The chance of having twins is influenced by whether they are identical or non-identical. The likelihood of having identical twins is relatively consistent across populations, whereas the chance of having non-identical twins varies considerably based on several factors.
Identical Twins: A Consistent Rarity
Identical twins occur when a single fertilized egg splits into two separate embryos. This event is considered random and is not significantly influenced by genetics or family history.
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Frequency: The approximate probability of having identical twins is about 1 in 250 births.
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Factors: Unlike fraternal twins, the chances of conceiving identical twins are not thought to be hereditary or related to ethnicity.
Non-Identical Twins: Influenced by Various Factors
Non-identical (fraternal) twins develop from two separate eggs fertilized by two different sperm. Several factors influence the likelihood of this occurring.
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Frequency: The frequency of non-identical twins varies significantly.
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Factors influencing the chance of non-identical twins:
- Ethnicity: The rate is highest among Nigerians and lowest among Japanese populations.
- Family History: Non-identical twins do run in families. If a woman has a family history of fraternal twins (particularly on her mother's side), she is more likely to have them herself.
- Age: Women over the age of 30 are more likely to conceive fraternal twins due to hormonal changes.
- Fertility Treatments: Assisted reproductive technologies, such as IVF, often result in multiple eggs being released and fertilized, increasing the likelihood of multiple births, including twins.
Summary
While the occurrence of identical twins remains relatively stable at about 1 in 250 births, the probability of having non-identical twins is highly variable and influenced by ethnicity, family history, maternal age, and fertility treatments.
