During the IVF procedure, there are times when there are no embryos derived from normally fertilized (2PN) eggs. So is it worth transferring embryos derived from 1PN eggs? What are the pregnancy rates after transfer?
For the study of embryos derived from 1PN eggs, in the past, due to the low prevalence of egg freezing and the difficulty in grasping the timeline of fertilization after thawing eggs, the vast majority of studies have investigated the formation of 1PN embryos from fresh eggs. Nowadays, with the prevalence of the concepts of egg freezing and egg collection, and the development of the technique of observing the spindle, the research on thawing eggs has been increasing gradually.
Previous studies on fresh eggs have shown that
- abnormally fertilized (1PN) embryos are less likely to develop blastocysts than 2PN zygotes.
- abnormally fertilized (1PN) embryos are more likely to have chromosomal abnormalities than 2PN zygotes.
- transfer of abnormally fertilized (1PN) embryos results in poorer pregnancy rates than 2PN zygotes.
To investigate the 1PN blastocysts from thawed eggs, the Stork Fertility Center calculated 571 recipient egg cycles from 2019 to 2020. The 571 cycles were divided into a 1PN group (1PN rate >15%) and a control group (1PN rate <15%) according to the rate of 1PN zygotes, and the results of embryo culture and the pregnancy rates of these two groups were calculated separately (study flowchart below).
To control the time point of fertilization after thawing and avoid damage to the spindle during ICSI, all eggs are observed for spindle before fertilization to confirm that they are MII mature eggs before ICSI. In addition, in order to ensure consistent egg quality, all eggs are from egg donors.
Statistical Results
- The 1PN group was less likely to form blastocysts than the control group (41.2% vs 63.1%).
The 1PN group had lower good embryo rates (D3 6II or better / 2PN), good blastocysts rates (D5 BC or better / 2PN), and the utilization rate (D5 BC or better / number of fertilized MII eggs) than the control group, and there was a statistically significant difference between the two groups (as shown in the figure below).
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Embryos derived from 1PN eggs had a higher aneuploid rate compared to those formed by 2PN (48.4% vs. 33.7%).
In this study, the results of Preimplantation Genetic Testing for Aneuploidy (PGT-A) of all embryos derived from 1PN and 2PN eggs in Stork Fertility Center were calculated. The results showed that embryos derived from 1PN eggs had a higher aneuploid rate than those from 2PN, and there was a statistically significant difference between the two groups (as shown in the figure below).
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The pregnancy rate of embryos derived from 1PN was worse than the control group.
The clinical outcomes of the transfer of blastocysts derived from 1PN were poorer than those from the control group in the β-HCG(+) rate, the clinical pregnancy rate (sac+ cycle number/transfer cycle number), and the implantation rate (fetal heartbeat number/embryo transfer number) (as shown in the figure below).
Conclusion
According to the statistics in Stork Fertility Center: The results of embryo culture and clinical outcome of 1PN blastocysts from thawed eggs were poorer than those of 2PN blastocysts, which were the same as the results of previous studies on fresh eggs. The results show that even when fertilization is performed at the precise time point (ICSI after observation of the spindle) and the quality of the eggs is ensured (egg donation), subsequent observation of the pronuclear (PN) formation with 1PN results in poorer embryo culture, chromosome aneuploid rate, and clinical outcome than blastocysts derived from 2PN. Even so, Stork Fertility Center still has records of the transfer of embryos derived from 1PN for live births. Therefore, Stork Fertility Center does not give up any opportunity to allow clients to have live births and has been retaining abnormally fertilized (NPN, 1PN) zygotes for culturing and freezing them independently after the formation of blastocysts since 2017 to provide clients with a second choice for transfer.
Reference
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Birth of nine normal healthy babies following transfer of blastocysts derived from human single-pronucleate zygotes. J Assist Reprod Genet (2015) 32:1401–1407
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Developmental potential of clinically discarded human embryos and associated chromosomal analysis. Scientific Reports | 6:23995 | DOI: 10.1038/srep23995
In the early stages of fertilization, the spermatozoa acrosome releases enzymes that penetrate the radial crown cells and the acrosome reaction occurs, the sperm head fuses with the egg cell membrane, releasing spermatozoa DNA into the egg, which activates the secondary oocyte to mature and produce the second polar body, the spermatozoa and the egg form a pronuclear respectively, which consists of two pronuclear, the so-called 2PN, which gradually move closer to the center and fuse together into a single one, which then disappears to form a diploid fertilized egg, also known as an zygote.
During fertilization, if the sperm and egg have an abnormal function or chromosome abnormality, only one pronuclear (1PN) or no pronuclear (NPN) is formed, which is also known as abnormal fertilization. The two pronuclear (2PN) of a normally fertilized egg can sometimes be misjudged as abnormally fertilized due to faster egg development, the fusion of the two pronuclear, or even the disappearance of the two pronuclear, which may cause the embryologist to miss the point at which the two pronuclear are observed. Although the status of fertilized eggs with abnormal pronuclear counts is more uncertain, there is still a chance of embryo formation, and transfer is recommended as a last priority.
According to Stork Fertility Center's records, between 2015 and 2020, one client transferred a 1PN embryo and had a successful live birth; three clients implanted NPN embryos and had successful live births. Although there is a higher chance of chromosomal abnormalities in embryos with abnormal pronuclear counts, and the number of clients with live births is few, Stork Fertility Center upholds the spirit of persistence to the end and does not give up on any embryo that has a chance of being born to live birth in the hope that it will continue to create miracles and touching feelings.