Birth of twenty-seven healthy babies following transfer of fresh and frozen-thawed embryos derived from monopronuclear zygotes: a retrospective study

Study question
What are the neonatal outcomes of fresh/frozen-thawed embryos derived from monopronuclear zygotes (1PN)? Is there any influence of the method of fertilization?
Summary answer
A total of 231 monoembryonic transfers (TF) of fresh or frozen/thawed embryos derived from 1PN zygotes allows to the birth of twenty-seven healthy babies.
What is known already
Zygotes were classified 16 to 18 hours post insemination according to the extrusion of the second polar body and the number of pronuclei (PN). The «normally fertilized» zygotes display 2 PN and those with 0, 1 or more than 2 PN (> 2PN) are considered as abnormal. If some zygotes are always discarded (>2PN) due to the increased risk of aneuploidy, the transfer of embryos derived from 1PN zygotes remains more controversial.
Study design, size, duration
Our retrospective study included all conventional in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI) and frozen embryo transfer (FET) cycles performed between january 2018 and december 2022.

Embryo and pregnancy outcomes were analyzed in a first group of 781 fresh cycles (731 patients) and in a second group of 167 FET (150 patients) with embryos derived from 1PN zygote. Only monoembryonic transfers were analyzed for live birth rate (LBR).
Participants/materials, setting, methods
Only embryos derived from 1PN zygotes were analyzed in our study. For fresh cycles (n = 2762), embryo outcome was analyzed for 1234 (1PN) zygotes (IVF = 648 and ICSI = 586). Fresh embryo TF was performed in 64 cycles. For frozen cycles, TF was achieved in 167 cases. For a total of 231 TF on day 2, 3 or 5, pregnancy and live birth rates were analyzed.
Main results and the role of chance
At first, 46% of 1PN zygotes from fresh IVF cycles gave rise to embryos of sufficient quality to be transferred or frozen (day 3 or 5/6). This rate decreased to 33% in the fresh ICSI cycles. Blastulation rate was also more important in IVF group (44%) in comparison to ICSI group (20%) without signification (p = 0.1).

Fresh TF of 64 patients (32 in each fertilization group) allowed 7 pregnancies in the IVF group (PR = 22%) as compared to 4 pregnancies in the ICSI group (PR = 12%). The 7 IVF pregnancies ended in 4 deliveries of healthy newborns, 2 miscarriages and one pregnancy still ongoing. In the ICSI group, 1 birth of a healthy newborn and 3 miscarriages were observed.

Secondary, thirty-six pregnancies were obtained in the 167 FET cycles. A non significative difference was observed between embryos derived from IVF cycles (PR = 26%) and ICSI cycles (PR = 16%) with respectively 15 and 6 healthy babies born. Two pregnancies in each group are still ongoing.
Limitations, reasons for caution
The main limitation of this study is that it was a retrospective, single-center study with a relatively small number of births.
Wider implications of the findings
In conclusion, we observed better outcomes in IVF cycles in comparison to ICSI cycles and our center policy to transfer a good quality embryo developed from 1PN zygotes allowed 26 deliveries of 27 healthy newborns during the period analyzed.