The occurrence of identical twins after a single embryo transfer

Even after many years of being a doctor, it still brings me immense joy to announce to my patient that she is indeed pregnant following a successful infertility treatment. And sometimes, the happiness is multiplied, as there isn’t one, but two buns in the oven!

In general, it is widely known that the occurrence of multiple births is much higher following assisted reproductive technologies (ART) as compared to incidences of twins in natural or spontaneous pregnancies. This is directly related to the number of embryos transferred back to the uterus. In general, if 2 embryos are transferred back into the uterus, the chance of having twin pregnancies is around 20-30%. As we all know, twin pregnancies carries more risks to the expecting mother as well as the babies. The risks of miscarriage, bleeding in pregnancy, high blood pressure, diabetes in pregnancy, Placenta Praevias, Caesarean Sections are some of the significant obstetric risks. The babies are also at risk of preterm births, stillbirths and complications associated with preterm deliveries such as cerebral palsies.

Therefore, in many countries especially in the west, it is mandatory that the patients are informed of these risks before an embryo transfer. Some countries had restricted the number of embryos transferred to one. This move had significantly reduce the incidence of twins following an IVF and had reduced the economic burden associated with medical care of pregnancies associated with twins

However, in recent years, it had been reported that there is a higher incidence of identical twins following single embryo transfer.

While the incidence of fraternal twins following the transfer of two embryos is an easily understandable consequence of IVF, the causes for identical twins after ART remain speculative at best. However, there is some evidence that attributes the phenomenon to the micromanipulation techniques, length of culture and culture medium used during treatment, as well as the genetics of the parents.

Identical twins, also known as monozygotic twinning or MZT, occur when an egg and a sperm unite to form a single zygote that divides into two embryos or separate individuals. It is a rare phenomenon, which occurs in only about 1% of natural conceptions. However, in assisted conceptions, the occurrence of monozygotic twinning almost doubles.

The splitting of the zygote into separate embryos can occur at any time during the first 2 weeks after fertilisation. However, the timing of the split will influence the type of identical twin. For example, in around 1/3 of MZT cases where the split occurs within 72 hours of fertilisation, the result is two placentas and two amniotic sacs. In the remaining 2/3 of MZT cases, the splitting occurs 4-8 days after fertilisation, resulting in one placenta and two amniotic sacs. And in about 5% of MZT cases, where splitting occurs 8-13 days after fertilisation, the outcome is one placenta and one amniotic sac. In very rare cases, the outcome is conjoined twins.

Although the prospect of having twins is certainly exciting, couples must also understand that twin pregnancies are considered high risk. They can lead to various pregnancy complications with long-lasting and even permanent effects for the children. These include caesarean section, premature birth, low birth weight, Twin-Twin Transfusion Syndrome (TTTS), pre-eclampsia, gestational diabetes, fetal abnormalities and congenital abnomalies. In view of the complications that can arise from a twin pregnancy, do speak to your infertility consultant about the risks involved should you have any worries.