The study suggests a high calorie diet at this time – and regular breakfasts – might increase the odds of a boy.
The study, by the Universities of Exeter and Oxford, focused on 740 first-time pregnant mothers in the UK, who were asked to provide records of their eating habits before and during the early stages of pregnancy.
The researchers found 56% of women with the highest energy intake around the time of conception had boys, compared to just 45% among women with the lowest energy intake.
Women who had sons were also more likely to have eaten a higher quantity and wider range of nutrients, including potassium, calcium and vitamins C, E and B12. They were also more likely to have eaten breakfast cereals.
The researchers say the modern trend to opt for low calorie diets might explain why the proportion of boys is falling in developed countries.
Over the last 40 years there has been a small but consistent decline, of about one per 1,000 births annually, in the proportion of boys being born in industrialised countries, including the UK.
Previous research has also shown a reduction in the average energy intake in the developed world, and there is also evidence that more people now skip breakfast.
Dr Allan Pacey, an expert in fertility at the University of Sheffield, said there was good evidence that nature had subtle ways of changing the sex ratio of a population in response to a variety of circumstances.
However, he said: “I would urge women to not to start starving themselves in order to try influence the sex of their baby.
“It has been observed in some animal studies that even small changes in female diet can affect the life long health of the offspring, so it is important that the mother has appropriate nutrition at the time of conception and throughout her pregnancy.”