“Parents take it personally and wonder, oh my God, why would my kid ever do that?” says Robbye Fox, who runs parenting workshops in Kensington, Md.

As more teens get involved in sexting, parents’ worries about the trend are mounting even faster. Some teens circulate sexually explicit selfies or videos, or capture and forward screenshots from intimate Instagram photos or FaceTime video chats.

As upsetting as this topic can be, parents should navigate it carefully. Those who react by erupting in anger or trying to control their adolescents’ behavior online risk shutting down communication altogether, research shows.

It’s better to be proactive. That means talking with children as young as 9 about preserving their privacy online and coaching them on how to avoid becoming either a victim or an active participant in abusive sexting.

More than one in four teens under 18 have received sexts. Nearly 15% have sent them. This is happening more with the increasing use of smartphones, according to a 2018 review of 39 studies of a total of 110,380 teens. Some 12% have forwarded sexually explicit images without the subject’s consent, and 8.4% have been victims of such behavior. The review, published in JAMA Pediatrics, surveys studies conducted in the past decade.

Coerced sexting by aggressors who pressure or manipulate their victims can be harmful, triggering guilt, shame and embarrassment. While boys are sometimes bullied or shamed over sexting, girls are more likely to be victimized.

Sexting also appears to be a gateway to future sexual activity. Teens who have sent a sext are 32% more likely to have had intercourse a year later, according to a 2014 study led by Jeff R. Temple, a professor and researcher at the University of Texas Medical Branch in Galveston.

All the sexting doesn’t mean sexual activity among teens is also increasing, however. The proportion of high school students who have had intercourse has fallen to 40% in 2017 from 48% in 2007, according to the Centers for Disease Control and Prevention. And the proportion of 15-to-19-year-olds who have had oral sex with someone of the opposite sex has fallen among girls to about 46% in 2015 from 54% in 2002, and to 51% from 55% for boys in the same period, according to the National Center for Health Statistics.

Still, sexting does make teens’ preoccupation with sex more visible to adults.

Parents often blame themselves for failing to instill morals. They shouldn’t, says Ms. Fox, a college consultant and parenting educator who specializes in teens and technology at Parent Encouragement Program, a nonprofit. “It has nothing to do with whether your child is good or bad or how you raised them,” she says. Many children and teens simply don’t yet have the developmental ability to control their impulses, regulate their emotions or exercise sound judgment, she says.

“Sending a picture to the love of your life when you’re 13 years old seems like a great thing to do,” Ms. Fox says. She encourages parents to remember how they felt at the same age.

Also, social media and dating apps tend to lower the psychological obstacles to intimacy. With repeated exposure, sexting starts to seem normal to teens.

Many teens who receive or send sexts are normal adolescents. But parents should take it as a prompt to talk with them about healthy relationships and safe sex, says Dr. Temple, who has co-written several peer-reviewed studies on the topic.

Parents can still influence teens’ behavior. But those who respond in a controlling, authoritarian way risk driving teens underground, research shows. Many adolescents use such tools as password-protected photo-storage apps that look like calculators to hide sexts from parents and others.

Parents who take a warmer approach, supporting their teen’s independence while coaching them on staying safe, will likely gain more traction.

Begin teaching children about the risks before they get a smartphone or are exposed to online pornography, says Caroline Knorr, senior parenting editor for Common Sense Media. Before they enter their teens, children should understand the importance of keeping their private parts private and refusing any requests to photograph them—no matter how much social pressure they face from boyfriends, girlfriends or other peers.

With teens, explain that sexually explicit images can come back to haunt them if they’re seen by coaches, colleges or potential employers. Explain the potential legal consequences for juvenile sexting offenders, from community service or remedial education under new laws in some states, to child-pornography charges in others.

Michelle Dennedy uses news stories about sexting to raise the topic with her two daughters, ages 12 and 17, and ask them what they think. When she learned last year that a classmate of her older daughter’s was posting sexually suggestive selfies, Ms. Dennedy, chief privacy officer for Cisco Systems in San Jose, Calif., urged her daughter to encourage the classmate to stop.

She also coaches teens to avoid becoming passive participants in abuse. She was driving her daughter and several other middle schoolers to the mall several years ago when she overheard them talking about a female classmate whose sexy photos were being circulated by a boy she liked without her knowledge. Ms. Dennedy pulled off the road, parked her minivan and turned to face the girls, telling them that such behavior is illegal and urging them to intervene.

“Whatever you have to do to help keep that girl safe, you do it,” she told them. “I want you to have each other’s back on this issue from now on.” White-faced with surprise, the girls agreed, and later offered support and encouragement to the victim.

Write to Sue Shellenbarger at sue.shellenbarger@wsj.com