A Sydney fertility expert has issued a startling warning to very fat women who want to be mothers: lose weight or risk losing your baby.

Dr David Knight said not only were obese women far less likely to get pregnant but any pregnancy was 80 times more likely to end in tragedy through miscarriage, stillbirth or the death of the baby in the first month after delivery.

The children of very fat women were also genetically programmed to be fat too, meaning they were destined to suffer a wide range of health problems including diabetes.

Women with a body mass index of 35 were 50 times less likely to get pregnant than women in the healthy range.

One in five couples had fertility problems due to obesity and pregnancy being delayed until an older age.

Dr Knight estimated 5 per cent now conceived through fertility programs.

He sees about 1000 women with infertility problems each year, and said a lack of education had created a generation of obese would-be parents who had no idea what they were doing to their bodies and any future babies.

"Bad food is cheap, convenient and addictive," Dr Knight said.

"We are dealing with a significant psychological addiction to a set of behaviours and we are all paying a price for it, whether it's through what we spend on the health system or because we don't have our whole seat on the plane because of the person next to us."

Society was telling fat people it was "completely OK" not to be accountable for their actions, he said.

Dr Knight and the State Opposition's Pru Goward said Mother's Day was the right time to raise the issue of growing female infertility.

"Infertility is a growing problem in Australia, and is stopping too many women from having a Mother's Day of their own," she said.

"Twenty years ago, approximately one in seven couples was infertile; today it is one in five."