Age Ivf Genetics as a Factor of Twin Pregnancy

Will I Have Twins? The Effects of Genetics, Age, and In Vitro Fertilization on the Conception of Twins

There’s nothing quite like the whirlwind of feelings that hit a woman and her partner during that prenatal visit when two little fetuses are first seen on the ultrasound monitor. Surprise, confusion, and above all, wonder are all normal things to experience. Although twin pregnancies are on the rise, there is still no doubt that twins are, somehow, special. How did this little miracle come to be?

Before diving into what influences your chances of having twins, it is useful to understand the difference between identical twins and fraternal twins.

Identical twins, known scientifically as monozygotic, or MZ twins, are the product of a single egg, fertilized by a single sperm, which split in half after fertilization, resulting in two babies with the exact same DNA. As the name indicates, it’s often difficult to tell the little ones apart, and identical twins are always the same sex. The rate of identical twins births is remarkably consistent throughout the world at about 1 in 250 births, and that rate has been steady since accurate records have been kept.(1) What causes a fertilized egg to split is not currently known.

Fraternal twins, known scientifically as dizygotic, or DZ twins, are the result of two eggs, released into the womb at the same time, and fertilized by two sperm. Fraternal twins are genetically no more related than any other pair of siblings, and may or may not be the same sex.(1) Fraternal twins, especially a same-sex pair, may look very different from each other, or may look enough alike to be mistaken for identical twins.

There are certain factors that increase the chances of having twins. The most well-known of these factors are genetics, maternal age, and participation in IVF treatment.

Many people know, or have heard of, a family where a set of identical twins each had a set of fraternal twins, one of whom gave birth to a set of identical twins…well, you get the idea. Such stories often make it into the media, and prompt us to wonder “do twins run in the family?” An article written by Nick Martin and Grant Montgomery, two senior research fellows at the Queensland Institute of Medical Research who specialize in twin studies, seems to answer the question:

“There is no doubt that twinning seems to run in some families. Closer investigation reveals that this is mainly confined to nonidentical, or fraternal (dizygotic – DZ) twins, although there are a few rare families with extraordinary numbers of identical (monozygotic – MZ) twins.”(2)

Since it is the tendency to release more than one egg at a time that runs in families, it is the number of sets of twins in the woman’s – not the man’s – family that increases the odds of that couple having twins.(2)

Another factor that greatly increases the chances of having fraternal – although not identical – twins, is the age of the mother at conception. Some sources report that a woman’s chances of having twins are about 3 per cent if she is under age 30, 4 per cent between 30 and 34, and 5 per cent between 35-39,(3) with her chances jumping to 17 per cent after age 40!(4) Martin and Montgomery state that “a woman who gives birth at 37 is four times more likely to have fraternal twins than at age 18.”(2)

It has been observed that as a woman ages, she naturally begins to produce more follicle-stimulating hormone, or FSH, which is what causes her ovaries to release eggs. Greater amounts of the hormone lead to a higher chance of more eggs being released at once. This means that although a woman in her 30s will likely take a longer time to conceive than a younger woman, her chances of carrying twins when she does conceive are significantly higher.(2) Since multiple eggs released at once result in fraternal twins, it is only the likelihood of having fraternal twins that increases.

The large increase in the number of twins born in recent years can be mostly attributed to in vitro fertilization (IVF) becoming more common. In fact, although your chances of having twins naturally is about 1 in 89, your chances of having twins with IVF is about 1 in 32, or almost three times higher!(1)

During an IVF treatment, 2-3 embryos will often be transplanted into the woman’s uterus at a time, to increase the chances of success. Often more than one embryo successfully implants into the wall of the uterus, which is why the rates of twin pregnancies are so high. In fact, nearly a third of in vitro births involve twins or more.(5)

Since IVF usually involves the transfer of more than one embryo, with each embryo being the product of one egg and one sperm, it’s logical to think that IVF could only be responsible for an increase in the birth rates of fraternal twins. It seems, however, that IVF also causes a slight increase in the birth rate of identical twins as well.(6) Although the reasons that an embryo splits into two are still unknown, research into why IVF causes a rise in the rate of identical twinning may be beginning to shed a light on the mystery.

It has been found that, when embryos fertilized for IVF are transferred to the uterus 5 days after fertilization, there is a higher likelihood that the embryos will split and result in identical twins than when the embryos are implanted at 3 days after fertilization.(6) The cause is not known, but scientists are currently trying to discover if it is related to the environment the cells are nurtured in before implantation. Discovering exactly why the eggs are more likely to split into identical twins after a longer time in the petri dish may provide new information on how and why identical twins are formed!

Another, less mysterious reason that IVF results in more identical twin births is that, in cases where very few viable eggs can be harvested, the embryos that result will be split in the laboratory to provide more healthy embryos and a greater chance of conception. An embryo split in a lab results in identical twins just as an embryo that splits on its own does, so if both embryos successfully implant – voila! – identical twins.(6)

Although scientists are discovering more information every day about how and why twins are conceived, it doesn’t distract from the wonderful, miraculous experience of unexpectedly seeing those two tiny, perfect little forms on the ultrasound screen. Although two babies can be twice the trouble, they are also twice the fun! After all, they don’t call babies bundles of joy for nothing!






5. “The Gift Of Life” Margot Williams, in the New York Times