Her face swollen and bleeding, Megan was taken to an emergency room and stitched up. No one realized she had sustained a severe concussion until three weeks later, when a player ran into her during another game and she fell to the ground with a seizure. Doctors say she experienced what is known as second impact syndrome, a sequence of events in which a child or teenager sustains a hit before a concussion fully heals, which can cause the brain to bleed or swell, even if the second impact is a moderate one.

“In retrospect, we hadn’t thought as much about her brain as we clearly should have,’’ said her mother, Barbara Wirtz, a nurse in East Lansing, Mich. “She doesn’t have lingering problems like some players do. We were very lucky in that regard. But the reality is if she continues to play, it could happen again.’’

New research in the latest issue of The American Journal of Sports Medicine shows that athletes such as Megan may be particularly susceptible to the damaging effects of a concussion. The research found that younger athletes and those who are female show more symptoms and take longer to recover from a concussion than athletes who are male or older.

More than 1.6 million Americans sustain a sports-related concussion every year, and a growing number occur among high school and college athletes. According to federal statistics, more than 150,000 teenage athletes sustained concussions on the playing field from 2001-05, though that figure accounts for only those who were taken to emergency rooms, so the true number, experts say, is most likely much higher.

While researchers have known that girls run a greater risk of sustaining concussions than boys playing the same sports, the new study is among the first to look at the effect of both age and sex on a range of symptoms.

The findings suggest that because of anatomical differences that make them more vulnerable, female athletes, and younger athletes in particular, may need to be managed more cautiously after a concussion, said Tracey Covassin, an associate professor of kinesiology at Michigan State University and the lead author of the report.

“Parents need to understand that if their daughter has a concussion, that they may potentially take longer to recover from that concussion than their son who is a football player,’’ she said.

Over two years, Covassin and her colleagues followed a large group of high school and college athletes from California, Michigan, Louisiana, and Tennessee. At the start of the study, the athletes were given baseline tests that looked at memory and other cognitive skills. Those who sustained concussions in the next two years, about 300 in all, were given three different post-concussion tests commonly used in professional sports.

Overall, after concussions, the high school athletes performed comparatively worse for their age than older college athletes on measures of verbal and visual memory, and female athletes reported more symptoms and showed greater declines in visual memory compared with their male counterparts.

The cognitive impairments were also more likely to persist over time in younger athletes, lasting an average of 10-21 days after a concussion in high school students. That is about two to three times as long as the five- to seven-day period of persistent symptoms that has been documented in college athletes.

Researchers say that younger athletes could be at greater risk of damage from a concussion because their brains are not fully developed. There is also some evidence that young women may suffer more symptoms than young men because of higher estrogen levels, which may exacerbate brain injury, as well as greater rates of blood flow and higher metabolic needs in the brain, which may make symptoms more pronounced.

But, says Mark Hyman, author of “Until It Hurts: America’s Obsession With Youth Sports and How It Harms Our Kids,’’ girls may also just be more willing than boys to admit to injury and to seek treatment.

“We don’t expect girls to be indestructible, as we do boys,’’ who may be more likely to play through pain to avoid being sidelined in their sport, Hyman said. “Attitudes are changing about that. But not fast enough.’’

The findings also highlight the dangers of treating children and teenagers as “miniature adults,’’ he added. “The brain and head of a small child are disproportionately large for the rest of the body,’’ he said. “The result is that their heads are not as steady on their shoulders. When they take a big hit in a football game or are slammed with an elbow in a soccer game, their brains move inside their skulls. That’s when concussions occur.’’

As for Megan, she continues to play soccer, but under strict rules. Her parents attend every game, and are more alert to potential problems.

“I think we’re better at assessing the symptoms of a concussion now,’’ Barbara Wirtz said. “We’re a little more watchful and demanding that coaches don’t keep her in if there’s any question at all that she got knocked around.’’