Since records began in 1838, the cries of babies born every year have been predominately male.

In not one year, stretching back to the start of Queen Victoria's reign, have girls outnumbered boys at birth.

In 2017, in England and Wales, for example, there were 348,071 live male births and 331,035 live female births - a difference of roughly 17,000.

And that higher tally of males compared to females born each year is a pattern that has repeated itself for nearly 180 years.

In fact, a ratio of roughly 105 male births for every 100 female ones is generally seen as natural and normal.

It is fairly consistent around the world, although in some countries like China and India the gap is wider because male offspring are more desirable.

More surprisingly, it is a ratio that has been known about since the 17th Century.

But why this ratio exists is not yet completely understood - although there are several theories.

It's all about evolution

The first theory is an evolutionary one which says that in order to have an equal number of males and female in adulthood, there have to be slightly more males born.

That is because being a male is a dangerous thing. Males are more likely than females to die in childhood and at all stages of life - from accidents, taking risks, suicide and from health problems.

"At every age, in almost every time and place, a man is more likely to die than a woman," says Prof David Steinsaltz, associate professor of statistics at the University of Oxford.

So more males than females at the start of life should mean equal numbers of men and women in adulthood, so the theory goes.

In fact, adult women always end up slightly outnumbering adult men in the UK, according to Office for National Statistics figures - and living longer.

Sperm and timing

There are lots of different factors that could determine whether a male sperm (carrying a Y chromosome) or a female sperm (carrying an X chromosome) is first in the race to fertilise the woman's egg.

These include the ages of the parents, the woman's ovulation cycle, levels of stress, diet and sexual position.

One popular theory is that the odds of having a girl increase by having sex several days before ovulation and then abstaining so that the female sperm, which live longer, but swim more slowly than male sperm, outlast their counterparts.

Conversely, if sex happens closer to ovulation or after it, the best swimmers get to the egg first and boys are produced.

Parents may swear by these techniques, but scientists say there is little evidence they make any difference.

There is also some research which suggests parental stress could lead to the birth of more girls, while living through wars and conflicts may give rise to more male conceptions.

Survival in the womb

If planning that sexual encounter does not have an impact on gender, then could something else be happening during pregnancy?

If there are equal numbers of female and male-producing sperm and roughly equal numbers of conceptions, then more female foetuses must perish to give males the upper hand.

Some research suggests females are more likely to be lost in the womb during early pregnancy, but other studies have shown males foetuses are more fragile later in pregnancy and lead to more stillbirths.

Scientists say it is hard to pin down what actually happens and the reasons why.

What we do know is that more male conceptions reach term and more boys are born.

BBC

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