When it comes to twins, one of the most common questions expecting parents ask is whether twins run on the mother’s side or the father’s side of the family. The short answer is that while there are genetic factors that make having fraternal (dizygotic) twins more likely for some women, identical (monozygotic) twins occur randomly and are not hereditary. Let’s take a closer look at the factors that influence twinning rates for both types of twins.
What Causes Fraternal Twins?
Fraternal twins occur when two separate eggs are fertilized by two different sperm. This results in twins who share 50% of their DNA, just like regular siblings. Fraternal twins can be different genders and don’t necessarily look alike.
Research has shown that fraternal twinning does run in families. Having a family history of fraternal twins on the mother’s side makes a woman more likely to have fraternal twins herself. This is because the mother’s family genetic history can influence the levels of certain proteins involved in ovarian function and egg release.
Specifically, some women inherit a gene variation that makes them produce more of the follicle stimulating hormone (FSH). FSH is key for stimulating the ovaries to mature multiple eggs in a single menstrual cycle. Releasing more than one mature egg at a time increases the chance that more than one egg will get fertilized.
Key Facts About Fraternal Twins:
- Runs in families on the mother’s side
- Caused by genetics influencing multiple egg release
- More common in women of African descent
- Chance increases with maternal age and number of pregnancies
Women from African countries are more likely to carry the FSH gene variant and have fraternal twins. However, women from all ethnicities can inherit this trait. Research shows that while the gene variant is more common in African populations, African American women have similar rates of fraternal twins as European American women when age, number of pregnancies and socioeconomic status is accounted for.
In general, a woman’s chance of having fraternal twins increases with age and number of pregnancies. Older women are more likely to release multiple eggs, increasing the chance of multiple fertilizations. Women over 35 have significantly higher rates of fraternal twins, especially if they have already had multiple pregnancies.
So in summary, traits inherited from the maternal side can increase a woman’s chance of hyperovulation and conceiving fraternal twins, but it does not guarantee twins. Fraternal twinning rates are influenced by a combination of genetic factors, age, and reproductive history. Fathers do not directly contribute to hyperovulation.
What Causes Identical Twins?
Identical twins occur when one fertilized egg splits into two separate embryos very early in development. They share 100% of their DNA and are always the same sex.
Research shows that identical twins do not run in families. The occurrence of identical twins is random and is not influenced by genetics, age or other factors in women. Identical twinning rates are very consistent across the globe, with only small variations between countries.
Scientists are still researching what causes the early embryo to split into identical twins, but maternal genetics and family history have been ruled out as factors. Leading theories look at potential influences in the uterine environment after conception that could impact cell division and splitting. However, there are still no definitive answers.
Key Facts About Identical Twins:
- Do not run in families – occurrence is random
- Rates are consistent globally
- Caused by embryo splitting post-conception
- Maternal factors like age and ethnicity have no influence
The bottom line is that identical twinning can happen to any woman, regardless of age, ethnicity or family history. Since the cause remains unknown, there are no proven ways to increase or decrease the chance of having identical twins. There are often large surprises in families when identical twins are conceived out of the blue!
Can Twin History on the Father’s Side Influence Twinning Chances?
Since conceiving twins relies heavily on factors within the mother’s body, such as releasing multiple eggs or embryo splitting, you may be wondering if a family history of twins on the father’s side has any influence.
Research has definitively shown that a family history of twins in the father’s family does not affect the chances of having fraternal or identical twins.
The father’s genes do not directly control ovarian function and egg release. For fraternal twins, only the mother’s hyperovulation trait is relevant. For identical twins, paternal genes play no role in the post-conception splitting of embryos.
So while having twins on the father’s side may indicate a shared family environment or lifestyle factors, it does not appear to be genetic. There are no special traits passed down from fathers that increase twinning rates.
The one exception where paternal genes could have an indirect influence is in sperm production. There is some emerging research showing potential links between high sperm counts and conceiving fraternal twins. If the father’s biology contributes to higher sperm counts, this could increase the odds that two sperm fertilize two eggs from the mother. However, this link is still considered weak and unproven at this time.
Can Fertility Treatments Increase the Chance of Twins?
The use of fertility drugs and assisted reproductive technology (ART) has significantly increased twin birth rates, especially over the last few decades. Procedures like ovulation induction, artificial insemination and in vitro fertilization (IVF) all heighten the chance of twins.
Fertility drugs work by stimulating the ovaries to mature and release multiple eggs. Just like natural hyperovulation, releasing more than one egg can lead to conceiving fraternal twins. Ovarian stimulation cycles aim for 2-3 mature eggs, which cuts the twin risk but does not eliminate it.
In IVF, there is also the option to intentionally transfer more than one embryo into the uterus to increase success rates. This dramatically raises twin rates, with more than 1 in 4 IVF births resulting in twins. Guidelines have tightened on allowing single embryo transfers for most patients to minimize multiple pregnancies.
Since identical twins arise from a random embryo split out of the doctor’s control, fertility treatments do not increase the chance of identical twins. However, treatments can make fraternal twins much more likely. Women undergoing fertility interventions have fraternal twin rates of 10-15%, compared to the natural chance of about 1-4%.
What Factors Do Not Influence Having Twins?
Over the years, many myths and misconceptions have popped up around ways to conceive twins. Here are some common examples of factors that do NOT influence twinning rates:
- Ethnicity – As discussed above, ethnicity only influences fraternal twinning due to differences in genetic traits for hyperovulation. When controlling for maternal age and number of pregnancies, twinning rates are similar across ethnic groups.
- Increased Sperm Count – Some believe high sperm counts at conception lead to twins, but most research has disproven this theory. Exceptionally high counts through sperm donations may show a slight effect.
- Breastfeeding – Despite claims that nursing or not nursing after a birth alters hormone levels to favor twins, no scientific evidence supports this.
- Birth Control Pills – Going off birth control does not somehow “boost” chances of twins in future pregnancies. The hormones do not persist to alter fertility long-term.
- Diet – Eating yams or dairy is not proven to increase hyperovulation or twin probability.
- Family Genes – As explained already, male genes do not directly cause hyperovulation or twin embryo splitting.
In all cases, claims around these influences lack rigorous data demonstrating a causal effect. Mothers of twins may understandably try to point to reasons after the fact, but the best scientific research discounts these factors impacting the likelihood of conceiving twins.
What Are the Odds of Having Twins?
Now that we’ve covered the essential factors that do and do not contribute to having fraternal and identical twins, what are the actual probabilities involved? Let’s look at the statistics:
Overall Twin Birth Rate
- Total: About 3% of babies born in the U.S. are twins.
- Breakdown:
- Identical twins – 1/250 births (0.4%)
- Fraternal twins – 1/30 births (3.2%)
Chances of Having Twins by Maternal Age
| Maternal Age | Chance of Twins |
|---|---|
| 20-24 | 1 in 143 |
| 25-29 | 1 in 86 |
| 30-34 | 1 in 63 |
| 35-39 | 1 in 42 |
| 40-44 | 1 in 21 |
| 45-49 | 1 in 17 |
As shown, a woman’s chance of twins increases steadily with age due to the rising rates of hyperovulation as women near menopause. At age 45-49, women are over 10 times more likely to have twins versus age 20-24.
Chances of Twins With Fertility Assistance
| Procedure | Twin Rate |
|---|---|
| Natural conception | 1 in 30 births |
| Ovulation induction | 1 in 10 births |
| IVF without embryo transfer limits | 1 in 4 births |
As noted previously, assisted reproduction heightens twin probabilities due to clinical efforts to release multiple eggs and transfer multiple embryos. Rates jumped from the 1970s to mid 2000s but have declined again with improved techniques and elective single embryo transfers.
While ethnicity, diet, and other popular myths do not impact odds of conceiving twins, a woman’s age, number of prior births, use of fertility treatment, and inheriting the maternal hyperovulation trait do significantly increase the chances. Couples should be aware of these likelihoods if aiming to avoid or achieve a twin pregnancy.
Conclusion
In summary, fraternal twins are influenced by maternal genetics and run in families on the mother’s side. Traits like hyperovulation that cause double egg release can be passed down from mothers to daughters. In contrast, identical twins arise purely by chance from random embryo splitting in very early development. The father’s family history does not directly impact the odds of having twins, since conceiving twins relies on factors within the mother’s body. While many myths persist around ways to increase twin chances, the best scientific evidence points to maternal age, reproductive history, fertility assistance and inheriting genetic tendencies for hyperovulation as the most important factors that raise the likelihood of conceiving both identical and fraternal twins.
