Birth rates in New Zealand were highest in the period between 1865 and 1885. They dropped in the late 19th century and especially during the 1920s and 1930s – a period of economic depression, first in the rural sector and then in urban areas. Birth rates rose steeply in the period after the Second World War. These high rates were sustained until the early 1970s, during an extended 'baby boom'. Since the late 1970s, the rate of births per 1,000 population has not exceeded 18, in contrast to rates of over 40 in the late 1860s and 1870s. However, the birth rate is a problematic indicator of fertility, because it is afffected by the overall age of the population and life expectancy, which rose steadily during the 20th century. A more accurate indication of fertility is the average number of births per adult woman.