It's natural to assume that if you're using contraception, you're protected against unexpected pregnancy.

But new research has revealed this isn't always the case.

In a study in the Medical Journal of Australia (MJA), almost half of women (4 in 10) surveyed who experienced an unintended pregnancy were using contraception at the time.

Among those women, more than 6 in 10 were taking the oral contraceptive pill when they became pregnant. The second-most commonly used method when they became pregnant was condoms, followed by long-acting reversible contraception (LARC) — such as the contraceptive implant or intrauterine contraceptive device (IUD).

Why does contraception fail?

No method of contraception is 100% effective. Some methods are referred to as 98% to 99% effective — meaning that for every 100 couples that use the method, only 1 or 2 will experience a pregnancy. But the method of contraception will only be this effective if used exactly according to the manufacturer’s — and possibly a doctor’s — instructions.

Some people call this 'perfect use' — where the rules of the method are followed rigorously, every time the couple has sex.

But not every couple uses contraception perfectly. In that case, the method of contraception will be less effective (sometimes called 'typical use').

For example, the combined oral contraceptive pill is 99.7% effective when used perfectly. This decreases to 91% with 'typical use'.

Some of the reasons for typical use — and contraceptive failure — include:

• being given the wrong instructions about how to use the method (e.g. starting the oral contraceptive pill at the wrong time)
• making mistakes when using the method (e.g. not using a condom properly)
• forgetting the requirements for using the method (e.g. being late for the next contraceptive injection)

How to find the right contraception

Choosing the contraceptive method that is right both for you and your partner is essential to avoiding an unintended pregnancy.

If you're a woman and you're unlikely or unable to take a pill at the same time every day, then the progesterone-only (mini) contraceptive pill probably isn't for you. If you're a man who is absolutely sure — after giving it plenty of thought — that you don't want any more children, then a vasectomy may be a better choice than condoms.

(However, remember that a vasectomy won't protect you against STIs whereas condoms do.)

To get a better idea of the method that suits you, take the quiz at MSI Australia's new website dedicated to contraception, Contraception.org.au. Both men and women can learn more about their contraception options and get one-on-one advice from a nurse or family-planning doctor.

You can also learn more about contraceptive methods here. Of course, always talk to your doctor to ensure you make an informed decision.