1.2.3 Mechanical methods of contraception
While hormone-containing pills represent a very sophisticated kind of contraceptive, mechanical contraceptives are a straightforward idea: they act by preventing sperm and egg from meeting. Mechanical contraceptives in their simplest form have been around since before Roman times; some are shown in Figure 4. The earliest ‘penis protectors’ were allegedly used less for contraception than as protection against disease, and as a badge of rank.
Male condoms, placed over the penis to catch the ejaculate and so prevent it from entering the woman, are probably the most widely used form of contraception, particularly as they also provide a barrier to pathogens. Unfortunately, they have very high failure rates: as high as 15% has been reported in some UK studies, although careful use can improve this to 2%. The high failure rate is partly because they are very thin and split easily during use, but also partly because they are often not put on soon enough, allowing sperm in pre-ejaculatory secretions to enter the vagina. Recently, female condoms have also become widely available. These are designed to fit over the vulva and inside the vagina.They are loose-fitting, and therefore less likely to break, but they are easily dislodged from the vulva, allowing the man's penis to enter the vagina beside, instead of inside, the condom. There are not yet any published failure rates for female condoms, but early results suggest that they are likely to be as effective as the male variety. Condoms have few physical side-effects, apart from occasional allergic reactions to rubber, but they are not popular with all men (or women), as they may be so tight-fitting as to dull sensation.
Although until fairly recently condoms have been the exclusive province of the man, other forms of mechanical contraceptive can be used by the woman. These are all devices that can be inserted into the vagina to prevent ejaculated sperm from passing through the cervix and entering the womb. These mechanical barriers include diaphragms, caps and sponges, early versions of which are shown in Figure 4c. These all fit tightly over the cervix, and caps and diaphragms come in different sizes to accommodate women of different shapes. In spite of this, they do not fit perfectly, and need to be used with a spermicide (a substance that kills sperm on contact) to be effective. Caps and diaphragms can be used repeatedly, but sponges are disposable, and are purchased already impregnated with spermicide. Depending upon how correctly they are used, failure rates of between 4% for caps and diaphragms and 25% for sponges are reported. The use of barrier contraceptives with spermicidal properties also has a long history. The ancient Egyptians, in around 1850 BC, were using a variety of pastes inserted into the vagina; honey and crocodile dung seem to have been popular, according to one source (Green, 1971). An alternative method was half a lemon placed over the cervix, which made an effective barrier, while the citric acid in the lemon juice acted as a spermicide.
Figure 5 shows the female reproductive tract, highlighting the places where barrier contraceptives are used.
There exists another kind of mechanical device that does allow the meeting of sperm and egg, but nevertheless prevents pregnancy: the intra-uterine device, or IUD. Strictly speaking, this is not a contraceptive as such, but an abortifacient (something that causes an abortion), and for this reason some people have reservations about its use. An IUD is inserted by a doctor into the uterus (womb), where it lies against the inner wall. This appears to set up a reaction within the wall which prevents a conceptus from implanting. Note that once the conceptus has implanted it is referred to as an embryo.
Q What other contraceptive already mentioned prevents implantation?
A This is one of the effects of progestogen.
Some IUDs contain high levels of copper, which is thought to dissolve very slowly in the uterine fluid. The high local levels of copper are thought to disable the sperm in some way, so these IUDs may, in fact, be ‘proper’ contraceptives, preventing fertilization. IUDs have low failure rates, of 1 or 2%, but, unlike barrier methods, they offer no protection against sexually transmitted disease. They can also cause very heavy menstrual bleeding, and users have a higher risk of infections of the uterus and Fallopian tubes, which can lead to infertility.