The paradox of declining fertility but increasing twinning rates with advancing maternal age.

BACKGROUND

Advancing female age is associated with declining fertility potential due to decreasing numbers and quality of oocytes but also with a distinct increase in dizygotic twinning rates, a phenomenon that has never been explained.

METHOD: An analysis of follicle development was made in 959 spontaneous ovulatory cycles of 507 women.

RESULTS

Multiple ovarian follicular development (>1 follicle >14 mm) and, by implication, multiple rather than single ovulations occurred in 105 women whose mean age (36.1 versus 34.6 years) and mean basal FSH concentrations (10.3 versus 7.7 IU/l) were significantly greater than those with monofollicular development (P < 0.01). The prevalence of multifollicular development increased with age.

CONCLUSIONS

Dizygotic twinning must be associated with the development of >1 large follicle, which we found to be a significantly more frequent occurrence in older women. It is hypothesized that the response of pituitary release of FSH to the decreased negative feedback induced by impending ovarian failure often 'overshoots', causing multiple follicular development. In the presence of two good-quality oocytes, a twin pregnancy may result.