Apart from complete abstinence, no method of birth control is 100% effective, there is a small chance that couples using contraception can still conceive. Here, then, are five key reasons why this may happen.1. Contraception not used properly.The main reason why contraception fails to work is that it is not used properly. For example, women forget to take the Pill, or if they are on the mini Pill they take it at the wrong time of day. Condoms are sometimes put on incorrectly – if there is any air in then friction can cause the latex to burst or tear. A condom that is too large can slip off, spilling semen. Similarly cervical caps need to be checked to ensure their position does not need adjusting. Even intrauterine devices (IUDs) need checking every so often.2. Inconsistent use of contraceptionHaving unprotected sex “just this once” can result in conception, as can sex while the Pill is being taken but inconsistently so (see above).3. AntibioticsAntibiotics in the blood stream can interact with the birth control pill, decreasing the effective levels of hormones and with that the efficiency of this method. This is the reason why women are advised to use a back-up form of contraception, such as condoms, when they are taking antibiotics.4. Split condomsSplit and broken condoms are an occasional occurrence. The main reasons are using the wrong size of condom in the first place and/or failing to expel air from the condom before it is put on. Using lubricants can help too as these reduce friction – but it needs to be a latex-safe lubricant. Other lubricants can actually weaken the latex and cause it to split so exacerbate the problem.5. Having sex during the “safe time”In fact there is no such thing as a safe time for unprotected sex – most conceptions take place during a woman’ mid cycle but they can also get pregnant on days considered as non fertile. Also, importantly many women don’t have regular cycles all the time. So sex without contraception is ill-advised unless you are actually trying to conceive, no matter what time of the month it is.