Monozygotic, or “identical,” twinning occurs when a single fertilized egg splits into two genetically identical embryos. In natural conception, the incidence of identical twins is rare and stable, occurring in approximately 0.4% of births globally. However, the rate of identical twins is significantly elevated in pregnancies achieved through in vitro fertilization (IVF) and other assisted reproductive technologies. This phenomenon increases the risk of identical twinning by two to four times, with reported rates in IVF cycles ranging from 1% to as high as 8.9% in certain patient populations. This increased risk points toward specific laboratory procedures and manipulations that encourage this uncommon embryonic splitting.
Understanding Monozygotic Twinning
Monozygotic twinning is fundamentally a developmental anomaly where the inner cell mass of a single embryo separates into two distinct masses, each capable of developing into a fetus. The timing of this division determines the shared structures of the resulting pregnancy, which defines its complexity. If the splitting occurs very early (within the first three days), the twins will develop in separate sacs and have their own placentas (dichorionic-diamniotic twins).
A later split, occurring between four and eight days after fertilization, is more common in monozygotic gestations. This results in twins that share a single placenta but have separate amniotic sacs. Splitting that happens even later, between eight and thirteen days, creates the highest-risk scenario where the twins share both the placenta and the amniotic sac. This significantly increases the potential for complications like twin-to-twin transfusion syndrome.
Procedural Triggers During Embryo Transfer
One of the most direct ways the IVF process triggers embryonic splitting involves physical intervention. Procedures that require breaching the zona pellucida—the protective outer shell of the embryo—are implicated in increasing the risk of monozygotic twinning.
Assisted Hatching and Biopsy
Assisted hatching (AH), a technique used to create a small opening, allows the inner cell mass to partially herniate and constrict, mechanically forcing a cellular division. Similar manipulation occurs during an embryo biopsy for preimplantation genetic testing (PGT). This procedure involves pulling a portion of the trophectoderm through a hole in the zona pellucida, subjecting the embryo to mechanical stress and the risk of cellular disruption.
Embryo Transfer Stress
The mechanical action of the embryo transfer itself is also considered a potential stressor. The process of drawing the delicate blastocyst into a narrow catheter and then expelling it into the uterine cavity creates hydrodynamic forces and physical pressure. This momentary mechanical compression could contribute to the destabilization of the inner cell mass, encouraging the separation that leads to identical twins.
The Role of the Culture Environment
Factors within the laboratory environment, distinct from physical manipulation, also play a significant role in increasing the risk of embryonic splitting. Extended culture time is one of the most consistently reported associations, as embryos are now routinely grown for five or six days to reach the blastocyst stage before transfer. This extended period of in vitro development, compared to traditional three-day culture, has been linked to a higher rate of monozygotic twinning. The theory suggests that prolonged exposure to an artificial environment stresses the embryo’s regulatory mechanisms, predisposing the inner cell mass to spontaneous division.
The specific chemical composition of the culture media may also contribute to this phenomenon by altering the embryo’s internal environment. Culture media contains necessary nutrients, growth factors, and buffering agents, but this artificial fluid can still induce cellular stress due to its non-physiological nature. Variations in media components, such as nutrient levels, pH, or high protein concentrations, are believed to disrupt cell-to-cell signaling and organization. This environmental stress can lead to cellular disorganization within the inner cell mass, resulting in spontaneous splitting.
Clinical Strategies to Reduce Twinning Risk
Recognizing the elevated risk of both identical and fraternal twins in IVF, clinics have implemented strategies to mitigate the chances of multiple gestations. The most impactful strategy for reducing the overall risk of multiple births is the practice of elective Single Embryo Transfer (SET). While SET cannot eliminate the chance of identical twinning, it removes the risk of fraternal twins, which are caused by transferring two separate embryos.
Clinics are also becoming more judicious about the application of invasive micromanipulation techniques. Procedures like Assisted Hatching are increasingly reserved for specific patient groups where a clear benefit is expected, rather than being used routinely. Ongoing research focuses on refining the artificial culture environment by optimizing the culture media composition. The goal is to develop media that more closely mimics the natural conditions of the fallopian tube and uterus, minimizing developmental stress and potentially lowering the incidence of spontaneous monozygotic splitting.