Joni Kirk has 3-year-old boy/girl twins, and they are currently in the fighting stage. "Biting, kicking, hitting – it's a normal day in my house," says Kirk, of Moscow, Idaho, who adds that her twins are very sweet to each other when they aren't slugging it out. Kirk thinks the fights likely started in utero, but notes that they've gotten worse in the last year, as her son and daughter became more competitive over toys, friends and attention.

The worst fight so far? Kirk's daughter (who is the larger of the siblings) gave her brother a black eye in a fight at daycare. "Wow! That was embarrassing when I went to pick them up!" she says.

Where does all the down-and-dirty fighting come from? If you're a parent of twins, triplets or more who's starting to worry about the bites, bruises and insults your children inflict on each other, keep reading.

Elizabeth Lyons' twin sons have been squabbling since around 6 months of age, when baby Jack began stealing baby Henry's pacifier through the crib slats. This quickly progressed to hitting, tackling and biting, says Lyons, the author of Ready or Not ... There We Go! The REAL Experts' Guide to the Toddler Years with Twins (Finn-Phyllis Press, Inc., 2006). "Jack has a scar on his back that is the perfect image of Henry's 1-year-old bite," Lyons says. These days, her now 5-year-old boys are more likely to fight over who gets in the car first, or who gets the blue cup at breakfast.

"Twins fight. Period," Lyons says.

Content on this site is not a substitute for professional medical or healthcare advice, diagnosis or treatment, and may not be used for such purposes. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical question or condition. Reliance on information presented on this site is at your own risk. This site contains the opinions and views of other users. Given the interactive nature of this site, we cannot endorse, guarantee, or be responsible for the accuracy, efficacy, or veracity of any content generated by our users.