Introduction:
Chances of having twins are becoming increasingly common in the last few decades. A recent study regarding births worldwide found that the twinning rate has increased threefold since the 1980s globally, with around 1.6 million twins being born every year. Researchers consider that one of the most essential explanations for this increase is the coincident rise in fertility treatments including in vitro fertilization (IVF).
What Is In Vitro Fertilization (IVF)?
IVF is one of the most widely known assisted reproductive technologies. IVF utilizes a combination of medicines and surgical techniques to help sperm fertilize an ovum and also help the fertilized egg implant in the uterus. IVF consists of many steps that take several months to finish. IVF sometimes works on the first try. However, many people require more than one round of IVF to get pregnant. It increases the chances of pregnancy in couples with fertility problems, but there are several limitations also.
What Is the Process of IVF Treatment?
The IVF treatment starts with taking fertility medications for several months, which assist the ovaries in growing mature eggs prepared for fertilization. This step is called ovulation induction. This may require regular ultrasounds or blood tests for measuring hormone levels and keeping track of ovum production. Once the ovaries have cultivated enough mature eggs, the doctor will extract the eggs from the ovaries, which is called egg retrieval, a minor surgical procedure.
After this step, in a lab, eggs are mixed with sperm cells. This is called insemination. In this process, the eggs and sperm are reserved in a particular container for fertilization. For sperm with lower motility, the doctor may inject it directly into the eggs to facilitate fertilization. As the eggs fertilize and they divide and become embryos, the doctor puts them under monitoring.
After three to five days of egg retrieval, one or more embryos are placed into the uterus, which is called embryo transfer. Pregnancy happens if any of these embryos implant themself into the uterine lining. The doctor may prescribe pills or daily injections of a hormone called progesterone for eight to ten weeks after the embryo transfer. Progesterone facilitates and supports embryo survival in the uterus.
What Is Twin Pregnancy in IVF?
Fraternal twins occur during IVF when the doctor successfully transfers two embryos, and they develop into two fetuses. Identical twins occur during IVF when a successfully implanted embryo divides and develops into two fetuses. Fraternal twins are more common compared to identical twins in IVF treatment. This is because the spontaneous splitting of an embryo is a rare event, happening only two or three percent of the time from an embryo after IVF treatment. Therefore, there are more chances of having twins from two embryos with IVF. Identical twins are also known as paternal or monozygotic twins.
Pregnancy with IVF twins varies from pregnancy with a single IVF baby as it is deemed higher risk, with six times more likely to be linked with preterm delivery (birth of a baby before 37 weeks of gestation). There are many risks linked with preterm delivery, such as learning disabilities and cerebral palsy, high blood pressure and diabetes during pregnancy, higher risk of miscarriage.
However, it is estimated that only six in ten twins will be born before 37 weeks. Hence there is a 40 percent chance that twin pregnancy in IVF will not happen.
Are There More Chances of Conceiving Twins in IVF Than Natural Conception?
The number of naturally conceived twins in existence is more than those born with IVF treatment because natural conceptions are more common than IVF conceptions. Researchers evaluate that fertility treatments account for even less than two percent of all births in the US currently. One study surveying multiple birth rates in the US assessed that only one-third of all twin births are because of fertility treatments. In addition, IVF accounts for around 17 percent of all twin births.
However, regarding the percentage of chance of twin pregnancy, the rate of twin births via IVF is higher compared to the rate of naturally conceived twins. IVF twins most often occur from the transfer of multiple embryos.
What Are the Factors Influencing the Chances of Having Twins via IVF?
Many factors can impact the chances of having twins with IVF, such as the followings:
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Age of the woman.
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If the embryos are fresh or frozen.
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Whether using own eggs or donor eggs.
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Embryos grow to the blastocyst stage (embryo of 5 days old) before transfer.
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Assisted hatching.
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Use of intracytoplasmic sperm injection.
Why There Are More Chances of Splitting of Embryo in IVF?
Many theories explain the twin monozygotic pregnancies associated with IVF that occur by splitting a single transferred embryo, but there is not much evidence to prove them. One theory suggests that ovarian stimulation favors the production of eggs that are more likely to create embryos tending to split.
Another fascinating theory focuses on the culture media used to grow embryos. This culture media likely makes the outer membrane of the zona pellucida embryo become harder, interfering with natural embryonic division. At the same time, Others speculate that the culture media differs from the environment of a female’s reproductive tract. These discrepancies include the lack of growth factors or cytokines and higher levels of free radicals. This may lead to changes to the embryo, making it more likely to twin.
However, there is also the possibility that during IVF, handling of the embryos damages the outer membrane covering slightly, making it more prone to splitting.
Conclusion:
According to many studies, there are increased incidences of twin pregnancies in IVF. Twin pregnancies are linked with a higher risk of morbidity and mortality in mother and fetus compared with a singleton pregnancy, and the twin pregnancy incidence has increased by 76 percent in the past three decades. Twin pregnancies after IVF are mostly dizygotic, but there is a raised incidence of monozygotic twins around two to 12 times the incidence of 0.4 percent. In addition, monozygotic twins pose a higher threat of complications and poor outcomes than dizygotic twins.
