Are Identical Twins Always the Same Sex and Gender?

Although there are some rare exceptions, identical twins are almost always the same sex. Because identical twins occur when a single fertilized egg splits and forms two embryos, each embryo has the same chromosomes—including sex chromosomes (such as XX for females and XY for males). But can identical twins be different genders? The short answer is yes.

Fraternal twins, on the other hand, do not share the same DNA. As a result, they can be assigned different sexes at birth and may also identify as different genders. Read on to learn why identical twins are usually assigned the same sex, why they can still have different genders, and how identical twins can have different sexes.

Why Identical Twins Are Usually the Same Sex

The terms "identical" and "fraternal" don't describe what twins look like, but how they form. The different types of twins are classified by zygosity, or how many fertilized eggs (zygotes) develop into fetuses.

Because they develop from a single egg that is fertilized by a single sperm, identical twins share all chromosomal characteristics, including sex. Fraternal twins, on the other hand, may have different sex chromosomes.

Identical twin characteristics

Identical (monozygotic) twins come from a single fertilized egg that splits into two. Of 1,000 births, about 31 produce identical twins. Here are the characteristics of identical twin development:

• Form from a single fertilized egg that splits into two
• Fertilized by a single sperm that carries either the male or female chromosome
• Babies almost always the same sex
• Babies look very similar, with slight differences due to their position in the uterus or environmental factors after birth

Fraternal twin characteristics

Fraternal twins are dizygotic. The "di" in dizygotic means two, as in coming from two zygotes (fertilized eggs). Of all sets of twins, two-thirds are fraternal. Here are the features of fraternal twins:

• Form from two separate eggs
• Fertilized by two separate sperm that may or may not carry different sex chromosomes
• Babies may be born of the same sex or be of different sexes
• Babies may look similar or not very similar, like any siblings
  

Some fraternal twins have an even more complex genetic makeup, often as a result of fertility treatments like in vitro fertilization (IVF). These fraternal twins can also be of the same sex or opposite sexes.

• Eggs from two parents: Twins could be conceived in vitro with donor eggs, which could come from two different people. If they are fertilized by sperm from the same parent, the twins would be half-siblings.
• Sperm from two parents: When two eggs from the same parent are fertilized by sperm from two different parents, fraternal twins may result. This can happen with unassisted conception and with in vitro fertilization. In this case, the twins would be half-siblings.
• Eggs from two parents and sperm from two parents: Donor eggs from two different people could be fertilized by sperm from two different donors. In this case, the twins may have no shared DNA unless the sperm and egg donors were related. They would be similar to step-siblings.

Why Identical Twins Can Be Different Genders

Though a set of identical twins will typically be assigned the same sex at birth because they almost always share the same sex chromosomes, that sex may not align with their gender identity as they age.

Gender is a personal identity that exists on a spectrum, can change over a person's lifetime, and is something that a person defines for themselves. Sex, on the other hand, is assigned at birth based on the appearance of a baby's genitalia or sex chromosomes.

While sex assigned at birth often matches a person's gender (called cisgender), sometimes, for transgender, intersex, and gender nonbinary people, it does not. It's for this reason that identical twins can ultimately have different genders.

Can Identical Twins Ever Be Different Sexes?

In extremely rare cases, identical twins may be of different sexes. This can happen due to a genetic mutation or unusual fertilization scenarios.

Genetic mutation

There have been case reports of a genetic mutation in monozygotic pairs that cause otherwise identical twins to be of different sexes. In some instances, after a zygote splits, one twin loses a Y chromosome and develops as a female. The female twin would have Turner syndrome, characterized by short stature and lack of ovarian development.

Other unusual scenarios of opposite-sex identical twins begin when an egg has two female chromosomes, known as Klinefelter syndrome. When this egg is fertilized by sperm with a male chromosome and splits, one twin can have XX (female) sex chromosomes and the other can have XY (male) sex chromosomes.

Semi-identical twins

Even fewer cases of semi-identical (sesquizygotic) twins have been reported in medical literature. In these cases, a single egg is fertilized by two sperm. The egg then divides and develops into two babies, who may be of different sexes. These babies share somewhere between 50% of the same DNA (like dizygotic twins) and 100% (like monozygotic twins).

Half-identical (polar body) twins

Scientists have discovered that a phenomenon called polar body twinning is possible, likely as a result of fertility treatments. In this (still-theoretical) scenario, a single egg splits into two separate, but unequal, parts before fertilization. Each part is fertilized by a separate sperm, which could result in opposite-sex twins who share most, but not all, DNA.

When You Can Find Out the Sex of Twins

You can find out whether you are having twins as early as 6 or 7 weeks of pregnancy with an ultrasound. But when can you find out your twin babies' sex?

Around 10 weeks of pregnancy, you can have a blood test that can determine whether your twins are both biologically female or whether there are one or two male babies in the mix. By your 20-week ultrasound, your health care provider should be able to tell with a high degree of certainty the sex of each of your twins by visualizing genitalia on ultrasound.