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Last year saw the lowest birth rate in the US since 1978, according to data from the US National Center for ��ҹ����1000���� Statistics. The decline in fertility in recent years means that the US population is not able to replace itself through reproduction alone.

The latest statistics are based on data collected from birth records across the US, which together account for over 99 per cent of all birth certificates recorded in the country. These suggest that the total number of births in the US in 2017 was down 2 per cent on the previous year. Today, over the lifetimes of every 1000 women, there are around 1764 births – not enough to replace the population.

“It doesn’t surprise me,” says of the Center for Assisted Reproduction in Texas. “Most developed countries are seeing the same phenomenon.”

Population stability is important, and has come to depend on immigration in places like the US, says Doody. “Immigration has allowed the population to increase at a healthy rate,” he says. “Without that, the population would shrink, and more of the population would be older – which we see in places like Japan.”

As a result of this, Japan is set to face economic problems due to a declining workforce and an ageing population in need of health support.

Fewer teen pregnancies

“The good news is that the decline is associated with a decrease in teen pregnancies, which we’re trying to avoid,” says of the University of Iowa. The birth rate for girls aged 15 to 19 has been steadily declining since the 1990s, and dropped 7 per cent between 2016 and 2017. Compared to 1991, the birth rate for this age group has now dropped by 70 per cent.

“With access to the Affordable Care Act, there has been greater access to birth control”, which has probably contributed to the decline, says Sparks. “It may also reflect better education,” she says.

However, the birth rate among women aged 40 to 44 rose by 2 per cent between 2016 and 2017. “Fortunately, assisted reproductive technologies are enabling many women to get pregnant,” says Doody. “But a lot of babies born to women in their 40s are not genetically related to them – they are often conceived with donor eggs,” he notes.

More pre-term births

Last year also saw a small rise in the number of babies who were born prematurely, and those that were born at medically low birth weights.

These factors may both be partially explained by the fact that women are tending to give birth at older ages, says Doody. “Even if you’re using eggs from a 23-year-old, you’re still more likely to develop high blood pressure in pregnancy, and that can lead to early delivery,” he says.

The rate of caesarean sections appears to be on the up – albeit by a fraction of a percentage on 2016 – even for women with low-risk pregnancies. This is surprising, says Sparks, because bodies like the American College of Obstetricians and Gynecologists have been pushing to , and C-section rates have generally been declining over the last few years.

The report also highlighted differences between ethnic groups in the US. Preterm birth rates are much higher in black women compared to their white peers, for example. And while nearly 83 per cent of white women received prenatal care in their first trimester, only 52 percent of non-Hispanic Native Hawaiian and North Pacific Islanders did. It is difficult to know whether these differences might be down to socioeconomic status or other factors, says Doody.

Read more: Fertility facts: How late can you leave it to have a baby?