Fats hidden in thousands of foods can harm a woman’s chance of having a baby, scientists said yesterday.
They can increase the risk of fertility problems by 70 per cent or more.
Eating as little as one doughnut or a portion of chips a day can have a damaging effect.
The scientists behind the study advised women who want to have a baby to avoid the fats, known as trans fats.
In the study, researchers from the Harvard School of Public Health in Boston looked at 18,500 women trying to conceive and found 438 cases of ovulatory problems.
If a woman does not ovulate there is no egg for sperm to fertilise and so she cannot become pregnant.
The scientists found that those who took two per cent of their energy intake from trans fats, instead of carbohydrates or polyunsaturated fats such as sunflower oil, had a 70 per cent greater risk of infertility through lack of ovulation.
Those whose energy came from trans fats instead of monounsaturated fats such as olive oil were twice as likely to have problems.
The amount of trans fats needed to reach the two per cent levels was just four grams a day in a 2,000-calorie diet.
People could easily eat that much in a meal of pie and chips or just one doughnut.
Lead researcher Dr Jorge Chavarro said: "It’s really a small amount of fats that we observed having a significant effect on infertility."
He said his findings, published in the American Journal of Clinical Nutrition, suggested that women wanting to conceive should watch their trans fat consumption, as well as giving up smoking and maintaining a healthy weight.
This was particularly true for those with known problems in ovulating such as Polycystic Ovarian Syndrome.
It is not clear how the fats affect ovulation - but they may affect sensitivity to insulin, which is already known to play a role in fertility problems.