(CNN) A healthy diet is just as important before pregnancy as during it. Women who eat more fast food and those who eat very little fruit take longer to get pregnant than women who include several portions of fruit in their daily diets, according to a study published Friday in the journal Human Reproduction.
Those who ate fruit less than three times a month took half a month longer to become pregnant than those who ate fruit three or more times a day in the month before conception. Similarly, women who consumed fast food four or more times a week took nearly a month longer than the women who ate several portions of fruit a day.
"Small modifications in dietary intake may have benefits for improving fertility," wrote first author Jessica Grieger, a post-doctoral research fellow at the University of Adelaide. "Our data shows that frequent consumption of fast foods delays time to pregnancy."
She added that further research is needed to assess the possible impact of a broader range of foods on pregnancy.
Grieger and her colleagues examined data from 5,598 pregnant women in Australia, New Zealand, the UK and Ireland. None of the women had previously had a baby.
During each woman's first prenatal visit -- occurring between the 14th and 16th weeks of pregnancy -- midwives collected information. Questions included how long it took to become pregnant and diet details from the month before conception. Specifically, the women were asked how frequently they ate fruit, green leafy vegetables, fish and fast foods: burgers, pizza, fried chicken and fries purchased from takeout or fast food outlets. Fast foods bought from supermarkets or eaten at home were not included.
When the researchers looked at the effects of diet on infertility, they found that in women with the lowest intake of fruit, the risk of infertility increased from 8% to 12%, and in those who ate fast food four or more times a week, the risk of infertility increased from 8% to 16%.
The researchers also found that pre-pregnancy consumption of either green leafy vegetables or fish did not affect time to pregnancy.
An admitted weakness of the study, the researchers noted, was that they did not collect dietary information from the fathers.
"A recent review on male diet and fertility markers indicated that higher intake of fruits and vegetables was associated with increased sperm motility whereas a higher intake of fat-rich foods and sweets may decrease semen quality," Grieger and her co-authors wrote.
Other unknown factors may have affected the results, they said.
Dr. Jennifer Wu, an obstetrician-gynecologist at Lenox Hill Hospital in New York, said the new study "highlights the need for good nutrition prior to conceiving."
The study required women to recall what they ate from weeks back, noted Wu, who was not involved in the research. However, the researchers queried women about their diets "toward the beginning of the pregnancy, so it's got a close proximity," she said. "The strength is, this is a really large study with a large number of women.
"We do know that women who are obese or overweight often have problems with irregular cycles, infertility, trying to conceive can take longer," Wu said. "So a woman who eats fast food more than four times a week has a higher chance of being obese."
The American College of Obstetricians and Gynecologists warns against excess weight during pregnancy and recommends a healthy diet both before and after pregnancy.
When the women were asked about fish and vegetables, they may have included fried fish and French fries, Wu speculated, so this may be why these food categories did not positively impact fertility.
"It may be that eating fresh fruit is a better measure of good nutrition," she said.
"Before getting pregnant, you want to achieve ideal body weight, and you want your diet to be as good as possible. It's really hard to go from eating a lot of junk food and then you're suddenly pregnant and you have to give up everything, including alcohol," Wu said.
"Get into good habits with eating before you get pregnant."