Since the 1970s, sex-selective abortions in China, India, and ten other nations have caused between 23 and 45 million women to go 'missing'.

By 2030, a new study predicts those very same nations will 'lose' another 4.7 million female births, skewing their sex ratios even more.

This study uses a model based on 3.26 billion birth records from 204 countries, and it identifies 12 nations with strong evidence of a skewed sex ratio and 17 nations at risk of heading in that direction.

The good news is that the 12 nations today with a skewed sex ratio show signs of recovery, especially China and India, where 95 percent of all current missing births are located.

By 2100, the model predicts these nations will lose 5.7 million women altogether – a fraction of what has been lost since the 1970s, when sex diagnosis became widely available, and a much faster decrease than other studies have predicted.

Still, that's a lot of women who will never be born, and it could cause lasting cultural and societal repercussions.

In China and India, where men now outnumber women by some 70 million, a marriage 'squeeze' has already led to a worrying uptick in loneliness, as well as a rise in violence, female trafficking, and prostitution.

In recent years, the sex imbalance in both of these nations has begun to slow, as government officials provide incentives for female births and restrictions on sex-selective abortions.

But more work is needed. The team behind the new model says we need to take immediate action if we want to rebalance the sex scales in places like China, India, Albania, Armenia, Azerbaijan, and Vietnam.

Especially because it could set precedence elsewhere.

If other countries with a preference for sons over daughters – like Nigeria, Pakistan, Egypt, Tanzania, and Afghanistan– begin to skew their sex ratios as well, the new model predicts we could lose 22 million more women by 2100.

In that particular scenario, nations in sub-Saharan Africa could contribute more than a third of all lost births.

Obviously, that's a hypothetical situation, but as sex diagnosis and abortion become more readily available around the world and as gender discrimination persists, it's not out of the question.

While abortion can give women power over their bodies, their health, and their future, those choices can also be dictated by societal attitudes and norms. Gender discrimination is a problem worldwide, but in some cultures, only men can work, carry on the family line or look after their aging parents.

Women, on the other hand, can sometimes not work or own property, and in certain cases, they need a dowry to marry. These cultural expectations condemn them to be a burden, through no fault of their own, especially on poorer families.

As well as sex-based abortion, researchers have also blamed female infanticide and poor female healthcare for the millions of missing women worldwide.

It's hard to predict how these attitudes will change in the future, but it's important that we know what's coming, given how much a nation's sex ratio can dictate the well-being of its population.

The new model isn't perfect, but it's the first attempt researchers have made to predict how many sons and daughters will be born in the years to come.

"These findings underline the need to monitor [the sex ratio at birth] in countries with son preference and to address the factors behind the persistence of gender bias in families and institutions," the authors write.

"A broader objective relates to the need to influence gender norms which lie at the core of harmful practices such as prenatal sex selection."

The study was published in BMJ Global Health.