Women who eat too much fast food and not enough fruit take significantly longer to fall pregnant.
They are also less likely to conceive a baby within a year after trying, an Australian study has found.
Published in journal Human Reproduction, the study adds to a large body of evidence linking poor diet to decreased fertility.
"The findings show that eating a good quality diet that includes fruit and minimising fast food consumption improves fertility and reduces the time it takes to get pregnant," said lead author Professor Claire Roberts, from the University of Adelaide's Robinson Research Institute.
For the study, more than 5500 women in Australia, New Zealand, UK and Ireland were quizzed about their diet.
The women had never had a baby before and were interviewed by midwives during their first antenatal visit.
Compared to the first-time-mothers who ate at least three piece of fruit a day in the month before conception, women who ate fruit less than one to three times a month took half a month longer to become pregnant.
Similar difficulty in falling pregnant was also found among the women who regularly ate burgers, pizza, fried chicken and chips bought from fast food outlets.
Compared to women who never or rarely ate fast food, women who consumed fast food four or more times a week took nearly a month longer to become pregnant.
The risk of infertility among the women with the lowest intake of fruit increased from eight per cent to 12 per cent, while it rose from eight per cent to 16 per cent for those who ate fast food more than four times a week.
"Our data shows that frequent consumption of fast foods delays time to pregnancy," said first author Dr Jessica Grieger, post-doctoral research fellow at the University of Adelaide.
Importantly, most the women did not have a history of infertility, said Dr Grieger.
"As diet is a modifiable factor, our findings underscore the importance of considering preconceptions of diet to support timely conception for women planning pregnancy," she said.
The new findings have prompted a call for all GPs and fertility specialists to refer their patients struggling to conceive to a fertility dietitian.
"With one in six couples now struggling to conceive, many couples are now asking what they can do to optimise their fertility," said fertility dietitian Melanie McGrice at St Vincent's Hospital in Melbourne.
"Avoiding smoking and achieving a healthy weight have been well established as important strategies. This new research supports the growing body of evidence that a nutritious diet is one of the most important strategies that a couple can employ to optimise their fertility," said Ms McGrice.
The expert also warned women hoping to fall pregnant against eliminating fruit from their diet in fear of consuming too much sugar.
"Fruit is rich in antioxidants, vitamins and phytochemicals, and should not be lumped into the same basket as sugar and soft drinks," Ms McGrice said.
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