Pregnancy & Baby Index: Trying to Conceive - TTC: Preparing to TTC: Why older women are more likely to have twins

Why older women are more likely to have twins

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Dutch researchers have demonstrated for the first time that older women are more likely than younger women to have multiple ovulations in the same menstrual cycle, thus explaining why non-identical twins become more common as women approach the end of their reproductive life.

According to co-authors Professor Roy Homburg and Dr Cornelius Lambalk from the Reproductive Medicine Division at Vrije University Medical Centre in Amsterdam, the findings give credence to previous theories that the rising concentrations of follicle stimulating hormone (FSH)1 that occur as women age can cause some ovaries to go into overdrive, tripping them into a state where they have a simultaneous double ovulation.

The report was published online in February 2006 in Europe's leading reproductive medicine journal Human Reproduction.

Professor Homburg, Professor of Reproductive Medicine in the Obstetrics and Gynaecology Department, explained: "Identical (monozygotic) twins occur when one fertilised oocyte divides to develop into two babies, but non-identical (dizygotic) twins develop from two separate fertilised oocytes. Two ovulations are obviously needed to produce the majority of twin pregnancies as non-identical twins account for up to three-quarters of twin pregnancies."

Senior author Dr Lambalk, who is head of research in the Reproductive Medicine Division, 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 twin rates -- a seemingly paradoxical phenomenon that has not been entirely explained, until now."

The research team analysed oocyte follicle development in 959 spontaneous natural cycles of 507 women who were undergoing intrauterine insemination because of unexplained infertility or mild male infertility.

They found that multiple ovarian follicular development i.e. more than one follicle over 14mm in size, and by implication, multiple rather than single ovulations, 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 base FSH rate also increased with age, both in women with single and multiple follicular development, but was higher in women who produced multiple follicles.

The authors explained that because of a decline in ovarian feedback capacity the pulse amplitudes of FSH increase in an attempt to counteract failing ovaries and FSH levels rise, overshooting the threshold needed for ovulation. This doesn't generally lead to multiple pregnancies because of the low number of remaining follicles with good quality oocytes. But, if there are two or more follicles with good quality oocytes available, then simultaneous double ovulation can occur, making a multiple pregnancy more likely.

Professor Homburg said, "These findings are a timely reminder to women not to delay their pregnancies if it can be avoided because of the difficulties of conceiving as ovarian failure begins to set in."

Over the last decade Dr Lambalk has documented the rise in multiple pregnancies in Holland. "This rise has not been entirely due to treatment for infertility," he said. "About half of the increase has been caused by the number of spontaneous multiple pregnancies, probably due to the fact that women are delaying childbirth to a later age."