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Which side is responsible for twins?

Having twins is an exciting yet challenging experience for many parents. When expecting twins, one of the common questions that come up is “which side of the family is responsible for twins?” Let’s take a closer look at the factors that influence twinning and which parent is more likely to pass on the genes for twins.

What causes twins?

Twins occur when two babies develop from a single fertilized egg. There are two types of twins:

  • Identical twins – Occur when one fertilized egg splits into two embryos. Identical twins have exactly the same genetic blueprint and are always the same sex.
  • Fraternal twins – Occur when two separate eggs are fertilized by two different sperm. Fraternal twins share about 50% of their DNA, just like non-twin siblings. Fraternal twins can be the same or different sex.

Having twins runs in families. However, the exact genes responsible haven’t been identified yet. Researchers believe that the mothers’ age, ethnicity, diet, and use of fertility drugs or procedures like IVF can influence chances of twins.

Does the mother or father determine twins?

Both sides of the family influence the chances of having twins but the maternal side is more influential. Some key points:

  • Having a twin sibling, cousin, aunt, or other close relative with twins significantly increases your odds of twins.
  • Fraternal twins are more common in women of African descent and in older mothers over 30.
  • Use of fertility treatments like clomiphene citrate tablets boosts the chance of twins by 10%.

Based on large twin studies, researchers found that moms are 2.5 times more likely to have identical twins than dads. For fraternal twins, moms have a 1.7 times higher chance of passing on twinning genes than dads.

Why mothers have higher odds of twins

There are a few key biological reasons why twinning rates are higher for mothers:

  • Hyperovulation – Women release more than one egg per monthly cycle, allowing two eggs to be fertilized.
  • Hormones – Higher levels of follicle stimulating hormone (FSH) can spur ovaries to release multiple eggs.
  • Age – Older mothers ovulate more, increasing the odds of releasing multiple eggs.
  • Genetics – Moms contribute mitochondrial DNA that may influence twinning.

In contrast, fathers don’t ovulate or produce hormones that affect eggs. The father’s age and family history have minimal impact on twin chances compared to the mother’s side.

Role of paternal genes

Although maternal genes play a greater role, fathers do contribute genetic factors that can influence twinning rates such as:

  • Y chromosome – Carries genes that help determine how many eggs are released.
  • MSH5 gene – Linked to hyperovulation and twinning rates.
  • Race – African ancestry boosts odds of twins.

The father’s family history can also give clues about twin potential. Having twin siblings or relatives on dad’s side suggests a higher predisposition.

How genetics influence different twin types

Twin Type Genetic Influence
Identical Maternal genes affect egg splitting. No direct paternal influence.
Fraternal Maternal and paternal genes influence hyperovulation. Maternal genes have greater effect.

For identical twins, moms pass on a tendency for her eggs to split after fertilization. Dads don’t directly influence identical twinning. For fraternal twins, both parents contribute genes that boost the release of multiple eggs or multiple sperm increasing the chances of twins.

Other factors that increase twin likelihood

While genes play a key role, several other factors can boost your chances of having twins:

  • Ethnicity – African-American and Nigerian heritage have higher twinning rates.
  • tall stature– Taller women are more likely to conceive twins.
  • Weight – Higher BMI is linked with elevated FSH and estrogen.
  • Parity – Having previous pregnancies may increase chances.

Environmental elements like nutrition and exposure to growth factors like IGF also influence twinning. A vegan diet and high dairy intake may increase twin likelihood.

Role of fertility treatments

Use of fertility drugs and procedures significantly increases the odds of multiples like twins. Some key effects include:

  • Ovulation-boosting tablets like clomiphene citrate cause release of multiple eggs.
  • Injectable gonadotropins FSH and hCG can lead to hyperovulation.
  • IVF involves surgically implanting multiple embryos.
  • IUI and ICSI boosts the number of viable eggs and sperm.

Without fertility treatments, natural twinning rates are about 1 in 250 pregnancies. With fertility interventions, this rate can spike to 1 in 40 or higher depending on number of embryos transferred.

Genetic testing for twin potential

Currently, there are no definitive genetic tests to check your risk of having twins. However, looking at family history on both sides provides useful clues:

  • Review extended family tree for fraternal twins, especially on mother’s side.
  • Ask about use of fertility treatments in family.
  • Consider parents’ age, weight, height as risk factors.
  • Note any cases of identical twins in mother’s family.

In the future, specific genetic variants associated with twinning are likely to be discovered through genome sequencing. But for now, family history remains the best indicator of twin potential.

Role of paternal age

Unlike maternal age, paternal age over 35 appears to have minimal effect on increasing twin likelihood. Some key points about dads and twinning:

  • Older fathers have slightly higher sperm mutations affecting embryo splitting.
  • Advanced paternal age alters hormonal balance, but not hyperovulation.
  • IVF studies show minimal impact of paternal age on twin rates.

While an older father’s DNA changes may have health impacts, these mutations don’t significantly influence the chances of twins. Maternal age is more influential on ovarian function.

Can medications influence twin chances?

Some prescription medications have been associated with a temporary increased chance of twins. However, effects are mild compared to fertility treatments. Possible effects include:

  • Sulfa antibiotics linked to elevated FSH levels.
  • Antibiotics like metronidazole associated with potential egg splitting impact.
  • Acne drug isotretinoin may temporarily boost twin risk.
  • Antiepileptic drugs increase hyperovulation in some cases.

However, these drug-related effects are mild and inconclusive. More research is needed. The most substantial medical influence remains specialized fertility drugs and treatments.

Lifestyle factors affecting conceiving twins

Certain diet and lifestyle factors may slightly sway the odds of twins. However, most effects are minor or inconclusive so far. Some potential influences include:

  • High dairy intake linked to elevated IGF which may affect ovulation.
  • Eating more yams, cassava root may slightly increase twin rates in some populations.
  • Taller or heavier women have higher estrogen and FSH levels.
  • Smoking and alcohol have been associated with higher twin birth rate in some studies.

While diet, height, and other lifestyle elements may contribute, genetics remains the most prominent factor for natural twinning outside of fertility treatments.

Role of maternal BMI and weight

Higher maternal BMI and being overweight has been associated with increased odds of twins. Some key findings on maternal weight include:

  • Higher BMI linked to elevated FSH and estrogen which control ovulation.
  • Obese women have up to 4 times higher chance of conceiving twins compared to normal BMI.
  • Heavier women produce more insulin growth factor (IGF) which may affect eggs.

However, the effects of higher BMI seem most pronounced in women undergoing IVF. For natural conception, a mother’s family history and age have a greater influence on twinning frequency.

Can a twin gene be inherited from father?

While less common, fathers can also pass down genetic factors increasing the odds of twins. Some ways dads may influence twin likelihood:

  • Carrying twin gene variants that boost sperm production and increase chances of fertilizing multiple eggs.
  • Having % African or Native American ancestry linked with elevated twin rates.
  • MSH5 gene mutations passed down via Y chromosome that affect ovulation.
  • Having twin siblings through father’s side of family.

However, for fraternal twinning the maternal influence is believed to be up to 5 times stronger. Fathers have minimal impact on identical twin rates.

Does paternal ethnicity matter for twins?

Research shows fathers of African, Native American, and Latino descent have slightly elevated rates of fathering twins compared to Caucasian men. Proposed reasons include:

  • Men with African ancestry have higher testosterone levels which may boost sperm count.
  • Native American men have high rates of variant FSHR gene that affects fertility.
  • Latino men have higher rates of twinning regardless of female partner’s ethnicity.

So while paternal ethnicity can contribute, it’s a minor factor compared to the mother’s family history, use of fertility treatments, age, and ethnic background.

Takeaway on which parent determines twins

In summary, while both parents genetically contribute, the maternal side plays a greater role in twin likelihood for these key reasons:

  • Maternal age affects hormone levels that control ovulation and egg release.
  • Mothers contribute more genetic material to offspring including mitochondrial DNA.
  • Maternal family history of twins is a strong indicator of risk.
  • Use of fertility treatments directly impacts the mother’s ovaries.

Fathers can still pass down genes that influence twin chances, but to a lesser degree. Understanding which side of the family carries twinning traits can help assess your risk, especially when considering fertility treatments.


Having twins is a complex interplay between genetic and environmental influences. While both parents contribute genes, research clearly shows that the maternal side is more influential in determining twin potential. From factors like hyperovulation, egg splitting, advanced age, and use of fertility treatments, mothers play a greater role in twin likelihood compared to fathers. Looking at detailed family history on the mother’s side provides the most telling insights into risks. Understanding these maternal and paternal influences can help parents plan and make informed decisions about multiple pregnancies.