Swedish women are having fewer children, according to newly released statistics, with local women having children at later ages on average while migrant women have a higher birthrate than native Swedes.

The cause of the downturn in Sweden’s birthrate is unknown according to Johan Tollebrant of the Swedish statistics agency SCB. “What the decline is due to is not clear… an explanation may be that the age at childbirth has increased slightly in recent years,” broadcaster SVT reports.

The statistics show the average birthrate at 1.78 for 2017, far below the replacement rate which is commonly said to be 2.2 children per woman. For native Swedes, the average is only slightly less than 1.78, while for foreign-born women the number rises to over 2 children per woman, around the replacement rate.

Tollebrant accounts for the differences in native and foreign-born women by claiming that many foreign-born women move to Sweden to start families.

The birthrate of native Swedes has fluctuated since its peak of 2.1 births per mother, and according to Tollebrant, much of the fluctuation over the years is connected to economic factors, with people having more children during good economic periods, but the theory has been challenged by the declining birthrate in recent years despite a fairly healthy economy.

Sweden’s Population Growth Second Highest in EU Due to Mass Migration

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While Sweden has a below replacement rate average birthrate, the country has still seen one of the largest population increases in Europe. In 2016, the country saw the second-highest population growth in the European Union due to mass migration.

Across Europe, birthrates have been falling among natives, including in countries with relatively high average birthrates like France, where the rate has dropped for the past several years in a row.

A study conducted by the Austrian Academy of Sciences (ÖAW) and the Vienna Wittgenstein Center released in June of this year suggested only France and Ireland had population growth due to birthrates when looking at figures between 1990 and 2017; every other Western European country that had seen population growth was driven by mass migration.