Eat less for a girl, more for a boy?

”Eat breakfast if you want your baby to be a boy”, the headline in the Daily Mail read today. Women are “more likely to have boys if they eat plenty and, crucially, have breakfast every day. And if it is cereal, the odds are even more in a boy's favour”, the newspaper explains. This new scientific study investigating whether calorie intake around the time of conception can influence the sex of your baby has received widespread press coverage. The Mail reports that “this is the first scientifically proven way of influencing the gender of a baby without the need for expensive medical treatment” while The Independent says that “the trend to skip breakfast could be altering the male/female balance in the population”.

The findings will undoubtedly spark keen interest among the population at large. However, although this study has been carefully designed and conducted to see whether it is possible to form a theory on how natural conditions may influence the sex of a baby, it has many limitations and the results cannot be considered conclusive. The biological processes of reproduction and fertility may, to some extent, be influenced by our general mental and physiological health, which may include eating a healthy diet. However, the sex of a baby is ultimately determined by the fertilisation of the egg by a sperm carrying either an X or Y chromosome, not by the mother eating a particular food.

The most important message for couples hoping for a baby is that women cannot be guaranteed a boy, or probably even increase their chances of having one, if they eat breakfast and increase their calorie intake, or guaranteed a girl if they do the opposite.

Where did the story come from?

Fiona Mathews of the School of Biosciences, University of Exeter, and colleagues of University of Oxford, carried out this research. The study was funded by the Sir Jules Thorn Charitable Trust. The lead researcher is a Royal Society Dorothy Hodgkin Research Fellow. It was published in the Proceedings of the Royal Society of Biology, a peer-reviewed scientific journal.

What kind of scientific study was this?

This was cross-sectional study of expectant mothers, designed to investigate the theory of whether parents can have any influence over the sex of their offspring. The researchers say that little is known about the natural mechanisms of sex allocation in humans, although one of the best known theories on how the sex ratio between males and females evolved historically, is that improved parental conditions “enhances the reproductive success of sons”. As sons are potentially able to produce a larger number of offspring than daughters, and therefore promote the human species, then “parents in good condition should favour male offspring”.

It is uncertain whether these patterns would be true in today’s society, where resources are more plentiful so that more parents should be in a ‘good condition’. Social and relationship structures are also different, and males are less likely to ‘mate’ with a large number of females than they may have done hundreds or thousands of years ago. This is what the researchers aimed to investigate.

Healthy women, who were about 14 weeks pregnant with their first baby, without any medical conditions and of a healthy weight, were recruited from a hospital in the south of England at their first antenatal visit. Recruitment was stratified to include a proportion of smokers representative of the numbers in the general population. A total of 740 women were recruited and they kept diaries of their seven-day food intake during early pregnancy. Of the total 97% also reported on their diet in the year prior to conception in a food frequency questionnaire and 89% completed another seven-day food diary later in the pregnancy, at 28 weeks. None of the women were aware of the sex of their babies.

The researchers summarised nutritional patterns from the three time points: preconception food intake, early pregnancy intake at 16 weeks and later pregnancy intake at 28 weeks. They used statistical tests to see whether nutritional contents remained the same over time, and how the sex of the baby related to the nutritional intake of the mother.

What were the results of the study?

The researchers found that there seemed to be consistencies over time in the nutritional content of the food that the women ate. When they looked at whether this correlated with the sex of the baby, they found that intake of protein, fat, folate, vitamin C and a range of other trace minerals (they called these factor 1 nutrients) across the three time periods were only just significantly related, while vitamin A components and vitamin B12 (factor 2 nutrients) were not.

Looking at the three time periods separately, they found that only the diet in the year prior to conception significantly related to the sex of the baby. The women who had higher intakes of factor 1 nutrients in the preconception period were more likely to have a boy. The researchers say these scores are highly correlated to energy intake and energy intake in itself was significantly related to whether the woman had a boy baby.

When the researchers divided the women up into three categories of energy intake during the preconception period, they found there was in increase in the percentage of male babies with an increase in energy intake, i.e. those in the highest third were 50% more likely to have a male than those in the lowest third of energy intake.

Of the 133 food items tested, they found that there was only a significant relationship between baby’s sex and cereals. They then looked at whether a similar relationship existed to that of total energy when they divided the women up into thirds of cereal intake, and found that those eating one or more bowls per day were more likely to have a boy than those having less than one bowl per week.

The researchers found no other relationships between baby’s sex and mother’s smoking history, folic acid use, age, weight, height, BMI or mother’s educational level.

What interpretations did the researchers draw from these results?

The researchers conclude that their results “support hypotheses predicting investment in costly male offspring when resources are plentiful”. They say mothers had a higher chance of having a boy if their nutrient intake was higher prior to pregnancy and that eating cereals seemed to be linked to having a male baby.

What does the NHS Knowledge Service make of this study?

This research has been carefully conducted. However, these results can only show a link between the recalled dietary patterns of a group of women before they became pregnant and the eventual sex of their baby. They do not prove that it was the nutritional intake or that eating one particular type of food actually determined the sex of the baby.

• Although the researchers used a validated method of dietary assessment, there are still likely to be some inevitable inaccuracies, both in the mother’s reporting of the foods that she has eaten and in the researchers’ detailed estimate of the nutritional content of the foods (e.g. breakdown into fat, protein, carbohydrate, vitamins and minerals). 
• In particular, the significant relationship – that of the preconception diet – is more likely to have inaccuracies in reporting. This is because the women are recalling their diet in a food frequency questionnaire of the year prior to birth, which may not be as reliable as the food diaries taken at the time in early or late pregnancy.
• It is not possible to relate these findings to a particular type or brand of food eaten as these are not detailed by the study.
• Cereals are the only specific food group of 133 tested that were found to be significant and therefore examined in more detail by the researchers (the results of which many of the papers have focused on). However, it is not possible to gain very much insight from this result, as there is no information on the type, brand or quantity given (people’s perception of serving size in one bowl may differ). Importantly, it also does not mean that the women who did not eat much cereal were skipping breakfast as they may have been eating other things. Therefore all of the reports that ‘skipping breakfast’ means you are less likely to have a boy and that “the trend to skip breakfast could be altering the male/female balance in the population” as stated in The Independent are not correct. 
• Although the researchers selected a reasonably large sample size, there is still the possibility that these are chance findings only. Particularly in the case of finding a link with cereals. In carrying out 133 food tests, it is not surprising that one would turn out significant results.
• The proportion of male and female babies in the overall sample was, as would be expected, 50:50. 
• The research was carried out in only white, healthy women from southern England in their first pregnancies; therefore results may not be applicable to other cultures, ethnicities, women with any medical conditions or those who have previously had a child.

At the current time it would seem ill advised to suggest to any couple that they could be guaranteed a boy, or even increase their chances of having one, if the woman eats breakfast and increases her calorie intake, or guaranteed a girl if she does the opposite.

Sir Muir Gray adds...

This type of study is full of pitfalls; so much information is collected that two factors can have a statistical association, occurring together more likely than would occur by chance, without one being the cause of the other; don’t change your cereal intake on the basis of this alone.