Birthweight of singletons after assisted reproduction is higher after single- than after double-embryo transfer

BACKGROUND: Single-embryo transfer (SET) has proven efficient in reducing multiple pregnancy rates after assisted reproduction technologies (ART). This study compares outcome of singletons after SET and double-embryo transfer (DET). METHODS: We studied 404 SET and 431 DET patients, who delivered a singleton child of > 500 g after fresh embryo transfer in a first, second or third cycle. Preterm birth and low birthweight incidences and gestational age and birthweight were compared between both groups. Adjustments were made for maternal age, parity, cycle rank number, treatment indication, ART method, embryo characteristics and sex of the child. RESULTS: Singletons born after DET have a significantly lower birthweight than that after SET (3204.3 +/- 617.5 g versus 3324.6 +/- 509.7 g, P < 0.01). Also preterm birth (< 37 weeks) [odds ratio (OR) 1.77, 95% confidence interval (CI) 1.06-2.94] and low birthweight (< 2500 g) (OR 3.38, 95% CI 1.86-6.12) are significantly more common in DET singletons. CONCLUSIONS: Singleton birth after SET is advantageous compared with DET. This sheds new light on the reasons why singleton births following ART do worse than spontaneously conceived singletons in IVF programs, where double- or multiple-embryo transfer is standard.