The frequency of monozygotic twins in IVF pregnancies is reported to vary between 1-2.5% in studies. In natural pregnancies, this rate is only 4 per thousand. Only live-born babies were taken into account when making the calculations. When early pregnancy losses are taken into account, the incidence may be higher. The reasons that increase the risk of identical twins in IVF treatments have been the subject of studies.
  • Embryo transfers performed at the blastocyst (5 or 6th day) stage are more common than embryo transfers performed at the cleavage stage (2nd and 3rd day). It is observed more frequently in cases with long protocol (pre-suppression with GnRH agonists).
  • Young female age (>35) may increase the risk.
  • Sperm fertilization techniques (IVF or ICSI) DO NOT increase the risk.
  • Assisted Hatching of the embryo shell with laser: There is no increase in risk in 2 meta-analyses conducted in recent years. Few studies have advocated an increased risk. Biopsy of the embryo for Pre-implantation Genetic Diagnosis (PGD) does not increase the risk.
There are 3 types of identical twins:
  1. Dichorionic diamniotic: There are two placentas (mate), two sacs. 20-25% of cases are in this group. It occurs when the egg fertilized with sperm divides into two 2-3 days after fertilization.
  2. Monochorionic diamniotic: There is one placenta (mate) and two sacs. 70-75% of cases are in this group. It occurs when the egg fertilized with sperm divides into two at the blastocyst stage (5-6 days after fertilization).
  3. Monochorionic monoamniotic: There is a placenta, a sac and 2 babies in the same sac. Approximately 1-2% of cases are in this group. It occurs when the egg fertilized by sperm divides into two within 7-14 days.
Multiple pregnancies, whether single or fraternal, have their own risks. Premature birth, preeclampsia, low birth weight, extreme nausea-vomiting, vanishing twin syndrome are among them. There are some other complications specific to identical twins. These:
  • Twin-to-twin transfusion syndrome (TTTS): When a single placenta cannot distribute maternal blood equally among babies, one baby gets more blood and the other less. It is seen with a frequency of 5-20%. If left untreated, it results in death with a probability of 95% for both babies.
  • Cord accidents: Especially when there are two babies in a single pouch, the cords of babies can become entangled and knotted, which can cause problems in the blood flow to the babies.

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