Birth rates in the United States are now the lowest they have been for three decades.

This is according to the latest figures to be released by the US National Center for Health Statistics, which has collected the data from 99 percent of all birth certificates registered in the country during 2017. It shows that the overall birth rate in the US is down 2 percent on the previous year, making it the lowest on record since 1987.

The birth rate in the US has been steadily declining over recent years, but the drop between 2016 and 2017 was the greatest yet seen, with 92,000 fewer births. This means that in total, the records show that 3.853 million babies were born in 2017. For a country that has a population of 325.7 million, that seems pretty low.

To many this has come as something of a surprise; the economy is doing pretty well and this is usually matched with a baby boom. But the data shows that other effects are in play.

The biggest observable change is that younger people are having fewer children. The birth rates among women in their teens, their 20's, and most dramatically in their 30's, have all fallen. In fact, the only age group that saw an increase in birth rates during 2017 was those in their 40's.

This is thought to be down to a number of different factors. One of the most significant is the fact that teen pregnancy is substantially lower than it has been historically. Even between 2016 and 2017 the rate has dropped 7 percent, but from 1990 this figure has come crashing down by an incredible 70 percent.

The causes behind this are likely varied, but better education among younger women, better access to contraception, and the Affordable Care Act are all thought to have had a role in this decline.

Interestingly, the number of women in their 30's having children declined, while those in their 40;s continued to rise. Experts think that there is probably a shift in attitudes occurring, where women are choosing to delay having children – particularly as assisted reproductive technology is ever improving – or simply deciding to have fewer.

The overall trend in declining birth rates is, however, not unexpected for the United States, as many developed nations see this downturn. Normally, the drop-off in births is picked up by immigrants replenishing the population. With the birth rate in the US now so low, it is thought that there are not enough children being born to replace and thus sustain the population, making the influx of migrants all the more necessary.

The alternative is a country like Japan, which is facing serious economic problems as an aging population matched with low birth and immigration rates mean that there simply aren’t enough working adults to support the population.