Can I trust my contraceptive?

contraceptive failuresWe all want to trust our contraceptive but how reliable are they really? There are statistics out there but how do we interpret them?

The background level of fertility varies naturally between couples. Many forms of contraception are affected by “user reliability”. Women’s natural fertility declines from the age of 27 and men’s from age 60, so there are lots of factors to consider. Statistics often quote a figure for “consistent and correct” use (often from clinical trials) but these are often very different from “typical” use, which is a more helpful figure to most people.

Contraceptive methods

Injections, implants and inserted devices

Some forms of contraception are longer acting, often referred to as LARCS (long acting reversible contraceptives). These don’t rely on the user and are generally administered by a doctor or family planning nurse. They are generally over 99% effective. These include progesterone injections, implants and intra uterine systems as well as copper coils. Surprisingly, male and female sterilisation are less reliable as nature has a way of getting round the surgery! In fact up to 1 in 2000 male sterilisations fail and ten times this number (1 in 200) female sterilisations fail.


Those forms of contraception relying on good technique and user reliability have the widest range of quoted failure rates. Many people will find it very difficult to be “consistent and correct” when using condoms leading to a drop from 99% reliability to 82% for “typical” use. Reliability of condoms can be affected by incorrect application, but also some lubricants, sharp finger nails, etc. However condoms are the only contraceptive which can also protect against STDs such as chlamydia, gonorrhoea , hepatitis B, and HIV.

If the condom is known to have failed, emergency contraception should be sought from a doctor, trained nurse or pharmacist as soon as possible. In the absence of hormone use, a woman’s most fertile time is mid-cycle, 14–16 days before her next period, and this is the most risky time for condom failures.

Natural family planning

Understanding your cycle and the life span of sperm – anything up to 7 days – is the basis for “natural family planning”. This relies heavily on user motivation and regular predictable cycles. It can result in 99% effectiveness, though “typical use” figures are closer to 75%.

Female condoms

In trials, female condom use can give 95% reliability, but in “typical use” this is much lower at 79%. Diaphragms and caps, which must be measured by a health professional, but are then user fitted, are typically 88% effective, though “consistent and correct use” can reach reliability of 96%.

Combined hormonal contraceptives

Combined hormonal contraceptives such as rings, patches and, most commonly, the combined pill, all contain a combination of synthetic oestrogen and progestogen, which fool the body into not producing an egg each month.

The hormones are not taken for a few days each month during which time the ovaries reawaken, closing down again on restarting the method. If this gap is prolonged by failing to take a pill towards the end or beginning of a pack, then there is a possibility of an egg being released and failure of the method.

mypill appThe most risky time to make a mistake with a combined hormonal contraceptive is close to the hormone free week (when a woman will often have a period like bleed). It is important to know what to do if you forget pills and sometimes emergency contraception is recommended. See NHS website for the rules concerning missed pills, delayed or detached contraceptive patches or vaginal ring failures.

Being overweight (over 90kg) can reduce the effectiveness of the contraceptive patch but has no effect on the combined pill or rings. Vomiting and persistent severe diarrhoea can affect absorption of pills. Studies have now shown that taking common antibiotics with the “pill” is not a problem. However some other drugs such as antiepileptics can have an impact, so always check with your doctor if you start a new medication. Typical reliability of combined pills is 91% but “perfect” use can give reliability of 99%. Mobile phone apps or alarms can give reminders to take tablets and so improve user reliability.

Progestogen-only pill

The “Minipill” contains only progestogen and is used by increasing numbers of women who cannot take oestrogens. Traditional minipills (Femulen, Micronor, Norgeston, Noriday) have a typical user reliability of 92%. It is very important that they are taken at the same time each day and only a 3 hour delay can lead to loss of contraceptive cover. Again phone apps and alarms can help to remind users. A more modern progestogen only pill which contains desogestrel, such as Cerazette, is very popular as it has a 12 hour missed pill window. This leads to an increased typical reliability of 99%. See NHS website for missed progestogen only pill rules. Vomiting and persistent severe diarrhoea can also affect absorption of Minipills and extra precautions or emergency contraception may be required.

Emergency contraception

Supposing you took emergency contraception because you forgot to use a method or failed to use it correctly – how effective would it be at preventing pregnancy?

There are three possible methods. The most reliable at 99% effective, is having an emergency copper coil fitted by a doctor or nurse. The two hormonal methods are Levonelle and ellaOne. Levonelle is however slightly less reliable (97%) and can only be used up to 72 hours post intercourse. Ellaone is 98% effective and can be used up to 120 hours post intercourse. It used to be thought these hormonal methods were more effective when taken closer to the time of intercourse but recent studies have suggested this is not the case, if they are taken within the recommended times and ovulation has not yet occurred.

Natural fertility

Women’s natural fertility declines from age 27 and by the age of 40 the possibility of pregnancy from each monthly cycle is only 5%. Therefore many older women will happily rely on a contraceptive with lower reliability rates. However if you are younger and more fertile make sure that you are using the best method that suits you, and make sure you know what to do if you forget a pill or make a mistake.

Further information