The rate of pregnancies for women in their 30s has overtaken that for younger women for the first time, official statistics show.
More babies are being born to older women
In 2004, the fertility rate for women aged 30 to 34 was 99.4 live births per 1,000 women, compared with 98.4 per 1,000 for those aged 25 to 29.
Rates for women in their 30s are the highest since the 1960s, but tend to be first, not third or fourth babies.
The data for England and Wales showed new fathers also tended to be older.
In 1974, the average age for men becoming fathers was 29.4, but this rose to 32 in 2004.
The figures also showed that, birth rate for women in their early 30s rose by almost 5% compared to 2003.
And the fertility rate among women aged 35 to 39 increased 48.9 live births per 1,000 women.
Among women over 40, there were 10.4 live births per 1,000 - an increase of just over 6% on the previous year.
In younger women the fertility rate was 72.7 for those aged 20 to 24 and 26.9 for those under the age of 20.
The ONS said: "As a result of rising fertility among women in their 30s, their fertility rates are now at levels last seen for this age group in the 1960s.
"However, in 2004 a greater proportion of births to women of these age were first or second births than in the 1960s, when a greater proportion of births were to women who already had at least two children."
The figures, from the Population Trends report, showed that there were 639,721 live births in England and Wales in 2004 - up by 2.9% since 2003 and the highest number of births since 1997.
The average age of women giving birth stayed at 29.4 years, while the average age for a woman to have her first child was 27.5.
The percentage of births outside of marriage also continued to rise, up to 42.2% from 41.4% in 2003.
A spokeswoman for the Family Planning Association said: "These figures show a continuation of the development for women to turn to parenthood when they have explored other areas of their lives."
She said women had been criticised for 'leaving it late'. But she added: "You become a parent when you're ready to become one. The best age is the age that's right for you."