Twins are more common for older mums because they are more prone to produce multiple eggs in a cycle than younger women, a Dutch study has found.
Eggs are produced from follicles
Increases in fertility treatment could not, on its own, explain the rate of births of non-identical twins.
Researchers from Vrije University, Amsterdam looked at egg follicle development in over 500 women in an effort to understand why so many occur.
The research is published in the journal Human Reproduction.
Identical twins occur when one fertilised egg divides to develop into two babies, but non-identical twins develop from two separate fertilised eggs.
Non-identical twins account for up to three-quarters of twin pregnancies.
In a normal cycle, a follicle stimulating hormone (FSH) triggers the maturation of an egg-containing follicle in the ovary. The process is controlled so that once one follicle matures sufficiently, FSH levels fall.
The Dutch scientists analysed the development of follicles - which produce eggs - in 959 natural cycles in the women.
All were undergoing intrauterine insemination - where sperm is inseminated directly into the womb - because of unexplained infertility or mild male infertility.
The researchers found that multiple ovarian follicular development - where more than one follicle was over 14 millimetres in size - happened in 105 women.
Of the 105, only five were under the age of 30, 45 were aged between 30 and 35 and 55 were aged over 35.
The concentration of FSH increases with age, both in women with single and multiple follicular development.
But those who produced multiple follicles had higher levels of the hormone.
In the last stage of a woman's reproductive life - the decade between 38 and 48, called the peri-menopausal transition' - the pituitary gland produces more and more FSH in order to stimulate the development of one of the few remaining viable egg follicles.
The study suggests that, in some women, FSH levels keep rising, overshooting the threshold needed to ovulate one egg.
In most cases, this does not lead to multiple pregnancies because of the low number of remaining follicles with good quality eggs (oocytes).
However, if there are two or more follicles with good quality oocytes available, then simultaneous double ovulation can occur, making a multiple pregnancy more likely.
Dr Cornelius Lambalk, who is head of research in the Reproductive Medicine Division at the Vrije University Medical Centre, one of the researchers who led the study, said: "Advancing female age is associated with declining fertility due to decreasing numbers and quality of oocytes.
"But at the same time there is a distinct increase in dizygotic [non-identical] twin rates - a seemingly paradoxical phenomenon that has not been entirely explained, until now."
Dr Gillian Lockwood, medical director of Midland Fertility Services, said the study confirmed what experts had suspected: "In effect, Mother Nature is doing what we deliberately strive for with 'ovulation induction' or IVF when we use injections of FSH to produce several follicles in one cycle.
"With the current concern over declining birth rates, especially amongst older professional women who 'leave it too late' it is interesting to speculate whether this natural mechanism may compensate to some extent for the 'baby gap'."