Arizona has passed a law which says pregnancy can begin two weeks before a child has been conceived, in another blow to reproductive rights in the US.

The law was signed in by Republican Governor Jan Brewer on Thursday, who said it would "safeguard our most vulnerable population–the unborn."

Aside from banning all abortions after 20 weeks, defined from the date of the woman's last period instead of conception except in the case of medical emergency, it will force women considering abortion because of fetal abnormalities to have counselling, and for women having an abortion to have an ultrasound.

It also says the age of a foetus is "is calculated from the first day of the last [woman's] menstrual period."

The Center for Reproductive Rights’ state advocacy counsel, Jordan Goldberg told the Daily Beast the new law "disregards women's health" . “The women of Arizona can’t access medical treatment that other women can.”

Six other US states have put similar curbs on late-term abortions in the last two years.