The Ancient Greeks and Romans allowed abortions. The early philosophers argued that a foetus did not become formed and begin to live until at least 40 days after conception for a male, and around 80 days for a female.
From the 16th century, the Christian doctrine of passive conception held that the foetus was only given a soul in the fifth month. Then, in 1869, Pope Pius X changed the timing of "ensoulment" to conception.
In 1803, the Ellenborough Act made abortion in Britain after the 16- to 20-week period in which life is first felt, an offence that carried the death penalty, though it later became life imprisonment. In 1938, Dr Alex Bourne was acquitted of performing an illegal abortion after claiming that it was to save a raped girl mental harm, setting a case-law precedent. Women wanting to terminate had illegal, backstreet abortions performed by unqualified abortionists. Women were often injured in the process and some died. At least 50 were killed each year from botched surgery and infection.
The 1967 Act
The private member's Bill introduced by the Liberal MP David Steel ended the scourge of backstreet terminations. Abortion was legalised if two doctors certified that continuing with the pregnancy would involve a risk greater than if it was terminated to the physical or mental health of the woman - or where there was a substantial risk of serious abnormality in the child. In 1969, the first complete year after the Act, there were 54,819 registered abortions. Doctors found abortion in the first few weeks was actually safer than continuing with the pregnancy. They began to interpret the law more liberally, taking increasing account of the mental health of the woman.
After 1967: the science
An upper time limit of 28 weeks for abortions was introduced under the 1967 Act. That was derived from the Infant Life Preservation Act of 1929 which had set it as the limit of viability - the age at which a foetus could survive.
Medical advances have seen the limit of viability fall. Today, neonatal units are equipped to save babies of 24 weeks gestation and below. Survival after birth has continued to improve since 1990.
In Britain, 1 per cent of babies born at 22 weeks survive and 11 per cent at 23 weeks. About a quarter survive at 24 weeks. Two-thirds of babies born at 23 weeks and more than a third born at 24 weeks suffer long-term disability.
After 1967: the politics
From the start, the Abortion Act came under sustained attack from opponents who sought to reduce the time limit and repeal the law. In 1990, the Human Fertilisation and Embryology Act introduced controls over techniques developed to help infertile couples and to monitor experiments on embryos. The abortion law was reviewed in the light of the new Act and the time limit for abortions was reduced from 28 to 24 weeks in 1991.
The key numbers
There were 181,600 legal abortions in England and Wales in 2003, a rise of 5,700 (3.2 per cent) on the year before.
The abortion rate for women resident in England and Wales in 2003 was 17.5 per 1,000 women aged 15 to 44. The abortion rate was highest, 31.3 per 1,000, among women aged 20 to 24.
The rise in the abortion rate in modern Britain is attributed to issues ranging from women wanting fewer children and wanting them later in life, to the decreasing popularity of marriage and the rise of "career women" who fear that children will hinder their job prospects.
The percentage of abortions performed at 20 weeks or later has remained at between 1 per cent and 1.6 per cent for years. Teenagers are more likely to have late abortions, usually because they do not realise they are pregnant.
Why cut the time limit?
The debate about the 24-week limit began with claims that the foetus showed evidence of consciousness and could feel pain from an early stage in the womb. The survival of babies at 22 and 23 weeks also showed that the limits of viability had fallen.
The foetus is sensitive to touch from about seven weeks and soon afterwards can move its limbs. But its movements are spinal reflexes and do not indicate awareness. After 26 weeks, actions become more defined, reflecting improved organisation in the nervous system. The structures necessary for pain to be felt are in place but there remains disagreement over when pain can first be experienced.
Many doctors and nurses feel uncomfortable performing late abortions and most over 18 weeks are contracted out by the NHS to the private sector.
Many doctors, MPs, medical ethicists and members of the public support a reduction in the time limit from 24 weeks to 22 or 20 weeks.
Why leave the limit?
Medical organisations say the law is humane, practical and working well. Pro-choice groups warn that any reduction in the time limit would be likely to affect the most vulnerable women - teenagers whose relationships have broken up and women waiting for the results of tests. Screening tests for foetal abnormalities in pregnancy identify women at high risk but they must be followed by diagnostic tests. Women may also have to wait until 20 weeks or more to get confirmatory test results of foetal abnormalities. After receiving the results they need time to consider their options.
Groups such as the British Pregnancy Advisory Service, the Family Planning Association and Antenatal Results and Choices say reducing the time limit will narrow the options for these women and lead to the birth of more unwanted babies.
The political football
Michael Howard made abortion an election issue by declaring last weekend that the upper limit for legal termination should be reduced from 24 weeks to 20 weeks.
Tony Blair, whose wife is a Roman Catholic, said abortion was a "difficult issue" but said he would not change the law.
Charles Kennedy said he had voted for the upper time limit to be reduced from 24 weeks to 22 but added that advances in medicine meant that "I don't know what I would do now".
Cardinal Cormac Murphy- O'Connor, the leader of the Roman Catholic Church in England and Wales, hinted that all Catholics should vote Tory but the Prime Minister has said he believes it is a matter for a free vote and conscience on both sides of the House.
So will this week's controversy take centre stage as the parties contest the election?Reuse content