Blastocyst culture is associated with an elevated incidence of monozygotic twinning after single embryo transfer

Clustering of monozygotic twinning in IVF

“…Over the past decade, there has been a continual shift towards blastocyst transfers, which now comprise the majority of embryo transfers in many countries including the USA and Australia. Cleavage stage transfers are generally reserved for older women, those with a poorer prognosis, and those with fewer embryos [12]. In our study, cycles performed in 2005 or later appeared to have a significantly increased risk of MZT compared to those cycles performed before 2005.…”

“…Extended embryo culture appears to be one of the most important factors contributing to MZ pregnancies [10,11], which may support the aforementioned theory that a breach of the integrity of the zona pellucida leads to a herniation of blastomeres and the splitting of the embryo [26]. This hypothesis led to concerns about ART procedures, which deliberately disrupt the zona pellucida, such as ICSI, assisted hatching, and PGD/PGS [12]. However, this concern has been refuted by several recent studies [9,13,16].…”

“…Of note, seven out of the eight women went on to deliver at least one live infant. In our retrospective study examining a large cohort at a single, urban, fertility center, MZT occurred at a rate of 1.1 % of all clinical pregnancies, which is on the lower side of what has been previously reported [9,[11][12][13][14]. Of note, only viable pregnancies (those with an identifiable fetal heartbeat) were included in our study.…”

“…Current evidence suggests that multiple factors might be responsible, including maternal age, ovarian stimulation, prolonged embryo culture, altered in vitro culture conditions, and zona pellucida manipulation [11]. However, a meta-analysis and a recent large study did not show an association between intracytoplasmic sperm injection (ICSI) or assisted hatching and MZT [9,12]. Blastocyst culture appears to be one of the most important factors contributing to MZ pregnancies [10,11].…”

“…Knopman et al [13] noted increased MZT rates among younger women when the maternal age of oocytes was <35 years; however, these results are confounded by the fact that younger women were more likely to have embryos transferred at the blastocyst stage [13]. While most studies suggest that blastocyst transfer is an independent risk factor for MZT [11][12][13][14], not all publications support this association [15]. For example, a recent study by Franasiak et al [16] failed to demonstrate increased rates of MZT when controlling for embryo cohort quality.…”