Neonatal health including congenital malformation risk of 1072 children born after vitrified embryo transfer

Study question: Does vitrification of Day 3 and Day 5 embryos adversely affect birth outcomes of singletons and twins in comparison with peers born after fresh embryo transfer?

Summary answer: Neonatal health parameters, including the prevalence of congenital malformations, in singletons and twins born after embryo vitrification are similar to or slightly better than after fresh embryo transfer.

What is already known: Although vitrification, rather than slow-freezing, of embryos is routine practice nowadays, convincing evidence regarding the safety for the offspring is sparse. Literature data comprise results from mostly small-sized studies or studies including only Day 3 or only Day 5 vitrified embryo transfers. Overall, better or comparable perinatal outcomes, in terms of higher birthweight and lower risk for small-for-gestational age or for low birthweight, have been reported for singletons born after vitrified embryo transfer compared with fresh embryo transfer. According to the single available study with sufficient sample size, the congenital malformation rate was found to be comparable after vitrified and fresh embryo transfers.

Study design, size, duration: Data were collected from 960 cycles after transfer of embryos vitrified on Day 3 (n = 457) or Day 5 (n = 503) and from 1644 cycles after fresh embryo transfer on Day 3 (n = 853) or Day 5 (n = 791), performed between 2008 and 2013 at the Centre for Reproductive Medicine of the university hospital UZ Brussel. Outcome measures were neonatal health in terms of birthweight, small-for-gestational age, prematurity rate, perinatal death and major/minor/total malformation rate.

Participants/materials, setting, methods: Perinatal health parameters of 11 stillborns and 1061 live borns (827 singletons and 234 twins) in the vitrified group and of 28 stillborns and 1838 live borns (1374 singletons and 464 twins) in the fresh embryo group are reported. Within 3 months after birth, children in the two study groups were assessed clinically with special attention to congenital malformations by a paediatrician blinded to the type of embryo transfer. Data were analysed by multiple linear and logistic regression, adjusted for treatment variables and maternal characteristics.

Main results and the role of chance: Mothers to infants in the vitrified group were on average slightly older and more often suffering from pregnancy-related hypertensive disorders than mothers to infants in the fresh transfer group. Singletons born after vitrification showed a higher birthweight standard deviation score (SDS) (-0.4 versus -0.7; 95% confidence interval (CI): 0.0-0.3, P = 0.001) and a lower small-for-gestational age rate (AOR: 0.55; 95% CI: 0.34-0.90) in comparison with peers born after fresh embryo transfer. Preterm birth rate and perinatal death rate were comparable between the two groups (AOR: 0.91; 95% CI: 0.57-1.43 and AOR: 0.97; 95% CI: 0.40-2.36). In twins, neonatal outcomes including birthweight SDS, small-for-gestational age and prematurity rates were comparable in the vitrified and the fresh groups, when adjusted for confounders. Furthermore, the rate of major congenital malformations in live borns was comparable between the vitrified group and the fresh group, both in singletons (2.6 versus 2.8%; AOR: 0.91; 95% CI: 0.47-1.78) and in twins (2.4 versus 2.7%; AOR: 0.51; 95% CI: 0.05-5.72). Also, the total malformation rate in the vitrified group (3.4%; 95% CI: 2.4-4.8) did not differ from the rate in the fresh embryo group (3.9%; 95% CI: 3.1-5.0). The embryonic stage at vitrification or fresh transfer (cleavage-stage embryo or blastocyst) did not influence the birth characteristics or malformation rate.

Limitations, reasons for caution: The main limitation of this study is the rather small twin group. Therefore, the outcome results for twins should be interpreted cautiously.

Wider implications of the findings: This study provides evidence that transfer of vitrified Day 3 and Day 5 embryos does not adversely affect the neonatal health of the offspring in comparison with transfer of fresh embryos. Furthermore, neonatal outcomes were not different after transfer of vitrified blastocysts compared with transfer of vitrified cleavage-stage embryos.

Study funding/competing interests: Educational grants for establishing and organizing the data collection have come from IBSA, Ferring, Organon, Shering-Plough and Merck. Merck Belgium funded the data collection for outcomes after vitrification between 2012 and 2015. All co-authors, except M.B., declared no conflict of interest. M.B. has received consultancy fees from Organon, Serono Symposia and Merck.