Older women may be more at risk of complications
UK scientists have produced further evidence to suggest delaying becoming a mother may be risky.
They found women who start their periods early were more likely to require medical assistance such as forceps, or a Caesarean section.
The effect was neutralised if these women had their first baby at an early age - but not for older mothers.
The University of Cambridge study features in BJOG: An International Journal of Obstetrics and Gynaecology.
The research was based on an analysis of data on 3,739 first-time mothers.
The Cambridge team found that the average age for the start of menstruation was 13.
An earlier start was more common among shorter girls and those with a higher body mass index.
Just over one in four (26.9%) women who did not start their periods until at least the age of 15 required medical assistance during delivery.
But for those who started menstruation early, from the age of 12 onwards, the rate of medical assistance was closer to one in three (32.4%).
The researchers believe that the key is that women who start menstruation earlier are exposed to the sex hormones oestrogen and progesterone for a longer time.
Both hormones are thought to impair the way the uterus contracts during labour.
The theory is bolstered by the finding that women who had their first child at an early age were not at a higher risk of requiring medical assistance during birth even if they started menstruation - known technically as menarche - early.
Previous studies have also shown that the risk of a medically-assisted delivery increases with a woman's age at the time of her first birth.
Researcher Professor Gordon Smith stressed that while the age at which women started having periods showed little variation, the age at which they started a family did.
He said: "The main significance of this study is not that menarche is usefully predictive of the risk of complications, but that the current finding sheds light on why advanced maternal age at the time of first birth might be associated with increased risks."
The researchers suggest the effect of one year earlier menarche would be cancelled by one year earlier first birth.
So, for instance, a woman experiencing menarche at 12 and having a first birth at 22 would be predicted to have the same risks, all other things being equal, as a woman having menarche at 15 and her first birth at 25.
Professor Philip Steer, BJOG editor-in-chief, said there was evidence to suggest that women were starting to menstruate at an earlier age - with rising obesity rates a possible factor.
He said the potentially damaging effect of too much - or too little - oestrogen on the female body was something that doctors had noticed over the years.
However, he stressed more, larger scale research was required before firm conclusions could be drawn about the impact of early menstruation.
In the meantime, he said women who have had early periods should not worry.
But he added: "It is particularly important for them to ensure they lead healthy lifestyles and maintain a normal body weight, as a high BMI during pregnancy is itself associated with poor uterine contractions and an increased need for operative delivery."
Scientists suspect that a woman's body is designed to have children shortly after becoming fertile.
In evolutionary terms, a long gap between puberty and childbirth was not desirable as life expectancy among our ancestors was short.
The early onset of menstruation has previously been linked to a raised risk of breast and womb cancer.