Image caption Before the pill, women were expected to get married and raise a family

The contraceptive pill has been called the greatest scientific invention of the 20th Century by some commentators. Arriving at a moment of social and political upheaval, it is now 50 years since it was made available on the NHS.

But what impact has this tiny pill really had?

In 1961, women's lives were very different.

Often married at an early age, most women were expected to stay at home and raise an expanding family while men went out to work.

Nowadays, women can choose to have children, further education and a career on their own terms.

The pill was instrumental in changing that.

"I don't think people thought it would be as revolutionary as it was," says Dame Valerie Beral, a professor of epidemiology at Oxford University, who has spent the last 40 years researching the pill.

She added that for women's health, it was the "most important thing in the latter half of the century - no question about it".

Case study Sarah, a 29-year-old from west London, has been using the pill off and on for eight years. Having initially decided to take it to ease heavy periods, she has had a mixed experience of using it. The first time the side-effects saw her put on weight and break out in spots, while another time a different brand of the pill made her feel more hormonal. While in a relationship, Sarah took the pill as she needed contraception and didn't want to use condoms. It was a joint decision and the pill made life easier. "Lots of guys expect you to be the one taking responsibility for contraception, and say 'why aren't you on a pill?' "They expect everyone to use it." Now using the coil, Sarah said there should be more awareness of possible side-effects and that doctors should discuss other options before handing out the pill.

But, of course not everyone supports the pill. Some faith groups are opposed to artificial birth control, saying it is a form of abortion.

When introduced on the NHS, the pill was prescribed mainly to older women who already had children and did not want any more. The government at the time did not want to be seen to be encouraging promiscuity or "free love".

Although there were not any restrictions on its use, the take-up of GPs prescribing it was slow.

That all changed in 1974 when family planning clinics were allowed to prescribe single women with the pill - a controversial decision at the time.

Many people had questions about whether GPs and parents had to be told when a young woman was prescribed the pill, says Dame Valerie.

Now, according to the latest prescribing review, two million women take it in England and Wales. It is estimated that 70% of all women in Britain have used the pill at some stage in their lives.

Shotgun marriages

The pill was liberating for both sexes as previously women had had to rely heavily on men for contraception, whereas, with its introduction, they had control, says Tracey McNeill, Marie Stopes' vice-president and director of UK and Europe.

"It has enabled individuals - women and men - to control their reproductive health and to choose when they want to have [a child], so I think from a social point of view, it's bound to have had an impact in families and relationships."

Image caption Health campaigns in the 1970s urged people to get contraception advice

It has also affected women who were never "on the pill", say economists George Akerlof, Janet Yellen and Michael Katz.

They say "courtship" used to involve an implied promise that if a woman became pregnant, the man would marry her, but as women were now able to control when they had children, the implied promise disappeared.

For women, the pill meant marriage became harder to come by.

They wrote in a study on the effects of the pill: "The pill encouraged the delay of marriage through routes such as reducing the necessity of marrying to have sex and lowering the incidence of shotgun marriages."

Jane Falkingham, the director of ESRC Centre for Population Change (CPC), agrees that the pill was part of a social change that separated partnerships and children.

Without the pressure to get married, many couples turned to co-habiting.

In Britain in the early 1960s, fewer than one in 100 adults under 50 were estimated to have cohabited, whereas nowadays about one in six do, according to a report by the CPC.

Changes in society and families Babies born to women aged under 25 in England and Wales fell from 47% (369,600 live births) in 1971 to 25% (180,700 live births) in 2008

Historically women's child rearing years were in their early 20s but they are later - in Scotland, the number of births in 2008 among those aged 35 and over exceeded births to women in their early 20s

In 2008, there were 320,800 live births outside of marriage in England and Wales (45% of all births), in 1971, this figure was less than 10% (67,500)

Of the women born in 1941, 94% were married by the age of 35, whereas only 60% of women born in 1973 had married by that age Source: Office of National Statistics

Christine Northam, a counsellor working for Relate, says the pill gave women the opportunity to be "freer in their own sexual habits, to have freedom to have sex as and when they wanted to".

She says the pill changed the dynamics in relationships.

"It instigated a change in the role of men and women. Men have had to make changes themselves because their roles have changed and some have found it easier than others to cope with this," she says.

Some women say they feel pressurised into taking the pill as men automatically assume that they are.

Ms Northam adds that one impact of the pill was that there were not as many children available for adoption as there had been in the past, as there were not as many unwanted children anymore.

"It's made us value children perhaps more than in the past," she says.

In terms of controlling the UK's population size, it did not have a significant impact as reproduction is controlled in other ways, says Ms Falkingham.

For example, the 1920s and 1960s had a similar number of childless women.

In the 1960s it was more through choice, but in the 1920s it was because women had not found a partner, partly because many were working as servants and were not able to set up home until later in life.

Male pill

The pill has also impacted on women differently at different stages of their lives.

For young women, its impact is mainly on their lifestyle, however studies have shown that as women age, those who have taken the pill for 10 to 15 years are less likely to get cancer of the womb or ovaries.

One criticism of the pill is that people only use that as contraception and can be at risk of contracting a sexually transmitted infection (STI).

Women who pioneered the pill's usage in the 1970s and were therefore not reliant on condoms, are now becoming increasingly at risk of STIs.

Now the pill has come full circle - if a male contraceptive pill becomes routinely available, women may again leave it up to their partner to control fertility - however, this time around, many may chose not to, saying they cannot trust men to take it.