The early abortions, which can be carried out before a woman has missed her first period, are being offered in 23 clinics around the US run by Houston Planned Parenthood.
British experts expressed surprise at the use of the technique which would require women, who suspected they might be pregnant, to have a blood test and then undergo vacuum aspiration, in which a hand-held syringe is used to extract the contents of the womb.
The Family Planning Association said women in the UK who had unprotected sex could take the "morning-after" pill up to 72 hours after intercourse which would trigger menstruation. If they missed that deadline, or wanted to confirm that they were pregnant before taking action, they would normally do a urine pregnancy test after their first missed period.
A spokeswoman said: "You can do a blood test earlier but it is not generally available and there is no point. You can't have an abortion as the foetus is so small there is a risk it could be missed by vacuum aspiration."
The attraction of the new method is that morning-after and "abortion pills" which can be effective up to seven weeks after conception , are not generally available in America. The early abortion may be politically more acceptable in a country where the "pro-life" lobby is very strong and wins support using photographs of almost fully formed foetuses.
Dr Wendy Savage, consultant gynaecologist at the Royal London NHS Trust, said that women who had very early abortions using the vacuum method must be warned that the pregnancy could be missed and that it might end naturally anyway. "However, I can see that it could be a good option in the US because of the ferocity of the anti-abortion movement."
Dr Michael Burnhill, vice president for medical affairs of the Planned Parenthood Federation of America, appeared to confirm that the early terminations were being offered in the hope of defusing anti-abortion opposition.
"With these very early abortions we are talking about a gestational sac the size of a matchstick head. It's nobody's picture of a little baby sucking its thumb," he said.Reuse content