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Twins that Don’t Look Alike: What You Need to Know About Fraternals

Twins that Don’t Look Alike: What You Need to Know About Fraternals

Twins. We often see and hear about them in books, TV shows, and movies. But how do they come to be? Why do some twins look alike? How come there some twins that don’t look alike? Are they really that rare? Read on to find out.

What Are Twins?

Twins are basically two babies born in the same pregnancy. There are basically two kinds of twins: fraternal and identical twins. To more easily identify the type: identical twins are twins ho look alike and that fraternal twins are twins that don’t look alike.

Identical Twins

Identical twins are also called monozygotic twins. That’s because identical twins come from the same egg during fertilization. At some point during the early stages of conception, this single egg splits itself into two separate individuals. What’s fascinating is that since these two came from the same egg, they share the same genes and will always be the same sex. That’s why we call them identical twins. They look alike because they are identical on a genetic level.

It is important to note, however, identical twins might not look completely alike. While they may be identical on a genetic level, their development inside the womb and other factors growing up might contribute to a slight difference in their looks.

Fraternal Twins

In contrast, fraternal twins come from the fertilization of two separate eggs in the same pregnancy. Doctors and scientists also call them dizygotic twins for this reason.

Since these twins come from two individual eggs, they only share half of their genes. This means that while they might look similar, they are twins that don’t look alike. This also means that they might have different genders.

Identical Twins or Fraternal Twins?

Besides the eye test of telling if they are twins that look alike or twins that don’t look alike, there are other scientific methods.

A DNA test is a surefire way to determine if twins are fraternal or identical. Identical twins will always have the same genes. The placenta can also indicate if twins are fraternal or identical.

An early ultrasound scan or examination of the placenta after childbirth can help. This method is not 100% accurate as opposed to a DNA test. It’s also always easier to tell if they have different genders because this automatically makes them fraternal twins.

Causes

No one really knows what causes identical twins. Every woman has the same chance of conceiving identical twins. Identical twins aren’t a hereditary trait so they can’t be passed on to family members down the line. Fraternal twins, however, have some factors that raise the odds of conceiving them.

Factors

There are multiple factors that contribute to a higher probability of fraternal twin birth. Here are some:

Mother’s Age

Women who are older than 35 are more likely to release two or more eggs during a single menstrual cycle compared to younger women.

Previous Pregnancies

A woman’s probability of giving birth to twins increases with the number of previous pregnancies she has.

Heredity

A woman who is a fraternal twin, has siblings who are fraternal twins, or has already given birth to fraternal twins has higher odds of giving birth to twins.

Race

African women have produced more twins while Asian women rank the lowest.

Assisted Reproductive Techniques

Procedures that stimulate the ovaries (usually with fertility drugs) tend to produce several eggs during ovulation. It’s also important to note that conceiving twins are more likely nowadays, because of more women relying on these reproductive techniques for conception.

Key Takeaways

In summary, there are two kinds of twins: fraternal and identical.

We can pretty much differentiate them by twins that look alike (identical) and twins that don’t look alike (fraternal).

On a genetic level, identical twins have the same genes and fraternal ones only have half. We don’t really know what causes identical twins but we do know that fraternal twins can be hereditary and are more likely depending on certain factors.

Learn more about other Conception topics here.

Ovulation Calculator

Ovulation Calculator

Tracking your period cycle, determines your most fertile days and increases your chance of conceiving or applying for birth control.

Ovulation Calculator

Tracking your period cycle, determines your most fertile days and increases your chance of conceiving or applying for birth control.

Ovulation Calculator

Cycle Length

(days)

28

Period Duration

(days)

7

Hello Health Group does not provide medical advice, diagnosis or treatment.

Sources

Fraternal Twins, https://www.genome.gov/genetics-glossary/Fraternal-Twins Accessed March 18, 2021

Identical Twins, https://www.genome.gov/genetics-glossary/identical-twins Accessed March 18, 2021

Pregnant with Twins https://www.nhs.uk/pregnancy/finding-out/pregnant-with-twins/. Accessed March 18, 2021

The Difference Between Identical and Fraternal Twins, https://www.healthychildren.org/English/family-life/family-dynamics/Pages/The-Difference-Between-Identical-and-Fraternal-Twins.aspx Accessed March 18, 2021

Twins – Identical and Fraternal, https://www.betterhealth.vic.gov.au/health/ConditionsAndTreatments/twins-identical-and-fraternal Accessed March 18, 2021

Twins, Triplets and Other Multiples, https://www.womenshealth.gov/pregnancy/youre-pregnant-now-what/twins-triplets-and-other-multiples Accessed March 18, 2021

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Written by Sky Abundo Updated Jun 10
Medically reviewed by Jobelle Ann Dela Cruz Bigalbal, M.D.
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