In the U.S., approximately 32.6 twins are born for every 1,000 births. Do you have twins in your family and are wondering whether you may have inherited this trait as well? Well, the answer isn’t definite.
Twins can occur from various biological processes, which may or may not be hereditary. We’ve outlined the different scenarios leading to having twins and determined if they are or aren’t due to genetic factors.
The U.S. Department of Health and Human Services has estimated that, since 1980, the twin birth rate has increased by 75 percent. It is, therefore, legitimate to wonder what the reason is behind this rise. Is this lucky event likely to happen to any of us?
This growth is the result of a few phenomenons. First, the number of women needing fertility treatments to conceive rose considerably. As this type of pregnancy carries a higher chance of having twins, it affected the twin rate.
Today, women also tend to have children later in life, positively impacting the conception of twins. We’ll expand on this further.
Before determining whether having twins is hereditary, let’s discuss the main underlying factor: whether they’re identical or fraternal.
They’re also called monozygotic twins—or MZ. Regular fecundation occurs when a female egg gets fertilized by a sperm. If, however, it divides into two embryos from the beginning of the fetus’s development, it may result in twins.
These babies are, as the name indicates, identical and often difficult to tell apart, even for parents. This is because they carry exactly the same genes.
Throughout the world, this isn’t a prevalent situation and typically happens every three or four per 1,000 births. Although it can sometimes seem that it runs within families, science hasn’t found that genetic factors play a role. To this day, the cause of the early cell division remains a mystery.
This type of twins is much more common than the previous type, about twice as frequent. They’re also referred to as dizygotic twins or DZ.
The biology behind this process differs from the one that happens for identical twins. During a regular menstrual cycle, only one egg is typically released.
Fraternal twins occur when, instead of one egg, two or more of them are released at the same time. This is also called hyperovulation. Each one can then be inseminated by a different sperm, which creates non-identical—but similar-looking—baby twins.
These twins don’t have the exact same genes, so you could, therefore, even have a boy and a girl. They’ll even have their own amniotic sac and placenta.
When it happens naturally, DZ are more likely to be hereditary, compared to MZ. Keep in mind, however, that only the mother’s genetics are considered. Even if the father carries the “twin gene”, he won’t impact the outcome of having twins, one way or another.
A woman with a sister or a mother who had fraternal twins is, however, twice as likely to have DZ herself. Indeed, she may have received the hyperovulation gene from her mother.
A study led by a scientist at Vrije University in Amsterdam looked for genetic variations implicated in giving birth to twins. They scanned people’s DNA using a technique called genome-wide association.
The study showed evidence that the presence of FSHB and SMAD3 genes in mothers increases the chance of conceiving twins. FSHB is the gene that prompts the production of FSH, which we’ll further discuss.
A woman can also hyperovulate without genetics being involved. Here are other causes that can lead to giving birth to fraternal twins:
- Diet and ethnicity
- Age and previous children
- Fertility drugs
Body composition or diet alone can cause a woman to produce two eggs or more per cycle. Precisely, fat increases the levels of estrogen in the body. This causes overstimulation of the ovary, with the potential release of several eggs at a time.
Generally speaking, obese women—with a BMI over 30—are more likely to have twins.
Diet and Ethnicity
What we eat can also be a contributor to having twins. Yams, for instance, are high in estrogen content, which could play a role in twin occurrences. It’s also a significant ingredient in the Nigerian diet.
Among the western population, it was also found that vegan women had a lower chance of conceiving twins than those who consumed dairy products. Indeed, non-vegans showed higher levels of the Insulin Growth Factor—or IGF—which positively affects twin rate.
Finally, studies have shown that African Americans are also more likely to have twins. On the other hand, Asians and Native-Americans have a much lower twinning rate.
In addition to body weight and composition, height also matters. Fraternal twins occur more frequently in tall and large women, rather than petite ones.
Although the exact reason is still uncertain, it’s thought to be related to nutrition. The twin rate decreased during World War II when food was scarce.
Women who get pregnant while breastfeeding can also increase the twin rate. Results showed 11.4 percent of nursing mothers had twins, compared to 1.1 percent during normal pregnancies.
Age and Previous Children
Women over 30 years old tend to produce more Follicle Stimulating Hormone or FSH in response to lower fertility. This stimulates the ovaries, and when producing several eggs within a cycle, it can lead to a higher chance of conceiving twins.
On top of that, having previous children also amplifies the odds of having twin babies. Let’s take the example of a 35 to 40-year-old mother with four kids. She is three times more likely to have twins, compared to a 20-year-old woman without children.
In Vitro Fertilization, or IVF
This procedure is often used when natural reproduction isn’t possible or is difficult. Most IVFs consist of artificially inseminating a fertilized egg into the mother’s womb.
Most of the time, several eggs are inseminated to increase the success rate. Almost 10 percent of these pregnancies result in having twins with women younger than 35 years old. This operation is human-made, and of course, doesn’t involve genetics.
Some medications can lead to having twins. Gonadotropins are fertility drugs containing FSH and/or Luteinizing Hormone.
Research showed that 30 percent of pregnancies under gonadotropin medications resulted in having twins. Five percent of them were even triplets.
Do Twins Skip a Generation?
Twins skipping a generation is a common saying that’s only partly true. Here is the idea behind it. A mother could pass the hyperovulation gene to her son, and his daughter may then inherit this marker.
The mother’s granddaughter could, therefore, have fraternal twins. In a way, the father’s generation would have been skipped.
The myth is only partially true because if the father has a sister, she could herself have inherited from the hyperovulation gene. She could then have conceived twin babies within the father’s generation.
Some future mothers try and hope to conceive twins, while others wish for a normal pregnancy with a single baby. Either way, this isn’t a controllable outcome. Statistics still indicate that having twins remains an exception to the norm, especially when it comes to identical twins.
Yet, genetic factors can come into play and increase your odds of conceiving fraternal twins. Falling into the categories with a higher twin rate—age, ethnicity, lifestyle, etc.—can also move the scale in your favor.