Bill Smith, technical director at Clinical Diagnostics Services (CDS) in London's St John's Wood, conducts thousands of fertility scans each year.

Over the past decade, several hundred Orthodox Jewish women with abnormal uterine bleeding have been referred to Mr Smith for scanning. A significant proportion have given birth by Caesarian section, and a number of them have had problems conceiving again.

Modern 3-D scanning reveals that this surgery sometimes leaves a defect in the wall of the uterus where the incision has been made. For these patients, during their menstrual period, while most of the blood comes away, some becomes trapped in this cavity and comes away only a couple of days later, or even after a gap of a few days.

"For our Orthodox Jewish patients," said Mr Smith, "that has serious ramifications. They're ovulating well, but because of the prolonged vaginal bleeding, they can't have intercourse during the fertile window."

The laws of niddah (family purity) require a Jewish woman to wait for seven days after menstruation has ceased before she can go to the mikvah and then resume sexual relations with her husband.

But for these women, by the time they have had a clean phase in their menstruation cycle, they will have passed their monthly ovulation. Mr Smith said: "Gynaecologists can't solve it, so they have to go back to their rabbi for advice."

But Virginia Beckett, of the Royal College of Obstetricians and Gynaecologists, and consultant obstetrician and gynaecologist at Bradford Royal Infirmary, does not accept Mr Smith's hypothesis. "If these women went to see him because they have prolonged bleeding, then the commonest reason would be a hormonal imbalance, not an anatomical problem caused as a result of their Caesarian section."

However, Mr Smith is now planning a research study to validate his findings. "I'd like to assess how serious this problem is within the Orthodox Jewish population."