The pill, and different contraception methods

The contraceptive pill, which has become the symbol of women's sexual liberation, has given birth over the past five decades to other, more practical hormonal methods to prevent unwanted pregnancies.

Invented by an American, the pill stops a woman from ovulating thanks to two synthetic hormones, estrogen and progesterone, which work to stop eggs from being released into the uterus by the ovaries.

Taken daily, usually around the same time, it is 92 to 99 percent effective in preventing pregnancy, according to the US Centers for Disease Control and Prevention.

The mini-pill is a variant of the original pill but contains only progesterone, and is designed for women who suffer side effects from taking extra doses of estrogen. Both hormones are naturally produced in women.

But tens of thousands of unwanted pregnancies - and abortions - every year are blamed on women forgetting to take the pill.

"Other forms of female hormonal birth control are really part of the legacy of the pill," said Andrea Tone, professor in Social Studies of Medicine at McGill University in Canada.

Other methods have been developed to lessen the risk of unwanted pregnancies, such as the Depo injection of progesterone, which lasts three months and is said to be 97 to 99 percent effective.

The contraceptive patch releases estrogen and progesterone into the bloodstream. It needs to be replaced weekly for three weeks, allowing women to have their monthly periods in the week without the patch.

The vaginal ring works by the woman's body absorbing doses of the two hormones through the vagina, and is withdrawn every three weeks to allow menstruation to take place.

According to doctors, a contraceptive implant is the most practical and surest method to avoid pregnancy. The small plastic tube, containing a reservoir of hormones, is inserted under the skin and remains 99 percent effective for three years.

Among other contraceptive methods available, the coil is an intrauterine device made of copper, which allows women's menstrual cycle to continue normally, while a mirena intrauterine system (IUS) is also T-shaped but releases small amounts of progestin daily.

Both methods are inserted into a woman's uterus by a doctor, and are said to be 99 percent effective. They can be left in place for several years.

They work by stopping a fertilized egg from implanting itself in the walls of the womb.

Other non-hormonal methods of contraception are referred to as barrier methods, such as the diaphragm or cervical cap. Smeared with a spermicide gel, they are placed over the cervix by the woman before intercourse to stop sperm from entering the uterus.

None of those forms of contraception are effective, however, in preventing sexually transmitted diseases (STDs), hence the continued popularity of male condoms which catch the sperm. But they have a lower rate of preventing pregnancy of 85 to 98 percent.

Female condoms are also available on the market, and may also help prevent STDs. In America, many men and women opt for sterilization.

"The pill has always been since the 1960 the most popular form of reversible contraception but male and female sterilisation has really been the method of choice," said Elizabeth Watkins, professor of the History of Health Sciences at the University of California.

"If you add together female sterilization and male sterilization, if you combine those, that's the most popular form of birth control in America. The third most popular are condoms," she added.

Experts explained that the popularity of sterilization, seen by some as an extreme method of birth control, had historically been due to the fact that bill control pills were not covered by US insurance companies.

"In the US the pill was very expensive. Not until the late 1990 after Viagra came on the market and was covered by insurance companies, did insurance companies start to pay for the birth control pill," added Watkins.

In 2006, the US government also authorized over-the-counter sales to adults of the "morning-after" contraceptive pill, known as Plan B. It consists of two hormone pills, taken 12 hours apart in the first 72 hours after unprotected sex.