How is it possible? Could it indicate that a diagnosis of the couple’s infertility must be required before starting an assisted procreation process?
A recent study published in Fertility and Sterility evaluated the rate of twin pregnancies – both monozygotic (identical twins) and dizygotic (fraternal twins) – after fresh or frozen single embryo transfer.
The practice of single embryo transfer is being widely adopted to reduce twin and multiple pregnancies, to thus try to avoid the risks of multiple pregnancies for both mother and child.
In any case, although controversy remains about whether single embryo transfer completely resolves the increase in twin pregnancies, there is no doubt that reducing it is very beneficial for both parties involved.
How is it possible that using IVF and single embryo transfer, dizygotic twin pregnancies (fraternal twins) can be obtained, because if the embryo transferred divides in two, the twins produced would be monozygotic (identical twins)?
To assess this, the aforementioned article used data obtained between 2010 and 2013 on pregnancies resulting from in vitro fertilization (IVF) in which only a single fresh or frozen embryo had been transferred. The study reported the outcomes of a total of 32,600 cycles. There were 15,143 pregnancies, of which 14,888 (98.3%) were singleton and 226 were twins (1.7%).
Although most twin pregnancies following single embryo transfer were monozygotic, one in five (0.31%) were dizygotic.
In view of these results, the question immediately arises: how is it possible that using IVF and single embryo transfer, dizygotic twin pregnancies (fraternal twins) can be obtained, because if the embryo transferred divides in two, the twins produced would be monozygotic (identical twins)?
Dizygotic twin pregnancy can be explained by the existence of two pregnancies at the same time: a spontaneous pregnancy (natural ovulation + coitus with subsequent fertilization and implantation) and another pregnancy derived from the single embryo transferred in vitro.
According to Gonzalo Herranz (personal communication), ovulation for spontaneous pregnancy may come from a mature follicle that was not aspirated and that released the egg, which is then fertilized; or from an egg that has been “lost” at the time of egg retrieval, which occurs more often in obese women.
Must a diagnosis of the couple’s infertility be required before starting an assisted procreation process?
Infertility women were not correctly diagnosed
These findings could suggest that, if there are women who become pregnant naturally during the IVF process, it may be because their infertility was not correctly diagnosed.
IVF in cases of couples with residual spontaneous fertility is something that has been known for some time, so one could ask: should a diagnosis of the couple’s infertility not be required before starting an assisted procreation process. Could it be that spontaneous pregnancies occurring during IVF processes are not properly reported, because this could harm the assisted reproduction industry? These are things that are worth pondering.
*See article: “Biological status of human embryo”
Justo Aznar
Bioethics Observatory – Institute of Life Sciences
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