Some are having children almost two years beyond the point they anticipated while an intended family size of at least two children has not, on average, proved a reality.
The effective baby gap was last night attributed to increasing numbers of women taking on careers, households not being able to afford large families or just not being able to find the right partner.
The average childbearing age in Britain now stands at 29.3 years, the highest level since records began in 1938, a study for the Office for National Statistics said.
Between 1991 and 2007, the research asked women how many children they intended to have and found they consistently gave a higher rate than the actual average fertility rate throughout that period.
The intended family size ranged between 2.0 and 2.16 children per woman when the actual fertility rate was around 0.3-0.4 children per woman lower until 2001, after which the gap narrowed slightly.
Women also expected, on average, to have their first child at a younger age than actually proved to be the case.
Those aged 22-25 in 1991/94 expected to give birth within 3.9 years – the actual average wait was 4.5 years.
For women aged 30-33, the anticipated two-year wait for a first child became 3.5 years on average.
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