Class rather than education was the main driver for British women delaying motherhood, a new study found.
It had been widely assumed a woman's desire to go to university or college to fulfil an educational aspiration before starting a family was the reason British women postponed motherhood.
But new research found education played a much smaller role in delaying motherhood than previously thought and a woman's family background was the major factor.
Since the end of the Second World War the average age of first-time mothers increased by as many as four to five years at the end of the 20th century throughout Europe and the US.
The study by the University of Oxford and the Universities of Groningen and Wageningen in the Netherlands found educational attainment for women also increased over the same period.
The average age of women when they left education rose steadily throughout the 20th century, however, the age of first-time mothers did not follow the same pattern but formed a U-shape instead.
New mothers were still relatively young after World War Two during the so-called 'baby boom' but were also generally becoming more highly educated
Only from the 1960s did women start to delay motherhood which coincided with the introduction of the Pill.
Those born after the 1960s postponed motherhood by around 2.7 years, on average, compared with women born at the end of the Second World War.
But researchers said longer educational enrolment only accounted for 6 per cent of this delay.
Using data from the Office of National Statistics for cohorts of women born in the UK between 1944 and 1967, the scientists tracked patterns of educational enrolment to see how they influence reproductive behaviour.
They also compared the fertility histories of more than 2,700 female twins from the largest adult twin register in the United Kingdom set up in 1992.
This acted as a controlled trial because this isolates the effects of different levels of education between siblings in pairs of twins who share so many other characteristics.
Their model calculated that for every extra year of educational enrolment after the age of 12, a woman delayed motherhood by an average of six months.
However, strikingly, they also found the main influence on whether a woman postponed having children was largely associated with her family background.
They concluded family environment, a combination of a woman's social, economic and genetic factors, was significant.
Education alone contributed to only 1.5 months of the total six-month delay.
Lead author Dr Felix Tropf, from the Department of Sociology at the University of Oxford, said: 'Our research casts doubt on previous studies that claim a strong link between educational expansion for women and the postponement of motherhood.
"We find that both education and a woman's fertility choices seem to be mostly influenced by her family background, instead of education influencing fertility behaviour directly.
"For example, families provide social and financial support, and pass on genes affecting reproductive behaviour.
"A large part of the observed association between education and age at first birth in other studies can actually be explained by the family environment.
"In isolation, education has a much smaller effect.
"We hope this important finding that a large part of the link between educational enrolment and fertility postponement is not causal but spurious may inform those forecasting future fertility trends or shaping family policy."
The study was published in the journal Demography.