The Twin Gene: 7 Surprising Factors That Determine If Twins Are From Mom Or Dad’s Side

The question of whether twins run in the family is one of the most common and enduring mysteries for expectant parents. For decades, the answer has been shrouded in anecdotal evidence and old wives’ tales, but modern genetic research has provided a definitive, clear, and surprisingly one-sided answer. As of the latest scientific consensus in December 2025, the genetic predisposition for having twins overwhelmingly comes from one biological parent, and it depends entirely on the type of twins conceived.

The short, to-the-point answer is that the ability to conceive non-identical twins is determined by the mother's genetics, not the father's. This article will break down the crucial difference between the two main types of twins—identical and fraternal—and reveal the specific genes, hormones, and surprising external factors that truly dictate your chances of a multiple birth.

1. The Genetic Truth: Why Fraternal Twins Are Only From the Mother’s Side

The key to understanding the heredity of twins lies in differentiating between the two primary types: fraternal and identical. The genetic link you hear about only applies to one category: Fraternal Twins, also known as Dizygotic Twins.

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Fraternal twins occur when the mother releases two separate eggs during a single menstrual cycle, and both are fertilized by two different sperm. This phenomenon is called hyperovulation.

• The Mother’s Role: Only a woman can hyperovulate. Therefore, the genetic trait that causes a woman’s ovaries to release multiple eggs is inherited through her maternal lineage. If a woman's mother or grandmother had fraternal twins, the woman has an increased chance of having them herself.
• The Father’s Role: A father’s genetics do not influence his partner’s ability to hyperovulate. He can, however, carry the hyperovulation gene and pass it on to his daughter, who may then have fraternal twins. He cannot pass the gene to his son to increase the son's chance of fathering twins, because the son cannot ovulate.

In essence, the father is merely a carrier of the gene, while the mother is the active participant whose ovarian function determines the outcome. This is why the "twin gene" is often said to "skip a generation" on the father's side—it passes from his mother (who hyperovulated) to him (a carrier) to his daughter (who can now hyperovulate).

The Specific Hyperovulation Genes Discovered

Recent breakthroughs in genetic research have moved beyond the vague concept of a "twin gene" to identify the specific genetic variants responsible for hyperovulation. This discovery provides the strongest evidence yet for the mother-side inheritance of fraternal twins.

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In 2016, scientists identified two key genes that significantly increase a mother's likelihood of conceiving dizygotic twins:

• FSHB Gene (Follicle-Stimulating Hormone Beta): This gene affects the production of the Follicle-Stimulating Hormone (FSH), which is crucial for stimulating the growth of ovarian follicles. A variation in the FSHB gene can lead to lower levels of FSH, which paradoxically means the ovaries need to produce more follicles to achieve the necessary stimulation, resulting in the release of multiple eggs.
• SMAD3 Gene (SMAD Family Member 3): This gene plays a role in the signaling pathway of hormones that regulate ovarian response. A variant in SMAD3 is also linked to the increased chance of hyperovulation.

The presence of these genetic variants can increase the odds of having fraternal twins by up to 29%. This groundbreaking research solidifies the mother's genetics as the sole biological determinant for hereditary twinning.

2. The Random Event: Why Identical Twins Do Not Run in the Family

The rules of heredity completely change when discussing Identical Twins, also known as Monozygotic Twins.

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Identical twins are formed when a single egg is fertilized by a single sperm, and the resulting embryo splits into two separate embryos shortly after conception. This splitting event is a biological anomaly that occurs randomly across all populations worldwide.

• Consistent Rate: Monozygotic twinning occurs at a consistent rate of approximately 3 to 4 out of every 1,000 births globally.
• Lack of Heredity: For the vast majority of cases, having identical twins is considered a spontaneous, non-hereditary event. It is not influenced by the genetics of the mother, the father, or any family history of twins.

While a few rare families have reported a higher incidence of identical twins, the scientific community maintains that the mechanism for the embryo splitting is still largely unknown and not a predictable genetic trait like hyperovulation. Therefore, if you are hoping for identical twins, family history is essentially irrelevant.

3. Beyond Genetics: 5 Non-Hereditary Factors That Increase Twin Chances

While the mother’s genes are the only hereditary factor for fraternal twins, genetics is far from the only element at play. Several external and biological factors can significantly increase a woman's chances of conceiving twins, often by inducing or mimicking the effect of hyperovulation.

A. Maternal Age

A woman’s age is one of the most significant non-genetic factors. Women over the age of 30, and particularly those approaching 35 to 40, have a higher chance of conceiving fraternal twins. As a woman ages, her body naturally produces higher levels of Follicle-Stimulating Hormone (FSH) in an effort to encourage ovulation. This surge in FSH can sometimes lead to the release of two eggs instead of one.

B. Assisted Reproductive Technologies (ART)

Modern fertility treatments are a major contributor to the rising rate of multiple births. These methods directly bypass or enhance natural ovulation, creating a higher likelihood of twins:

• In Vitro Fertilization (IVF): Often, multiple embryos are transferred to the uterus to increase the chance of a successful pregnancy. If more than one implants, a multiple birth occurs.
• Ovulation-Inducing Drugs: Medications like Clomiphene Citrate or Gonadotropins are used to stimulate the ovaries to produce eggs. This stimulation often results in hyperovulation, leading to fraternal twins.

C. Race and Ethnicity

The rate of fraternal twinning varies dramatically by ethnic background, suggesting a widespread genetic influence within certain populations:

• Highest Rate: Women of Nigerian descent have the highest rate of fraternal twinning in the world.
• Lowest Rate: Women of Asian descent (e.g., Japanese or Chinese) have the lowest twinning rate.
• Intermediate Rate: Caucasian and African-American women fall in the middle.

D. Body Mass Index (BMI) and Height

Studies have shown that women with a higher Body Mass Index (BMI) (specifically those who are taller and heavier) have a slightly increased chance of having fraternal twins. This is theorized to be related to higher levels of estrogen or insulin-like growth factor (IGF), which can affect ovarian stimulation.

E. Prior Pregnancies

A woman who has had previous pregnancies, particularly if she has had five or more children, has a higher probability of conceiving fraternal twins in subsequent pregnancies. The body's reproductive system appears to become more "primed" for multiple ovulations with each successful pregnancy.

Summary of Twin Inheritance

To definitively answer the question of "Are twins from mom or dad?", it is essential to remember this distinction:

• Fraternal Twins (Dizygotic): Primarily from the mother’s side. The mother must possess the gene (like FSHB or SMAD3) for hyperovulation. The father can pass the gene to his daughter, but it does not affect his own chances.
• Identical Twins (Monozygotic): A random event that is generally not hereditary. Family history of identical twins is typically a coincidence.

Understanding these genetic and environmental factors provides a clear, modern perspective on the fascinating biology of multiple births, proving that while a father can certainly hope for twins, the ultimate genetic key rests almost entirely with the mother.