Nearly half of all births in England and Wales are to mothers aged 30 or older, according to the Office for National Statistics.

The average age of mothers has continued to increase for almost four decades. It is currently 29.7, but 49% of women are over 30 when their baby arrives. However, the overall percentages for the three main age groups for mothers are similar to the level they were just before and after the second world war. This is because births among young mothers have plunged from their late-60s to early-70s peak, a time when there were significantly fewer older mums.

The ONS bulletin detailing the latest figures for 2011 says that between the mid-1940s and mid-1970s the average age of mothers decreased by just under three years, from 29.3 in 1944 to 26.4 in 1973.

It said on Thursday: "The overall rise since 1973 reflects the increasing numbers of women who have been delaying childbearing to later years. Possible influences include increased participation in higher education, increased female participation in the labour force, the increasing importance of a career, the rising opportunity costs of childbearing, labour market uncertainty, housing facts and instability of partnerships."

The average age for a mother's first birth over the past 70 years mirrors the all-birth trends since 1940 – it is estimated at 27.9 compared with 27.7 in 2010 and 26.6 in 2001.

Actual numbers for women giving birth aged 45 and older are comparable to the 1940s, despite the rise in infertility treatment. In 2011, 1,832 babies were born to women of that age, out of 723,913 births. Before, during and after the second world war, it was common for that figure to top 2,000 in years when the total number of births varied from 579,091 in 1941 to 881,426 in 1947. Numbers sometimes fell to well below 500 between the 70s and mid-90s.

Marriage, or civil partnership in recent years, remains the most common family setting for births, despite the steady fall in births registered to married couples since the 1960s, down to 53% compared with 60% in 2001 and 94% in 1961. Mothers between 30 and 39 are most likely to be married or in a civil partnership, with only 31% of babies born outside these arrangements.