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.
Overall, the figures revealed there was a degree of uncertainty about fertility intentions for women throughout their childbearing years, including a significant minority of women who did not make firm decisions about future childbearing, the study said.
It was undertaken by the Economic and Social Research Council (ESRC) Centre for Population Change at the University of Southampton.
The report suggested the gap could be due to "change with age due to learning, altered preferences associated with the experience of childbearing, competition with other activities, retrospective rationalisation and a variety of constraints including fecundity, housing, economic factors, difficulties in partnership formation, partner preferences, as well as period influences".
Anastasia de Waal, the director of family and education for the think tank Civitas, said: "With education opportunities being much better now for women, many are going in to higher level professions and become higher status and therefore have to be careful about when they leave the workforce to have a child."
She added there was also a strong economic link either because most households now needed both people working or because families had to balance wanting a large family with how much it will cost to raise one.
Article: 23rd September 2010 www.telegraph.co.uk
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