One twin can be taller than the other due to differences in the womb and environmental factors, despite sharing nearly identical genetics.
The Role of Genetics and Environment
While genetics account for 60-80% of height, the remaining 20-40% is influenced by environmental factors. This means that even identical twins, who share almost the same DNA, can have height differences.
Differences in the Womb
The primary reason for height discrepancies in twins often stems from unequal access to nutrients and space within the womb.
- Unequal Placental Sharing: Twins who share a placenta may not receive nutrients equally. One twin might get a larger blood supply and more nutrients than the other, leading to accelerated growth in the better-nourished twin.
- Twin-Twin Transfusion Syndrome (TTTS): In monochorionic pregnancies (where twins share a placenta), TTTS can occur, causing one twin to receive too much blood and the other too little. This can affect their growth trajectories and lead to height differences.
- Crowding: Simple space constraints can also play a role. One twin might have more room to grow within the uterus than the other.
Post-Birth Environmental Factors
Even after birth, environmental factors can contribute to height variations:
- Nutrition: Differences in diet and nutrient absorption can impact growth. Even subtle differences in feeding habits or a twin's ability to process nutrients can contribute to stature variations.
- Illness: If one twin experiences more frequent or severe illnesses, their growth might be temporarily or permanently stunted.
- Activity Level: While twins often share similar activity levels, small differences in physical activity or participation in sports could subtly affect bone development and growth.
Summary
Height differences in twins are typically a result of a combination of factors, primarily unequal resource allocation during gestation and environmental variations post-birth. Even with nearly identical genes, these factors can lead to noticeable differences in stature.
