What makes a twin a twin?

Excluding the often uncanny phenotypical resemblances, what makes a set of twins identical chiefly comes down to their common origin story—namely, when sperm met egg.

To understand this, let’s briefly break down the critical difference between fraternal and identical twins:

• Dizygotic twins is the scientific name for fraternal twin—”di” meaning “two,” originating from two separate zygotes. Fraternal twins or dizygotic twins are made when two eggs are dispatched in a single ovulatory period (hyperovulation), and subsequently fertilized by two different sperm.
• Monozygotic twins is the scientific name for identical twins. In this case, two future babies will develop from a single zygote—the fusion of one egg and one sperm.

As far as we’ve come in the field of genetics, scientists still aren’t sure what causes a single fertilized egg to grow two separate babies. Monozygotic twins are rare, resulting in just 3 to 4 of every 1,000 pregnancies.

Throw a single sperm cell and a single egg together, and you’d think that the same cache of genetic material would mean equivalent reserves of DNA. But by understanding the basic ABCs (or really ACGTs) of genetics, it’ll be easier to see why that’s often not the case.

Human Genetics Basics

Think of DNA, short for deoxyribonucleic acid, as the smallest possible unit of genetic material. DNA contains chemicals that provide micro-bytes of information to instruct your body on how to develop and behave. This information is “coded” in various permutations of four nucleotide bases:

• Adenine (A)
• Cytosine (C)
• Guanine (G)
• Thymine (T)

When these compounds latch together, they form the double helix structure we associate with DNA. Scaling up the ladder, genetics also involve:

• Gene – Genes are little units of DNA that contain different parcels of hereditary information to govern different aspects of biology. It’s estimated humans have between 20,000 and 25,000, with two versions of each (one from each parent). Less than 1% of your total genes are different than other humans’.
• Allele – Alleles are pairs of genes built to pass on the same type of information, but the genetic “instructions” are different. For instance, a child whose father has dark skin and whose mother has light skin might inherit two genes related to melanin production, but their encoded instructions would be different.
• Chromosomes – Chromosomes are our biggest bundles of genetic material, containing many genes, that live in the nucleus of every cell in your body. 23 are courtesy of the sperm, which match with 23 carried by the egg, making for 46 in total for most humans.

Let’s circle back to the tiniest increment of genetic material: DNA. Every human’s genome contains some 3,000,000,000 As, Cs, Gs, and Ts. That’s a lot of room for variation—even between babies that sprung from the same two sex cells.

Now the question is: how do those genetic differences arise in the first place?

What causes genetic differences between identical twins?

Only recently did scientific advancements make it possible to analyze such enormous stores of data to test out the myth of identical DNA.

In a study published in 2021, scientists analyzed the genomes from 381 monozygotic twins and discovered that most “identical” twins diverge by 5.2 early-development genetic mutations on average. Some of their twin subjects varied more, and others did in fact exhibit the same DNA.

In their analysis, the scientists traced those cases of DNA differences to two main phenomena.

Naturally Occurring Mutations

When babies develop in utero, they undergo a process of rapid cell division. Every human cell must house a copy of the organism’s DNA in order to function properly.

Enzymes are responsible for leading the charge when it comes to DNA replication—but it’s not uncommon for them to make a mistake. Sometimes, a mutation can occur before the monozygote splits in two; other times, it can arise after to affect one twin but not the other. In short, there are many opportunities for an enzyme to fumble the ball—a forgivable offense, given they’re doing twice the work for twins!

Epigenetics

As if the sheer volume of DNA wasn’t enough to cause the exquisitely complex genetic relationships between identical monozygotic twins, epigenetics complicate matters even further.

The epigenome consists of chemical markers that let genes know whether, how, and how much to express themselves. How genetic information at the level of DNA manifests depends on both extrinsic and intrinsic input, including:

• Nutrition
• Climate or environment
• Exposure to toxins
• Stress and trauma
• Relationships and communities
• Positive emotional experiences

One might think that identical twins growing up in the same household—with the same geographic environment, same family dynamics, and so on—would arrive at a pretty similar place developmentally.

But certain genes, like ones that carry genetic disorders, often express themselves differently in different twins—a phenomenon known as disease discordance. Moreover, epigenetics can control the proverbial on-off switch in certain genes over the course of a lifetime.

Pregnancy, Parenting, and Motherhood with Identical Twins

It’s one of the great miracles of nature that, even with the same biological instruction manuals for life, so much diversity exists within the human species, within families, and between siblings. If you’re expecting a twinning twosome, you may have already done plenty of research on the nitty-gritty of navigating pregnancy with multiple buns in the oven.

But what does it look like to raise siblings with the same (or almost the same) DNA?

Your Babies Might Differ in Size

If your babies are sharing a single placenta, it’s not uncommon for them to have different birth weights. In one study from MIT and Boston Children’s Hospital, researchers observed that certain areas of the placenta may have better access to oxygen than others. This can mean one fetus can enjoy more nourishment than the other in-utero, leading to a higher birth weight.

Unfortunately, this can’t be corrected during pregnancy. However, many identical twins are born perfectly healthy (and hungry to hit their first growth spurt when it’s time to nurse!).

You May Notice Behavioral Changes Instantly

Human infants begin to display behavioral traits out of the gate, even though many will change over time (that’s epigenetics for you!). Many parents report noticing conspicuous differences between twins early on, like differences in sleep patterns, baby food preferences, and even how prone to crying they are.It’s possible some of these differences start in-utero, particularly in identical twin pregnancies where twins occupy two separate placentae (dichorionic twins, which include 30% of pregnancies).

Yes—You May Have Trouble Telling Them Apart

If you’re one of the lucky parents with carbon-copy identical twins, it may be difficult to separate the two—and that’s totally normal. Even if your twins are born at different birth weights, one can quickly catch up to the other. So how do twin parents tell them apart?

You might try:

• Color coding – A tried and true organization hack, opting for certain hues for each baby is an easy way to keep track of who’s who. Think of this method as your training wheels: by visually separating the two, you’ll be able to hone in on the details and traits that set them apart (soon, you can choose clothes strictly based on whichever colors make them look cutest!).
• Using the nail polish method – Use a non-toxic nail varnish with a light hand to daub a color on one twin’s toenail. Just be sure to apply them on the feet, and not the hands (toes tend to be munched on much less frequently).
• Making personalized baby books – Many moms keep scrapbooks or new-mom diaries in their baby’s first or first few years. Why not use them to document the differences between your twins? You could even take a cue from this twin mom and add a page that shows each of your babies’ perfectly unique fingerprints.

An added bonus? When they’re older, there’s no better way to show them how much you love them than by presenting each with a special keepsake from their babyhood.

Get to Know Your Babies Individually with SneakPeek

Genetics is an incredibly intricate discipline, and, if you’re pregnant with twins, a fascinating subject to dive into.

If you’re curious to get to know your little one(s) even earlier, check out SneakPeek’s DNA-based Early Gender Blood Test. This at-home test kit lets you learn your baby’s gender with DNA-based, over 99% clinically proven accuracy¹—as early as 6 weeks into your pregnancy.

Our privacy-protected process is trusted by more than 1 million moms across the US: You’ll take your sample at home, mail it to SneakPeek labs, and we’ll triple-review your results and send them to you on the same day we receive them.

Pick up your test kit and learn why SneakPeek Test remains OBGYN’s #1 recommended at-home early gender test by visiting SneakPeek today.