In Vitro Fertilization (IVF) is a medical procedure that assists in achieving pregnancy. An observed phenomenon in IVF is the increased occurrence of identical, or monozygotic, twin pregnancies compared to natural conception. This elevated rate has prompted research into the IVF process’s contributing factors. Understanding these influences provides clarity on why a single fertilized egg might unexpectedly develop into two embryos.
Understanding Twin Development
Twins form through two primary pathways. Fraternal (dizygotic) twins develop when two separate eggs are fertilized by different sperm, making them genetically distinct, similar to any other siblings. Identical (monozygotic) twins originate from a single fertilized egg that divides into two separate embryos. This division occurs in the earliest stages of development, so identical twins share nearly identical genetic material.
In natural conceptions, identical twins occur at about 0.4% of births. With IVF procedures, the rate of monozygotic twinning significantly increases, ranging from 0.9% to 2.35%.
Embryo Manipulation Techniques
Several laboratory procedures performed on embryos during IVF influence the likelihood of identical twinning. Assisted Hatching (AH) is one technique where a small opening is made in the outer shell of the embryo, known as the zona pellucida. This opening is intended to help the embryo emerge from its shell and implant in the uterine wall. This manipulation, by weakening the zona pellucida, could make the embryo more prone to splitting.
The physical stress from creating this opening or the embryo hatching through a hardened zona pellucida might cause the inner cell mass to divide. This division then leads to the formation of two distinct embryonic structures. While the exact mechanism is still being investigated, this mechanical intervention during assisted hatching contributes to the elevated rates of monozygotic twinning in IVF.
The Influence of Culture Conditions
The laboratory environment where embryos are grown, known as culture conditions, also influences identical twinning. The culture media’s composition can impact embryo development and induce stress. If the media lacks specific growth factors or contains high levels of free radicals, it might affect the embryo’s integrity and predispose it to splitting.
Additionally, the duration of embryo culture, particularly extended culture to the blastocyst stage (typically Day 5 or 6 of development), has been associated with an increased risk of identical twins. Prolonged exposure to an artificial environment, even under optimal conditions, can lead to subtle changes in the embryo’s cellular structure or function. This extended culture period may contribute to the hardening of the zona pellucida, potentially leading to abnormal hatching and subsequent embryo division. This suggests that while extended culture is beneficial for selecting viable embryos, it may also inadvertently increase the chance of monozygotic twinning.
Mechanical Stress During Transfer
The physical act of transferring the embryo into the uterus during an IVF cycle is another factor considered a potential contributor to identical twinning. The process involves drawing the embryo into a thin catheter, which is then gently guided through the cervix and into the uterine cavity. The forces involved, such as the gentle suction and pressure exerted by the catheter, could potentially induce minor trauma or stress on the delicate embryonic cells.
This mechanical stress, though subtle, might be sufficient to disrupt the developing embryo, leading to an unplanned division of cells. While the precise mechanisms are not fully understood, the transfer procedure represents a moment of external interaction with the embryo that could trigger the splitting event.