Monozygotic Twinning and Assisted Reproductive Technologies
Summary
Monozygotic twinning occurs when a single fertilised egg divides to form two genetically identical embryos. In spontaneous conception, the incidence is approximately 0.3–0.5%, but assisted reproductive technologies (ART) such as in vitro fertilisation (IVF), intracytoplasmic sperm injection (ICSI) and blastocyst culture have been associated with a two- to threefold increase. Splitting may occur at different stages: early cleavage can yield dichorionic–diamniotic twins, while later division of the inner cell mass (ICM) generates monochorionic–diamniotic twins. Factors implicated include prolonged in vitro culture, zona pellucida manipulation, blastocoel expansion and assisted hatching. Monozygotic twin pregnancies carry elevated risks of preterm birth, twin-to-twin transfusion syndrome, growth discordance and perinatal morbidity and mortality. Single embryo transfer policies have reduced dizygotic twinning, but cannot fully prevent monozygotic events. A detailed understanding of cellular and clinical mechanisms underpins efforts to refine stimulation, culture and transfer protocols, with the aim of balancing success rates against the hazards of multiple gestations.
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Monozygotic Twinning and Assisted Reproductive Technologies publication trend
The graph below shows the total number of articles in monozygotic twinning and assisted reproductive technologies across all publications each year (not limited to Nature Index journals).
Technical terms
Monozygotic twinning: Formation of two embryos from a single fertilised egg, yielding genetically identical offspring.
Assisted reproductive technology (ART): Medical techniques to achieve pregnancy, including IVF, ICSI, embryo culture and transfer.
Blastocyst: Embryonic stage (~day 5–6) comprising an outer trophoblast layer and an inner cell mass (ICM) surrounded by a fluid-filled cavity (blastocoel).
Inner cell mass (ICM): Cluster of cells within a blastocyst that gives rise to the embryo proper; its splitting underlies monochorionic twinning.
Chorionicity: Number of placental structures in a twin pregnancy—monochorionic twins share one chorion, dichorionic twins have two.
References
- Cellular mechanisms of monozygotic twinning: clues from assisted reproduction. Human Reproduction Update (2024).
- Eight-Shaped Hatching Increases the Risk of Inner Cell Mass Splitting in Extended Mouse Embryo Culture. PLOS ONE (2015).
- Risk factors associated with monozygotic twinning in offspring conceived by assisted reproductive technology. Human Reproduction Open (2023).
- Maternal and perinatal risks for monozygotic twins conceived following frozen-thawed embryo transfer: a retrospective cohort study. Journal of Ovarian Research (2024).
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