Scientists have long argued that the age at which women have their first child is largely defined by environmental, cultural and educational factors.
But research by a team of British, Australian and American scientists has found genetics plays a part in whether teenagers are likely to become pregnant.
But the new research emphasises genetic as well as social factors in the timing of human reproduction and maintains they are influencing evolution more than at any time in the "pre-history of humans".
Although many women in western societies are having children later for career reasons there is a simultaneous increase in teenage pregnancies.
Natural selection
Published in the latest edition of the scientific journal Evolution, the study examined female reproduction in 2,710 pairs of twins and found women reproducing earlier in their lives have higher "Darwinian fitness" - the ability of an organism to reproduce itself because it is well adapted to its environment.
The study suggests the trend is being passed on from generation to generation through natural selection.
Author Dr Ian Owens, from the Biology and Biochemistry Department at Imperial College, said: "Changes in society, such as freely-available birth control for women and eradication of several important childhood diseases, which have taken place in the last 20 to 30 years will probably lead to genetic changes in humans through evolution."
The findings were made after a three-year analysis of questionnaires from 2,710 pairs of twins held in the Australian Twin Registry (ATR).
But in addition to genetic differences, the researchers found reproduction influenced by social factors such as religion and education.
Genetic differences
They found Roman Catholic women had 20% higher "reproductive fitness" than other religions.
University-educated women had 35 per cent "lower fitness" than those who left school as early as possible.
But after removing factors such as differences in religion, education and the effect of the baby boom, the researchers found "inheritable" genetic differences in the age women started their periods, reached menopause and had their first baby.
The researchers suggested that genes influencing early reproduction in women will become more common, predisposing more to start reproducing earlier.
Dr Owens believes the study has stumbled upon a "genetic phenomenon" that is likely to have implications for changes in human behaviour over the next few hundred years.
Religious affiliation
It is likely to add to the debate on whether changes in human culture are still influencing natural selection.
Dr Owens said: "I was staggered by the results we got.
"There was a massive difference in the number of children born to families with a religious affiliation.
"We also found that mothers with more education were typically having just one child at an older age.
"Their reproductive fitness was much lower than their peers who left school as early as possible.
"Even after we controlled for social factors, there was still lots of genetically heritable genetic variation in the life history traits - this is a really unexpected finding.
"Looking to the future, I would expect to pick up genetic changes within the ten generations since industrialisation."