Sibling rivalry can apparently begin even before birth and researchers in London have remarkable footage of twins fighting for legroom in the womb.
The footage shows the legs of the smaller of the two fetuses extending into the space of the larger fetus, as if trying to push it back or kick at it, although it's not certain what is really going on between the fetuses.
"If you've got two fetuses in the womb, they can't possibly stay in their own space," Dr. Marjorie Greenfield, division chief of general obstetrics and gynecology at University Hospitals Case Medical Center in Cleveland, told ABCNews.com.
Greenfield is hesitant to call the movement fighting, but she still finds the video interesting.
"Twins have been kicking in wombs for millennia," said Greenfield. "Only now we're able to experience it like this."
The scientists used cinematic-MRI, which utilizes magnetic resonance imaging to string together moving pictures from inside the body. Unlike CAT scans, the procedure does not use radiation and is far safer for the fetus.
The video comes from a study aimed at diagnosing a rare but increasingly more common condition unique to identical twins. Called twin-to-twin transfusion syndrome, the condition causes the twins' blood flow to essentially become interconnected. One baby inevitably loses out to the other, receiving less blood, and stops growing. The other baby then grows dangerously fast, putting it at risk of cardiovascular complications and premature birth.
Doctors hope cinematic-MRI technology, which has been used on pregnant women before, will become a more common tool for identifying twin-to-twin transfusion syndrome as more innovations that fix the life threatening issue become available.