on day 14 and women with a 35 day cycle begin ovulation on day 21. Therefore, a woman who has a 28 day cycle is generally fertile from day 11 through day 17 with the most fertile days being days 13, 14, and 15. Conversely, a woman who has a 35 day cycle is fertile between day 18 and day 24 with the most fertile days being days 20, 21, and 22.

Basal Body Temperature 

There are other ways for a woman to calculate her fertility also. Charting changes in the basal temperature may help a woman assess her fertility. During ovulation the body temperature raises from .6 degrees up to 1.6 degrees, and keeping a log of normal daily temperature can help a woman chart her ovulation. An examination of the cervix can also help determine a woman's fertility status because during ovulation the cervix is open.