Researchers have found Asian-American women are less likely than white women to successfully have a baby through IVF, but were unable to pinpoint why.

Evidence for the difference has been building up. 'In the infertility literature, Asian ethnicity has emerged as a risk factor for poor outcomes after IVF treatment, without a clear understanding of the biologic mechanism', write the authors, led by Dr Elizabeth Langen, at Stanford University in California, USA. While their work couldn't illuminate the cause, it does eliminate some of the proposed explanations.

The team studied 180 women undergoing IVF in 2005 and 2006, and found only 31 per cent of the Asian women gave birth successfully compared to 48 per cent of the white women. Asian women were also less likely to become pregnant; 43 per cent against 59 per cent.

A variety of data were collected, including number of previous pregnancies, body mass index, history of prior IVF and thickness of the endometrial lining - the lining of the uterus where a fertilised egg is implanted. Even after adjusting for these factors, the difference between the groups persisted.

The researchers particularly looked for differences in embryo quality, but found no evidence for this. As the women had similar responses to stimulating the ovaries to produce eggs, comparable rates of successful fertilisation and the same average of two eggs implanted in the uterus, differences in the initial part of treatment were ruled out.

Being over- or under-weight has been linked to poor IVF outcomes. Generally the Asian women were thinner but, as both groups were on average in the normal weight range, the team say it's unlikely this had an important clinical impact.

Another proposed theory involves a suggested higher level of endometriosis in Asian women - a condition where the endometrial lining grows outside the uterus, often affecting the functioning of the uterus and ovaries. The team didn't assess the women they studied for this, but they found no racial differences in the rates of past diagnoses.

However, they did find a difference in thickness of the endometrial lining. Whilst the clinical relevance of this isn't known, the authors hypothesise that hormone levels created during IVF may affect Asian women differently, and lead to a less hospitable environment for implantation.

Further avenues the authors note for investigating the causes of racial variation were looking at different subgroups from Asia, paternal influence and lifestyle.