Monochorionic twins after single blastocyst transfer: retrospective cohort and blinded time lapse annotation analysis.

RESEARCH QUESTION:Is it possible to identify monochorionic twin pregnancies before blastocyst transfer based on maternal or embryo characteristics registered by time lapse? DESIGN:A retrospective cohort study including women who received fertility treatment (n = 6501) between 2010 and 2019 at two fertility clinics in central Denmark. The treatment resulted in 2239 pregnancies after single embryo transfer (SET) and, of those, 43 (1.92%) were monochorionic twins. Baseline characteristics, information on assisted reproductive technology and ultrasonic findings at gestational week 8 were collected on all women. Furthermore, a blinded time lapse annotation analysis was conducted by two independent laboratory technicians on a total of 85 embryos. A total of 22 embryos leading to monochorionic pregnancies were matched with 63 embryos leading to singleton pregnancies. RESULTS:A monochorionic twin rate of 1.92% was found. No significant difference was found in maternal age, the use of intracytoplasmic sperm injection compared with IVF, indication for treatment or other maternal characteristics. In the blinded annotation analysis, inner cell mass grade A was associated with an increased risk of twinning (P = 0.04) in fresh embryos. The s3 division timing was found to be significantly shorter in fresh twin compared with singleton embryos (P = 0.006). No other time lapse parameters were found to be characteristic of twin embryos. CONCLUSION:To the best of our knowledge, this is the first blinded annotation study to identify aspects in time lapse resulting in monochorionic twins after SET. Whether inner cell mass grade A and s3 reflect a biological background for monochorionic twinning merits further investigation.