STUDY QUESTION: Does the manipulation of gametes or embryos during ARTs increase the risk for monozygotic twinning (MZT)?
SUMMARY ANSWER: Frozen embryo transfer (ET) is associated with a lower MZT rate, while blastocyst culture is associated with an increased risk of monozygotic pregnancy.
WHAT IS KNOWN ALREADY: Monozygotic twins have a higher risk for perinatal complications. Although an increased incidence of monozygotic pregnancies after ART has been previously reported, data regarding the possible impact of different laboratory procedures are conflicting.
STUDY DESIGN, SIZE, DURATION: All clinical pregnancies after single ET carried out in our centre between 2004 and 2013 (n = 6096) were retrospectively analysed for the incidence of MZT. The effect of different laboratory procedures on the incidence of MZT was evaluated.
PARTICIPANTS/MATERIALS, SETTING, METHODS: The following ART risk factors were assessed: maternal age, type of ET (fresh versus frozen), zona pellucida (ZP) manipulation (specifically, ICSI, embryo biopsy and assisted hatching), use of donor oocytes, embryo stage at time of ET (cleavage, compaction, early or advanced blastocyst) and culture media.
MAIN RESULTS AND THE ROLE OF CHANCE: The overall MZT rate was 2.2% (136/6096). Frozen ET was associated with a significant reduction in MZT incidence (adjusted odds ratio (aOR) 0.48, 95% CI 0.29-0.80), while blastocyst transfer (early or advanced blastocyst) was associated with a significant increase in MZT risk (aOR 2.70, 95% CI 1.36-5.34; aOR 2.05, 95% CI 1.29-3.26, respectively). No significant differences were found between the MZT and singleton (non-MZT) groups regarding maternal age, the use of different ZP manipulation techniques, not type of culture media used.
LIMITATION, REASONS FOR CAUTION: This study is limited by its retrospective nature and the fact that monozygosity was not confirmed by genetic testing. Furthermore, since monozygotic pregnancy is a rare event, other ART parameters that may influence its incidence could not be assessed during our analysis.
WIDER IMPLICATION OF THE FINDINGS: Our findings warrant future studies designed to investigate the association between specific ART procedures and MZT, namely the potential risk of blastocyst transfer to increase MZT.
STUDY FUNDING/COMPETING INTERESTS: No external funding was used for this study. There are no conflicts of interest.