In cycles of in vitro fertilization, it is routine to stimulate the female’s ovaries so that many eggs will be obtained. Many eggs are needed in order to provide a good chance of success because most human eggs do not become viable embryos.
In many cases, we obtain more than one good embryo to consider for transfer back to the uterus of the female parent or a gestational carrier. The availability of multiple good embryos allows us to grade them and then choose the best among them for transfer. It also brings on a common dilemma – how many embryos to transfer. Some patients prefer the transfer of multiple embryos in order to increase their chance of at least one embryo resulting in a live birth.
A critical statistic in IVF is the implantation rate. The implantation rate is the proportion of transferred embryos that actually implant in the uterine lining and continue to develop so that a fetal heartbeat is detected. It is therefore the ratio of fetal hearts to transferred embryos.
In IVF, implantation rates are highly dependent on the age of the female that provided the eggs, as egg quality declines rapidly with age. Embryos derived from eggs from a 40-year-old woman might have an implantation rate of 10-15%, while those from a 30-year-old might have an implantation rate of 40-50%.
The implantation rates can be increased by genetically screening the embryos before transfer or by transferring the embryos into a uterine environment that has not been subjected to ovarian stimulation, such as with frozen embryos, oocyte donation cycles, or gestational carrier cycles. Under ideal conditions, some embryo transfers can have implantation rates of 70-80% or even greater.
The greater the implantation rate, the less need to transfer multiple embryos and the greater the risk of twin pregnancy will be when multiple embryos are transferred.
Is twin pregnancy really a bad thing?
Many couples I see, long frustrated with their struggles with infertility, are ecstatic with the thought of having twins. The birth of healthy twins is an enormous contrast against years of failure to achieve pregnancy or birth.
But twins are not always born healthy. While any pregnancy has risks, many risks are greatly increased with twin birth. While singleton deliveries are associated with a 9% risk of low birthweight, a 2% risk of very low birthweight, and a 14% risk of prematurity, these same risks increase to 57%, 9%, and 65% with twin birth. With triplet birth, these risks are 96%, 34%, and 97%, respectively, according to a 2006 report by the Centers of Disease Control.
Triplets are relevant to the discussion because some patients actually want three or more embryos transferred, and also because about 1% of implanting embryos will split into identical twins, so that even when “only” two embryos are transferred, triplet pregnancy can still occur.
The risks of low birthweight and prematurity are not to be taken lightly. These in turn incur numerous increased risks, which in cold medical terms include prolonged hospitalization, intellectual disabilities, cerebral palsy, respiratory distress, visual issues, digestive disorders, hearing loss, jaundice, bleeding in the brain, inability to regulate body temperature, neonatal death, and increased risks in adulthood such as adult death from heart disease.
There are also increased risks to the birth mother when multiple infants are delivered. The cost of a multiple pregnancy is also greatly increased when compared to a singleton, so that the average cost of twin pregnancy is about fourfold that of a singleton, and the cost of a triplet is about tenfold that of a single.
The urge to transfer two embryos is sometimes based on the subtle fear that embryos not transferred are somehow wasted. Embryos not transferred can be frozen, and modern techniques with frozen embryos can yield success rates that equal or even exceed those with fresh embryos.
Multiple studies have found the infants from frozen embryo cycles to be healthier than their fresh counterparts. This might be because fresh embryos are typically returned to a uterine environment that was exposed to ovarian stimulation and resulting hormone levels far exceeding natural conditions, and this altered uterine environment might affect the embryo’s ability to implant and grow properly. Oocyte donation and gestational carrier cycles also avoid such exposure, whether using fresh or frozen embryos.
The intended parents often have the freedom to choose whether to transfer one or two embryos. But they should not make this decision lightly or naively. They should make this decision only after carefully considering the risks to infants and mother, their desire for multiple children, their finances, their clinic’s performance with frozen embryos, and the expected implantation rate.