Why transfer only one IVF embryo but pass twins?

Although very rare, it is still possible to transfer an embryo during IVF resulting in identical twins after embryo cleavage.

Doctor Pham Thi My Tu, Reproductive Support Center of Tam Anh General Hospital in Ho Chi Minh City, said that currently, IVF centers prioritize transferring an embryo into the mother's uterus to avoid multiple pregnancies and risks in pregnancy. multiple pregnancies such as stillbirth, miscarriage, premature birth, low birth weight, intrauterine growth retardation, increased maternal and child perinatal mortality. However, even with an embryo transfer, a mother can still have twins or triplets due to embryo cleavage.

The case of twins after in vitro fertilization at IVFTA. Photo: Provided by the hospital

During IVF, an embryologist takes a sperm and fertilizes an egg to form an embryo. During development, an embryo can split into two (or more) separate embryos, which develop into two (or more) fetuses in the uterus. This phenomenon is twins (or triplets...) of the same egg, the children will be genetically identical and have the same sex.

Depending on the stage of blastocyst separation, embryonic structures such as the lamina propria, amniotic membrane, umbilical cord, and placenta may be fused or separated. There will be 3 cases:

First, if the separation of the embryo occurs soon after fertilization to the embryo on day 4, it will form twins with the same egg, 2 placenta and 2 amniotic chambers. This case accounts for about 20-30% of natural fetuses.

Second, if the separation occurs at the blastocyst stage to day 8 after fertilization, it will form twins with the same egg sharing the same placenta but with 2 amniotic chambers, accounting for about 70-75% of cases in natural pregnancies.

Third, if the separation occurs after 1-2 weeks, it will form monochorionic, monoamniotic twins, accounting for about 1-2% of cases in natural pregnancies.

According to doctor My Tu, the rate of twins with the same egg according to statistics around the world is about 4/1,000. Embryo cleavage often results in a twin pregnancy. However, the embryo can also split into triplets, resulting in identical twins, similar to identical twins. The process of embryo separation is difficult to assess and observe with the naked eye. In a small number of cases, "embryo split" can be clearly seen at the blastocyst stage.

Studies have shown that the frequency of twin pregnancies after transferring a day 5 embryo is higher than that of a day 3 embryo. Therefore, the European Society of Human Reproduction and Endocrinology (ESHRE) as well as the American Society for Reproductive Medicine ( ASRM) recommends transferring only one good embryo on day 5 or day 6 in women with good prognosis, under 35 years of age.

Doctor Tu said, in addition to the phenomenon of identical twins, fraternal twins can also occur when transferring an embryo. Then one embryo fertilizes from the transferred embryo, the other is a natural pregnancy. This could be explained by spontaneous ovulation of oocytes remaining after aspiration, which increases the likelihood of spontaneous pregnancy. Patients should be informed of the possibility of spontaneous pregnancy during the assisted reproductive cycle.

According to doctor My Tu, pregnancy with twins for infertile mothers is a joy but comes with concerns related to the mother's health and the safety of the fetus. In particular, identical twins also face the risk of twin-twin transfusion syndrome, with a frequency of 5-20% in monochorionic twins and the risk of stillbirth up to 95%. Complex knotted umbilical cord is more likely to occur in single amniotic sac, one placenta twins because the two umbilical cords are in one amniotic sac.

In some cases where the mother's health is not guaranteed to carry twins, the doctor will consider reducing the pregnancy. The plan to reduce pregnancy will help prevent complications of twin pregnancy such as premature birth, premature rupture of membranes, low birth weight, perinatal diseases, and protect the mother from the risk of hypertension, diabetes. , preeclampsia.

To reduce pregnancy, doctors will selectively keep the strongest fetus, aspiration one(or) of the remaining fetus(s). The technique of reducing multiple pregnancy is usually performed at 7 weeks of pregnancy. This technique is not too complicated, but requires experience, ingenuity and high precision in manipulation, especially in cases of multiple pregnancies with accompanying fetal pathologies. Therefore, women carrying twins or multiple pregnancies with indications for pregnancy reduction should be performed at large hospitals and medical facilities with full modern equipment and experienced and skilled obstetricians.