Births registered outside marriage or civil partnerships in England and Wales have outnumbered those registered within them for the first time.
The percentage of births registered to cohabiting parents has also increased compared with a decade earlier, while mothers aged 30 years or over were almost twice as likely to be in a marriage or civil partnership compared with those in their 20s or younger.
The latest figures from the Office for National Statistics (ONS), for 2021, show 624,828 live births in the two nations.
Of these, 320,713 (51.3%) were registered to women not wed or in a civil partnership.
The average age of mothers giving birth rose to 30.9 years in 2021, up from 30.7 the previous year, the ONS said.
The average age of fathers stayed at 33.7 years.
The ONS said 2021 is the first year the number of live births registered outside marriage or civil partnerships has exceeded those registered within them since records began in 1845.
It follows a long-term trend of declining marriage rates and increasing numbers of cohabiting couples seen in recent decades, the ONS added.
The statistics body said that as the full impact of the pandemic on marriage and civil partnership statistics is not yet known, caution should be applied when interpreting its latest figures.
Harry Benson, research director of Marriage Foundation, said the rise was “almost certainly a knock-on effect of the ban and restrictions on marriage during lockdown in the previous year”.
He said “many” couples had had to delay their weddings in the pandemic and said he expected “to see some sort of rebound in births within marriage” in future.
Some 36.5% of all births were registered to parents living together, compared with 31.2% in 2011.
In 2021, new mothers aged 30 years and over were almost twice as likely to be married or in a civil partnership (60.5%) than those under 30 (31.2%).
Stillbirths were most common in women aged 40 and over, with a rate of 5.9 per 1,000 births, the ONS said.
This was followed by women aged under 20, at 5.0 stillbirths per 1,000 births.
A stillbirth is a baby born after 24 or more weeks which did not breathe or show signs of life, the ONS said.
The highest stillbirth rate was among babies from the black ethnic group, at 6.9 per 1,000 births.
Stillbirths remained more common in the most deprived areas, the ONS said, adding that the rates have been higher for mothers living in these areas consistently since 2015.
The stillbirth rate in the 10% most deprived areas in England was 5.6 stillbirths per 1,000 births, compared with 2.7 per 1,000 births in the 10% least deprived areas.
The most popular date of birth in 2021 was September 23 while the least common was December 26.