Why older mums expect more twins

OVARIES in overdrive are responsible for the relatively large number of twins born to older mothers.

Though fertility declines with age, older women who do have children are more likely than younger mothers to give birth to non-identical twins. "It's a paradox that nobody could explain," says Roy Homburg of the Free University Medical Centre in Amsterdam, the Netherlands.

Non-identical twins develop from separate fertilised eggs, so the increase among older women was attributed to their greater use of IVF, in which several embryos may be implanted. However, when Homburg and his colleagues analysed multiple birth records in the Netherlands over the past decade, they found that more than half the cases involved natural pregnancies.

The team used ultrasound to monitor the growth of ovarian follicles, from which the eggs are released, in 507 women. Of the 105 women who produced multiple eggs in one cycle, 95 per cent were more than 30 years old. The older women also had increased levels of follicle stimulating hormone (FSH). Women who produced multiple eggs had the highest levels of all (Human Reproduction, DOI: 10.1093/humrep/de1009).

"From the age of 35 to the menopause, FSH goes up steadily to counter failing ovaries, which have difficulty releasing eggs," says Homburg. "In some cases the body overcompensates, shooting hormone levels so high that two are released."

Via: New Scientist