Japan will drastically expand its clinical study on genetic screenings of fertilized eggs to cover several thousand women, the Japan Society of Obstetrics and Gynecology has said.

More than 3,500 women at dozens of medical institutions will take part in the expanded study on preimplantation genetic screenings, which check eggs fertilized in vitro for chromosome abnormalities and put back into the uterus only eggs without abnormalities.

The medical society long banned the screenings, which are often criticized on ethical grounds. But it eased the rule to accept only clinical studies, including those to check their effects in reducing the risk of miscarriage.

The society has started a small-scale clinical study involving 80 women at four institutions. For comparison, the society created another group of women and transplanted into them fertilized eggs that were not put through the screenings.

In the upcoming large-scale study, the society will gather more than 3,500 women by easing participant qualifications.

A paper on the small-scale study said the screenings did not lead to a decrease in miscarriages.

The society says the expansion is needed to evaluate the effects of the screenings more accurately. Meanwhile, critics say the expansion is an effective full lifting of the ban on the screenings.