From conception to birth and beyond, twins carry a certain mystique with them. Let's demystify the fun genetic two-for-one that are fraternal twins by comparing them to their identical counterparts and getting to the bottom of what makes them so special.
Fraternal vs Identical Twins: The Major Differences
Fraternal twins occur when a woman hyperovulates or releases two eggs during ovulation instead of one. Separate eggs are then fertilized by separate sperm. Fraternal twins are not DNA clones, they each have their own DNA. They are no more alike than any other set of siblings; other than the fact that they share a birth date (most of the time at least).
In comparison, identical (monozygotic) twins are born from the same split fertilized egg and are always the same gender as well as DNA clones.
These are some of our favorite interesting facts that are unique to fraternal twins:
- Fraternal twins occur more often than identical twins. They occur in 20 out of every 1,000 births, while identical twins occur in 3 out of every 1,000 births.
- Chromosomes from the father's sperm determine the gender of the twins (XX for a girl, XY for a boy), therefore the twins can be both boys, girls, or one of each.
- Fraternal twins can have different fathers. This happens when a woman hyperovulates and one partner fertilizes an egg and then a different partner fertilizes another egg even up to days later. This phenomenon is called superfecundation.
- Fraternal twins are basically siblings that share the same uterus. While the fetuses gestate together, they each have their own amniotic sac and their own placenta.
- Fraternal twins share about 50% of the same DNA which is the same amount of DNA they would share with any of their siblings. Whereas, identical twins share the exact same DNA.
- Fraternal twins may or may not look alike. This depends on the inherited genetics from their parents.
- Each twin will have its own blood type which is determined by genetics.
- Fingerprints of fraternal twins will be different, just as it would be with any of their other siblings.
- Paternal twins is another name for fraternal twins.
Fraternal Twins Have Two-to-One Odds
You can calculate the odds of having twins but there's always some uncertainty. In 2016, the U.S. twin birth rate was 33.9 per 1000 births, way up from 18.9 per 1000 in 1980. Much of the increase is due to fertility therapies such as in-vitro fertilization (IVF), which is used by couples to increase the chances of becoming pregnant.
The hormone stimulations also increase the chance of releasing multiple eggs. That can mean multiple fertilizations and multiple births. These twins are always fraternal. However, IVF also increases the chance that one fertilized egg will split into two — and thus you'll have identical twins.
Fraternal Twins Skip a Generation
Identical twins are never hereditary. So if your parents had identical twins, you have as much chance of conceiving identical twins as anyone else in the world does. However, with fraternal twins, there is a genetic predisposition to hyperovulation.
These are some of the interesting aspects of hyperovulation as it relates to fraternal twins:
- If you hyperovulate, this makes it more likely you will have fraternal twins.
- It seems that twinning skips a generation because the gene doesn't express itself in men. So while women in the family may hyperovulate and are more likely to have twins, sons merely pass the gene on.
- Being predisposed to hyperovulation doesn't mean you'll have twins, it just increases the chances.
- A woman who's a fraternal twin or who has twin siblings is two-and-a-half times more likely to have twins herself.
- You have a higher probability of twins if you're of African descent and a lower probability if you're of Asian descent.
- Older women and women who have already given birth are more likely to carry fraternal twins.
Being a Fraternal Twin Doesn't Ensure a Close Bond
Fraternal twins may be extremely close - even sharing a secret language or an inexplicably close bond. However, don't count on it just because they're twins. Like anyone else in the family, they may fight, argue, and be as close or not close as they are with any other sibling in the family. Regardless, experts generally agree that it's important to let twins determine their attachment needs at their own pace.
Fraternal Twins May Not Share Milestones
It's important to remember that fraternal twins are two different people who happen to have been born at the same time. It would not be unusual if their milestone markers were different. It's not inconceivable that one twin will walk at 9 months, while another waits until their one-year birthday.
Success Strategies for Managing 'Twindom'
While your twins will have an undeniable bond, you should guide them to grow into confident separate people.
There are a variety of ways you can facilitate this:
- As your twins grow, be wary of treating two little personalities like a single entity.
- Celebrate each twin's triumphs and skill masteries. Fraternals don't always hit milestones in tandem.
- Give each twin "alone time" with their parents. Fraternals are not a single unit and each one needs individual parent bonding.
- Encourage individuality as much as possible, even (or especially) if they look alike.
All Twins Are Double the Fun
Fraternal twins are a twofer. They may exhibit all the behaviors of indistinguishable intuitively linked carbon copies you expect from identical twins. Or they may be as chill and independent as their other non-twin siblings. Either way, they're blessed with a built-in buddy for life.
