Researchers from the Create Fertility Center in Ontario, Canada, recently published a study comparing preterm birth rates in pregnancies conceived with blastocysts versus day 5/6 embryos. Results of the study were published in the journal Human Reproduction.

Noted as part of the study were results from recent US and Swedish studies that reported an increased risk of preterm births and fetal malformations when embryos older than day 5/6 were used. The purpose of this study was to compare day 3 blastocysts with embryos implanted on day 5/6.

The cohort used data collected as part of the Canadian Assisted Reproductive Technology Register as the sole means of information. More than 12,000 singleton births were noted. About 9,500 were conceived with day 3 (blastocyst) embryos and about 3,200 were conceived with day 5/6 embryos.

Conclusion: Researchers found a 3% increase in preterm births after day 5/6 embryo transfer compared to day 3 embryo transfer; 17.2% versus 14.1%, respectively. Preterm birth was the only clinically significant difference between the two study groups.

Source: Dar S, Librach CL, Gunby J, Bissonnette F, Cowan L; on behalf of the IVF Directors Group of the Canadian Fertility and Andrology Society. Increased risk of preterm birth in singleton pregnancies after blastocyst versus Day 3 embryo transfer: Canadian ART Register (CARTR) analysis. Hum Reprod. 2013 Jan 24.