Voters in the US state of Mississippi have voted against a proposed constitutional amendment that would have defined life as beginning at fertilisation.

The proposed amendment would have afforded embryos and fetuses – whether conceived naturally or artificially –similar legal protection as that covering all US citizens but was rejected by over 55 per cent of voters on 8 November.

Initiative 26, also known as the 'personhood amendment', asked voters: 'Should the term 'person' be defined to include every human being from the moment of fertilisation, cloning, or the functional equivalent thereof?'

The initiative would have effectively outlawed abortion, even in cases of rape or incest. Its implementation would also have had wider implications for any infertility treatments that resulted in the loss of embryos and for certain birth control methods.

'The message from Mississippi is clear', Nancy Keenan, president of NARAL Pro-Choice America, said in a statement. 'An amendment that allows politicians to further interfere in our personal, private medical decisions, including a woman's right to choose safe, legal abortion, is unacceptable'.

Proponents of the amendment collected more than 135,000 signatures in order to trigger the ballot. In an information pamphlet on the amendment available to all voters, the 'Yes to Initiative 26' campaign stated: 'by voting "yes on 26" we...will ensure that the preborn receive equal protection under the law regardless of their size, location, developmental stage or method of reproduction'.

The passing of this amendment would have built support for similar laws in other states and could have ultimately threatened the precedent set by Roe v Wade, the landmark 1973 US Supreme Court decision that legalised abortion. Such a goal is not disputed by the amendment's proponents, who are trying to outlaw abortion in Mississippi.

In response to their defeat at the polls, Personhood USA, the main campaigning force behind 'Vote Yes on 26', vowed to continue the fight for the protection of human embryos from the moment of conception throughout the country. They hope to put the question to voters in other US states including Florida, Montana, Oregon and California in 2012. However, similar measures were already rejected twice by wide margins by voters in Colorado in 2008 and 2010.