The number of pregnancies among women aged 30 and above in England and Wales has surpassed the number among women in their 20s for the first time since records began, the latest figures show.

The long-term rise in pregnancies of older women, which have more than doubled for those aged 40 and over since 1990, has been driven by women spending more time in education and in work, and by the rising opportunity costs of childbearing, according to the Office for National Statistics.

The ONS data covers England and Wales in 2017, the most recent year for which figures are available, and covers conceptions and abortions, not whether the pregnancies end in birth or miscarriage. Age-specific data has been recorded since 1990.

There were 395,856 pregnancies among women in their 20s and 398,284 among women aged 30 and above in 2017, according to the ONS.

The increase in older women becoming pregnant was accompanied by all-time-low pregnancy rates for women aged 25 and under. For the second year running, women aged 40 and over were the only group to see an increase in their pregnancy rate, rising 2.6%.

Natika H Halil, the chief executive of the sexual health charity FPA, said the figures “could show that women are waiting longer to have children, which might be due to a number of reasons, including but not limited to: higher costs of living, fewer young people able to afford mortgages, or perhaps feeling less pressure or desire to start a family”.

The figures also show that teenage pregnancy rates continued a decade-long downward trend, with 18 teenagers out of every 1,000 becoming pregnant in 2017. The figure represents a two-thirds fall from the start of the 1990s, when 48 out of 1,000 teenagers became pregnant.

But experts pointed to stark regional inequalities in terms of outcomes for young parents and their children. Middlesborough and St Helens in Merseyside had teenage pregnancy rates of more than double the national average.

Alison Hadley, the director of the Teenage Pregnancy Knowledge Exchange and teenage pregnancy adviser to Public Health England, welcomed the trend but warned against complacency. “Stark inequalities persist between and within local areas, and outcomes for young parents and their children remain disproportionately poor, with notably higher rates of poor maternal mental health, low birth weight and infant mortality,” she said.

“Effective implementation of statutory relationships and sex education in all schools in 2020 is key for sustaining progress in the long term, but councils continue to require sufficient resources to focus on the other aspects of effective teenage pregnancy programmes – including youth-friendly contraceptive services, targeted help for young people most at risk and high-quality support for young parents.”

The ONS figures also revealed the highest proportion of pregnancies outside of marriage or civil partnership ever recorded in England and Wales, at 58.7%. Around a fifth of all conceptions in England and Wales led to an abortion in 2017.

Kathryn Littleboy, from the ONS’s vital statistics output branch, said: “Conception rates for women aged under 18 years in England and Wales decreased for the tenth year running in 2017. Possible reasons for the continued decrease in teenage conception rates include improved sex and relationship education, better access to contraceptives and increased participation in higher education.

“By contrast, for the second year in a row, women aged 40 years and over were the only age group for whom conception rates increased. This could relate to the rising costs of childbearing and housing, among other reasons.”