- Diet of more calories before pregnancy raises likelihood of a child being male, study says
- Parents with stressful occupations are also more likely to have a baby girl
- These associations cannot predict the sex of a child with certainty
Maybe Henry VIII should have been watching what his many wives ate if he wanted to improve the odds of having sons.
The question of a child's sex may not carry the same weight in today's British monarchy, but the world is watching to find out whether Prince William and Catherine, Duchess of Cambridge, are expecting a baby boy or girl this month.
Based on studies done on factors influencing whether babies are born as male or female, one expert would wager that the odds are against the duchess giving birth to a little prince.
"It does look like the balance may be tipped slightly in the favor of a girl," said Fiona Mathews, director of the program in biosciences and animal behavior at the University of Exeter in the United Kingdom.
Mathews and colleagues have studied how the sex of a child isn't totally random. Worldwide, slightly more boys are born every day, Mathews said -- 105 to 106 boys for every 100 girls.
One part of the story seems to be dietary habits before conception, Mathews told CNN's Elizabeth Cohen. Scientists don't know if diet or other lifestyle factors cause the sex of the baby, but research has found some connections.
Mathews conducted a study looking at 740 women early in their pregnancies. Researchers asked the women what they ate before becoming pregnant. Those who had higher caloric intakes were more likely to give birth to a boy than a girl.
More calories may be needed for baby boys because male embryos develop faster than females, and perhaps boys even at the earliest stages of life need a richer food supply because of their faster development rate, she said. Male-carrying sperm swim faster and, after conception, the embryo will divide faster if the baby will be male than if it's female.
Calorie consumption may provide the body with cues that influence the sex of the child, she said. Women who are generally gaining weight are more likely to have a boy baby, Mathews said.
"What we seem to get is this general idea that when conditions are really favorable, then that male embryo can survive and can keep developing," Mathews said. "Whereas maybe when conditions are a bit tighter --- less nutrients are available -- then it's only the female embryos that manage to survive."
Stress also seems to be a key factor in determining the sex of the child. More female babies are born after times of crisis, such as earthquakes and other natural disasters, Mathews said. Women who have highly stressful jobs also seem to be more likely to have a female child, she said.
The father's occupation may also play a role. A small 1999 study in the journal Norsk Epidemiologi found that the proportion of male children of men who were military air pilots was significantly lower, supporting the results of American studies in 1961 and 1987. But a German study in 1976 did not find this result. None of these studies proves that the father's occupation causes the sex of the child to be any different, and more research would be needed to strengthen the connection.
Of course, pilots and stressed women do have male children -- it's just that they have a lower statistical probability of doing so, according to research.
Devoted followers of the British monarchy are probably now thinking: William is an air force pilot. Catherine is probably stressed about being in the public eye. It all points to the baby being a girl, right?
But the royal couple also have a characteristic that makes them more likely to have a boy. A study looking at billionaires found that they are more likely to have male offspring than female -- particularly if the wealthy parent is the dad. It's unclear whether the royal couple fall into this category, but they are certainly wealthy.
Bringing this back to the case of Will and Kate, "It's looking like a girl -- apart from, they're very rich, which would predict a boy," Mathews said.
On the other hand, the Duchess of Cambridge has another point in favor of having a girl: morning sickness. At the beginning of her pregnancy, she had a condition called hyperemesis gravidarum, which is essentially nausea that leads to vomiting and dehydration. This condition is strongly associated with having a female child.
One study conducted in Washington state found that pregnant women who were hospitalized for this condition in the first trimester had a 50% higher likelihood of giving birth to a girl, compared with women who gave birth to a single baby and were not hospitalized in the state for the condition. Those hospitalized for at least three days had a whopping 80% greater odds of a female infant.
It's unclear whether all of these factors associated with a girl baby would neatly add up to predicting accurately whether any given couple is having a baby girl. These are all probabilities, not certainties.
Too bad Henry VIII's wives were so stressed.
Follow Elizabeth Landau on Twitter at @lizlandau and on Google+.