To compare the perinatal outcomes of babies conceived by single embryo transfer (SET) with those conceived by double embryo transfer (DET).

A retrospective population-based study of embryo transfer cycles in Australia and New Zealand between 2002 and 2006, using data from the Australia and New Zealand Assisted Reproduction Database.

Proportion of SET procedures; comparison of SET and DET procedures with respect to multiple births, low birthweight (LBW), preterm birth and fetal death.

The proportion of SET procedures has increased from 28.4% in 2002 to 32.0% in 2003, 40.5% in 2004, 48.2% in 2005 and 56.9% in 2006. The multiple birth rate for all babies conceived by SET (4.0%) was 10 times lower than for those conceived by DET (39.1%) (P < 0.01). The average birthweight for all liveborn babies conceived by SET (3290 g) was higher than for those conceived by DET (2934 g) (P < 0.01). The preterm birth rate of all DET-conceived babies (30.3%) was higher than for SET-conceived babies (12.3%) (adjusted odds ratio [AOR], 3.19 [95% CI, 3.01–3.38]). All babies conceived by DET were more likely to be stillborn than those conceived by SET (AOR, 1.49 [95% CI, 1.21–1.82]). Singletons conceived by DET were more likely to be born preterm than singletons conceived by SET (AOR, 1.13 [95% CI, 1.05–1.22]). Liveborn singletons conceived by DET were 15% more likely to have LBW than liveborn singletons conceived by SET (AOR, 1.15 [95% CI, 1.05–1.26]). There was no significant difference in fetal death rate between DET- and SET-conceived singletons.

The increase in proportion of SET procedures has resulted in a lower rate of multiple births and in better perinatal outcomes in Australian and New Zealand assisted reproduction programs.