Condoms don't usually break, but they have been know to do so on occasion, and with disastrous results. Certain brands may be more prone to breaking than others. Check the expiration date on the package. Expired condoms may be more likely to break and for people who are sensitive to the lubricants or latex may experience more irritation, but an old condom is better than no condom!!
When lovemaking is done check the condom for leaks, by carefully removing the condom from the penis without spilling any of the semen, holding the top of the condom carefully placed between two fingers, slide your fingers down to the tip. All the semen should stay right in the tip. If theres a hole it will squirt.
If you find any leaks or a condom breaks, to prevent pregnancy, immediately place two tablets of ascorbic acid (Vitamin C, with no bio-flavanoids) inside to vagina, right next to the cervix, this is where the sperm are making a bee-line for. The ascorbic acid has been reported as irritating, it it becomes too much gently rinse the vagina with water. You could take a warm bath. Douching forcefully can push the sperm closer to the cervix. If you happen to have spermacide on hand, using an applicator and placing it as far into the vagina as possible can also kill the sperm, and may be more effective than the vitamin c, and less irritating.
If you want, place a drop of spermicide inside the tip of the condom, to kill the sperm right away. If a condom does break, the spermicidal lubricant on a condom is not enough to prevent pregnancy
Ascrobic acid can be taken internally 6-10 grams throughout the day, for the next 5-9 days. Someone who takes vitamin c on a regular basis or smokes may need to take more than somone who doesn't. Progesterone prepares the uterus to receive a fertilized egg. Ascorbic acid interferes with the production of progesterone, therefore preventing the uterus from becoming a nourishing place for the egg, thus preventing implantation.
If condoms are you sole form of birth control always be prepared for an emergency. A prescription of emergency contraceptive pills can be obtained at any time, and the refills used as needed, if needed. I also think it makes sense for every woman keep a bottle of ascorbic acid on hand, just incase.