Embryo reduction and birth weight discordance in dichorionic twins.

Background: Twin birth weight discordance is associated with a poor perinatal outcome. The aim of this study was to analyse the risk factors of growth discordance among dichorionic twin pregnancies.

Methods: A cohort of 346 dichorionic twin pregnancies delivered at one perinatal centre between January 1996 and December 1999 was analysed. Two groups were created, according to the presence or absence of intra-pair birth weight discordance (n = 75 and 271 respectively). Birth weight discordance was defined as a difference of >/=20% of the weight of the heavier twin. The two groups were compared by uni- and multivariate analysis, with regard to the woman's characteristics, risk factors for growth restriction or discordance, and outcome of pregnancy.

Results: Pregnancies with birth weight discordance had a poor outcome compared with pregnancies without discordance, with a 4-fold increase in neonatal mortality. The rate of iatrogenic embryo reduction was significantly higher in discordant pregnancies (14.7 versus 6.6%, P = 0.03). The risk of birth weight discordance was increased with a larger starting number of embryos before reduction [20.2% (64/317), 28.6% (6/21), 57.1% (4/7) and 100% (1/1) respectively, for an initial number of two (no reduction), three, four, and five embryos, P = 0.02]. In multivariate analysis, embryo reduction was the only significant risk factor for the occurrence of birth weight discordance [adjusted odds ratio (OR) = 2.3 (1.0-5.2)].

Conclusions: Birth weight discordance carries a poor perinatal outcome. Embryo reduction is an independent risk factor for birth weight discordance in dichorionic twins. This finding emphasises the need for better control of assisted reproductive technology in order to avoid high-order multiple pregnancies.