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Nine months ago, two lines showed up in your pregnancy test. Then, popped two babies – twins! Double the fun – or is it double trouble? But then, why aren’t you seeing double?? Aren’t twins supposed to look alike? Or be of the same gender? Find out all the answers to your questions about fraternal twins in this article!
What Are Fraternal Twins, and How Are They Formed?
To put it simply, fraternal twins are like any other siblings, except that they are birthed in the same pregnancy. They are dizygotic, meaning they are formed from two, different eggs fertilized by two, different sperm cells. Hence, they usually have separate amniotic sacs and placentae during pregnancy.
In some cases, the placentae fuse and appear to be a single placenta. Since they are formed from different eggs and sperms, uncommon circumstances may occur in which the twins have different fathers (superfecundation, in which they are called bi-paternal twins) or different gestational ages (superfetation). In an article in Everyday Health, superfecundation occurs in around 1 to 2 percent of fraternal twins.
Fraternal twins have diverse characteristics, just like how a regular set of siblings can differ or resemble each other. Some characteristics may overlap, but they generally look different from each other. They may have different hair and eye color, stature, and personality. They can have the same or different genders and gender orientations. Yet, sometimes, fraternal twins can look so much alike, they’re thought to be identical!
Do Fraternal Twins Have the Same Blood Type?
Fraternal twins share half of their genes, and hence may or may not share the same characteristics – including blood type. As stated earlier, they are like any other siblings. They can have the same blood type, but this is not always the case, so don’t be alarmed if your twins have different blood types!
How Common Are Fraternal Twins?
Fraternal twins are the most common type of twins, making up 75 percent of the twin population. In 2019, the Centers for Disease Control and Prevention (CDC), reported that twins occur in 33 out of 1000 live births. In an article in Twins Magazine, the rate of fraternal twinning was estimated to be 22.8 per thousand in the world, occurring more frequently than identical twins.
Fraternal twins were also noted to be more common among African women, occurring at 16 per thousand. All over the world, the highest twinning rate is found in southwest Nigeria, in a tribe called Yorubas. The tribe has 1 fraternal twin birth out of 22. Asians have the lowest rate, at 3 out of 1000 births. For Caucasians, the rate is about eight per thousand births.
Northern areas of the world also have greater rates of fraternal twins than those in the farther south.
What Causes Fraternal Twins?
The main cause of fraternal twins is hyperovulation, in which two or more eggs are released during ovulation. Genetics may play a role in this phenomenon; hence you are more likely to produce twins if you come from a family with twins in the first place. Interestingly, increased chances of twinning may also occur if it is your husband who holds the twin gene.
Menopause is usually regarded as the ending of a woman’s reproductive years, and advanced maternal age is usually met with skepticism. Reports of increased pregnancy risk and difficulty bearing a child at the outset are common. But who says that advanced maternal age doesn’t have its perks? The hormonal changes associated with menopause may actually cause hyperovulation. According to Healthline, women who are 35 years or older have increased chances of having twins.
Fertility treatments may also cause fraternal twinning. Intrauterine Insemination (IUI) uses ovulation-stimulating drugs like clomiphene, letrozole, and follicle-stimulating hormone. The use of these drugs may result in hyperovulation, and voila! You may just have fraternal twins on the way!
In InVitro Fertilization (IVF), on the other hand, depends on the number of embryos you wish to transfer. If more than one embryo is transferred and successfully implanted and developed, you get fraternal twins – or maybe more!
The US Department of Health & Human Services noted an increase in twin and triplet babies in the United States from 1980 to 2005. They attributed this increase to the rising number of women having babies after 30 years old, as well as to the increased frequency of fertility treatments.
Fraternal vs. Identical: What’s the Difference?
Whereas fraternal twins are dizygotic, identical twins are monozygotic. Identical twins form from a single fertilized egg that splits in two during embryonic development. This results in the development of two individuals. Also, the propensity to have identical twins is not hereditary. Identical twins are most likely to happen by chance, and everyone has equal chances of having twins! Their likelihood remains constant around the world, which is about 3 in 1000 births.
As stated earlier, fraternal twins often have separate placentae and amniotic sacs in utero. Identical twins, on the other hand, commonly share a single amniotic sac and placenta. A certain complication, Twin to Twin Transfusion Syndrome, may more likely be seen among identical twins due to the shared placenta.
Unlike fraternal twins who have diverse characteristics, identical twins share the same DNA and the same genes. Hence, they always look the same, have the same gender, and have the same blood type. They share the same eye color, hair color, and skin tone. Talk about a clone! Variation in environmental exposure may cause slight differences in the physical features of identical twins, but they could still easily be doppelgangers of each other!
Height and weight may vary for some identical twins, but perhaps what can really tell them apart are their fingerprints. The formation of fingerprints in utero is affected by both genetic and environmental factors. Hence, identical twins do not have the same fingerprints. They may have similarities, but when analyzed closely, not even identical twins can have the same, exact set of fingerprints. Differences may also be seen in their retinal scan.
What Are the Chances of Having Fraternal Twins?
The chances of having fraternal twins vary from person to person. As you may have realized, there are non-modifiable factors such as race and genetics that affect the outcome. But, good news! Additional factors may be agreeable to some tweaking in order to increase your chances of having fraternal twins!
You might find it interesting that taller height and an increased BMI can up the chances of having fraternal twins. While height is non-modifiable, weight can be modified. According to healthline.com, a body mass index (BMI) of 30 and above offers the best chances. Adipose tissue carries increased levels of estrogen, hence the better chance of having fraternal twins. But that’s not to say that being overweight or obese is a good thing! It can come with a range of complications, pregnant or not, so it is still prudent to maintain a normal BMI while you’re trying to conceive.
Consumption of dairy products has been shown to increase the rate of fraternal twinning. Cows secrete a hormone called insulin growth factor into their milk, which can affect human reproduction. So, don’t forget to stock up on your dairies!
Yams, on the other hand, are believed to contain phytoestrogen, a substance similar to estrogen and thought to induce multiple ovulations. The yam species Dioscorea rotunda was particularly seen in large amounts in the diet of the Yoruba tribe, which, as mentioned earlier, had the highest twinning rate in the world.
The chance of conceiving fraternal twins is also increased if you have had previous pregnancies. The more, the merrier, right?!
Another interesting fact is that July seems to be the most auspicious month for conceiving fraternal twins. It may be due to the length of daylight, which can affect the Follicle Stimulating Hormone. January has the least number of fraternal twins conceived.
So, there you have it! All of your questions about fraternal twins – answered!
List of some famous fraternal twins!
Who knows? Your twins might just be next!
- Mary-Kate and Ashley Olsen
This one’s a little tricky because most people can’t tell them apart! But the most famous – and possibly the wealthiest – twins in America are actually fraternal, not identical twins!
- Scarlett and Hunter Johansson
While Scarlett Johansson rocked Hollywood, her twin brother, Hunter, was more focused on politics, though he’d done a bit of acting himself. In 2008, he worked as Barack Obama’s campaign adviser. He also founded an environmental solar organization called Solar Responders.
- Vin Diesel and Paul Vincent
Paul Vincent, Vin Diesel’s fraternal twin, is a film editor. Wanna know his off-screen name? Vin Diesel is Mark Sinclair Vincent by birth name!
- Aaron and Angel Carter
Aaron Carter is more commonly associated with his older brother, Nick Carter, from the legendary band, Backstreet Boys. But Aaron also has a fraternal twin sister, Angel, who is a model!
- Ashton and Michael Kutcher
Ashton Kutcher’s fraternal twin, Michael, has cerebral palsy. He also underwent a heart transplant in his teens, which is probably why he chose to be an advocate for disability rights and organ donation.