Introduction The purpose of the present study was to estimate the effect of fertility treatment; both assisted reproductive technology (ART) and non-ART ovulation stimulation, on the number and rate of multiple live births during the last 30 years in Japan.

Methods Japanese vital statistics according to maternal age class and plurality of live births published by the Ministry of Health, Labour and Welfare from 1974 to 2009 and Japanese ART statistics published by Japan Society of Obstetrics and Gynaecology from 1989 to 2008 were gathered and reanalysed. Total iatrogenic multiple births were estimated by vital statistics assuming that spontaneous multiple-birth rates according to maternal age class would be constant. The number and rates of ART between 1977 and 1988 were estimated using approximation formulae. The number of non-ART iatrogenic multiple births were estimated by subtracting ART multiples from total iatrogenic multiples.

Results The birth rates of iatrogenic multiples increased dramatically between 1977 and 2005, and decreased. The estimated maximum percentage of multiple births that were iatrogenic was 50.0% in 2005. Non-ART multiple births increased during past 30 years overall, whereas ART multiples tended to increase from 1983 to 2005 and then decreased rapidly. The number or percentage of ART multiples were almost constantly lower than that of non-ART multiples.

Conclusion The rapid increase of numbers and rates of multiples born to women over 30 years in Japan is mainly attributed to iatrogenic rather than spontaneous multiple births. The effect of non-ART fertility treatment is large recently.