Increased risk of preterm birth in singleton pregnancies after blastocyst versus Day 3 embryo transfer: Canadian ART Register (CARTR) analysis

Are the fetal outcomes of singleton pregnancies that result from cleavage stage embryo transfer (ET) different from the outcomes from Day 5/6 blastocyst stage ET? There was a significantly higher risk of preterm birth (37 weeks) in singletons after extended embryo culture (Day 5/6) compared with cleavage stage (Day 3) transfer. Two recent studies, from Sweden and the USA, reported an increased risk of preterm birth in singleton pregnancies after Day 5/6 ET compared with Day 3 ET. The US study also showed increased early preterm births and the Swedish study showed increased fetal malformations in this group. A retrospective cohort study was performed. Data were collected from the Canadian ART Register database for all singleton births after fresh IVF/ICSI ET cycles (20012009). A total of 12 712 singleton births were included. Of these, 9506 resulted from a Day 3 ET and 3206 resulted from a blastocyst (Day 5/6) ET. Preterm birth rate 37 weeks (unadjusted by potential confounding factors) was higher with Day 5/6 versus Day 3 transfers (17.2 versus 14.1, P 0.001). Using logistic regression analysis to adjust for confounding factors, preterm birth rate 37 weeks was the only outcome significantly increased after Day 5/6 compared with Day 3 transfer (odds ratio 1.32, 95 confidence interval 1.171.49). The following confounding factors were adjusted for: year of treatment (20012009), maternal age (continuous), parity (0 versus 1 birth), diagnosis category, number of oocytes retrieved [20 versus 20 (high responder group)], insemination method (IVF versus ICSI), number of embryos transferred (1, 2 or 3) and the presence of a vanishing twin (1 fetal heart on the initial ultrasonographic examination). Post-natal follow-up studies will be required to determine if this difference we observed translates into adverse long-term effects on these offspring. The rate of early preterm births (32 weeks) was higher in Day 5/6 versus Day 3, but the low number of cases in this category did not have the power to show a difference (3.0 versus 2.7, P 0.34). We found a significantly higher risk of preterm birth (37 weeks) in singletons after extended embryo culture (Day 5/6) compared with cleavage stage (Day 3) transfer, even when adjusting for confounding factors. Our findings are in agreement with the previous two studies; however, we did not show a difference in the very preterm deliveries (unlike the US study) or in fetal malformations (as in the Swedish study). We hypothesize that there may be a deleterious effect of prolonged in vitro embryo culture on subsequent placentation. Longer term follow-up studies will be required to determine if prolonged in vitro culture to the blastocyst stage has an adverse effect on the long-term health of offspring when compared with shorter cleavage stage culture. None.