P-408 Monozygotic twins from in vitro fertilization: can bring the rate closer to that of natural conception?
Authors: Wang W 
Published: July 2024

Study question

Whether the occurrence of monozygotic twins in human in vitro fertilization is associated with zona manipulations, and can we explore approaches to reduce its rates?

Summary answer

Monozygotic twinning is partially caused by zona manipulation, and it can be lowered by creating a larger opening in the zona before embryo transfer.

What is known already

While the rate of monozygotic twins in spontaneous conception is 0.5% of live births, the frequency of monozygotic twins after in vitro fertilization-embryo transfer can be a few times higher. Various factors have been suggested to contribute to this phenomenon, including ovarian stimulation, intracytoplasmic sperm injection, embryo cryopreservation, extended culture, embryo quality, maternal age, genetic factors, and micromanipulation of the zona pellucida, such as assisted hatching and biopsy. Due to the fact that the overall rate is low, it is difficult to pinpoint the main reason(s) contributing to it, and there are no effective approach(es) to lower this rate.

Study design, size, duration

We studied monozygotic twins in 8,063 clinical pregnancies resulting from single blastocyst transfers from 2016 to 2023. Blastocysts, whether fresh or frozen, were transferred with or without biopsy for preimplantation genetic testing and/or blastocyst collapse. Clinical pregnancy, embryo implantation, including twin implantation, and clinical outcomes such as live birth, ongoing pregnancy, and birth weight were further evaluated. Monozygotic twin rates were compared between partial zona removal (larger hole) and no zona removal (small hole)

Participants/materials, setting, methods

Monozygotic twins were initially evaluated in blastocysts with or without biopsy in 732 clinical pregnancies after fresh or frozen embryo transfer. Subsequently, monozygotic twins were further evaluated in 7331 clinical pregnancies after an approach to open a larger hole in the zona before blastocyst transfer. Additionally, 56 monozygotic twins were evaluated for live birth and birth weight outcomes.

Main results and the role of chance

In the context of fresh blastocyst transfer, the monozygotic twin rate following biopsy was significantly higher than that for blastocysts that did not undergo zona manipulation (3.4% vs. 0.29%; p = 0.025). After blastocysts underwent biopsy and cryopreservation/warming, the monozygotic twin rate was 2.79%, a similar rate as observed in fresh blastocyst transfer after biopsy. However, when frozen blastocysts (either biopsied and/or collapsed) underwent partial zona removal before transfer, the rate of monozygotic twins significantly decreased to 0.6% (p = 0.00001). When we further examined whether the biopsy procedure increased the monozygotic twin rate (28/4947) under the conditions that partial zona removal was performed by comparing those with blastocysts without biopsy (16/2384), we did not find a significant difference (0.57% vs. 0.67%, p = 0.587) between the two treatments. In this study, there were 34 twin live births (60.7%) including 5 ongoing twin pregnancies, and 9 (16.1%) singleton live births, contributing to an overall live birth rate of 76.8%. Ten cases (17.8%) experienced pregnancy loss with heartbeat cessation during the first trimester. Furthermore, 3 cases (5.4%) had abnormal pregnancies. When the birth weights were evaluated among the monozygotic twins, it was found that lower birth weight (&lt; 1500g) was only observed in twin live birth.

Limitations, reasons for caution

This study only included data from one large clinic in the US under the conditions and protocols (embryo culture, embryo biopsy, and cryopreservation) used in this clinic. Different patients and protocols would affect the rate differently.

Wider implications of the findings

Zona manipulations are associated with an increased monozygotic twin rate in both fresh and frozen embryo transfers. This effect can be mitigated by creating a larger opening in the zona before embryo transfer, and this approach can bring the monozygotic twin rate closer to that of natural conception.

Trial registration number

NA