New Zealand's natural growth was its lowest in a decade last year as more women opted to have children later in life, the government statistics agency said Wednesday. "Nationally, 58,717 live births were registered in 2013, down 2, 461 (4 percent) from 2012," Statistics New Zealand population statistics manager Andrea Blackburn said in a statement. Last year women aged 35 to 39 years had 71 babies per 1,000 women, for the first time overtaking births by women aged 20 to 24, who had 67 babies per 1,000. Deaths in 2013 were down 2 percent to 29,568 in 2013. The natural population increase live births minus deaths was 29,149, the lowest natural increase from 28,124 in 2003. Last week the independent New Zealand Institute for Economic Research (NZIER) called on the government to allow in an extra 40, 000 immigrants each year, saying they were needed to significantly grow the national economy. Almost one-in-four New Zealand's 4.5 million people were born overseas, but the current policy of 45,000 to 50,000 immigrants a year was "too low and very arbitrary" and more migrants would lift incomes, said the NZIER.