New figures from the Office for National Statistics (ONS) show 49 per cent of births in 2012 were to mothers in this age group.
Some 383,189 of all live births in 2012 were within a marriage or civil partnership and 346,485 were outside.
Within marriage or a civil partnership, 148,403 births were to mothers aged 30 to 34, 78,689 to mothers aged 35 to 39 and 17,380 to mothers aged 40 to 44. A further 1,240 births were to mothers aged 45 and over.
Outside marriage, 67,839 of births were to mothers aged 30 to 34, 36,108 were to those aged 35 to 39, 10,639 were to those aged 40 to 44 and 735 were to those aged 45 and over.
Most births in this category were to younger women, with 32,520 to those aged 20 and under, 102,829 to those aged 20 to 24 and 95,815 to those aged 25 to 29.
In 2012, nearly two-thirds (65 per cent) of fathers were aged 30 and over, excluding births registered solely by the mother. Overall in 2012, the average age of mothers in all categories was 29.8 years. The average age for first births was 28.1 years.
Some 84 per cent of all babies were registered by parents who were married, in a civil partnership or living together.
The percentage of births to women aged 30 and over was down slightly in 2012 compared to 2011. In 2012, 49 per cent of all births were to women aged 30 and over, down from 51 per cent the previous year.
ONS data released last week showed one in 10 mothers aged 40 and over gives birth to a premature baby.
Some 10.1 per cent of babies born to mothers aged 40 or over were delivered before 37 weeks in England and Wales in 2011.
This is up slightly on the 9.5 per cent figure for 2007 and compares to 6.7 per cent of 25 to 29-year-old women giving birth prematurely in 2011.
The number of live births to mothers aged 40 and over has more than quadrupled over the last three decades from 6,519 in 1982 to 29,994 in 2012.
Cathy Warwick, chief executive of the Royal College of Midwives, said of the new data: “This demonstrates very starkly the changing demographics of childbirth.
“More older women are giving birth and this has an impact on maternity services.
“Older mothers tend to have more complications in their pregnancy and therefore often need additional care and attention from midwives and maternity services.”