CAMBRIDGE, Mass. — Three-year-old twins Zyler and Kadyn Sharpe scurried around the boys and girls clothing racks of a narrow consignment store filled with toys. Zyler, wearing rainbow leggings, scrutinized a pair of hot-pink-and-purple sneakers. Kadyn, in a T-Rex shirt, fixated on a musical cube that flashed colorful lights. At a glance, the only discernible difference between these fraternal twins is their hair — Zyler’s is brown and Kadyn’s is blond.

Is Zyler a boy or a girl? How about Kadyn? That’s a question their parents, Nate and Julia Sharpe, say only the twins can decide. The Cambridge, Mass., couple represent a small group of parents raising “theybies” — children being brought up without gender designation from birth. A Facebook community for these parents currently claims about 220 members across the U.S.

“A theyby is, I think, different things to different people,” Nate Sharpe told NBC News. “For us, it means raising our kids with gender-neutral pronouns — so, ‘they,’ ‘them,’ ‘their,’ rather than assigning ‘he,’ ‘she,’ ‘him,’ ‘her’ from birth based on their anatomy.”

Parents in the U.S. are increasingly raising children outside traditional gender norms — allowing boys and girls to play with the same toys and wear the same clothes — though experts say this is happening mostly in progressive, well-to-do enclaves. But what makes this “gender-open” style of parenting stand out, and even controversial in some circles, is that the parents do not reveal the sex of their children to anyone. Even the children, who are aware of their own body parts and how they may differ from others, are not taught to associate those body parts with being a boy or girl. If no one knows a child’s sex, these parents theorize, the child can’t be pigeonholed into gender stereotypes.

This type of parenting received widespread attention in 2011, when a Toronto couple announced that they were raising their child, Storm, without gender designation, sparking a media frenzy. Progressive parents, who see their child’s gender as fluid rather than binary, took notice. A Brooklyn couple runs a blog featuring their 2-year-old, Zoomer, and offering advice on how to navigate the world while raising a “theyby.” Others have taken to Instagram to share photos and support.

Some developmental experts see gender-open parenting as a noble goal, but they also wonder how it will hold up once kids enter a gendered world that can be hostile to those who don’t fit clearly into categories. Gender-nonconforming children are more likely to be bullied. Last year, 10 states considered “bathroom bills” requiring people to use bathrooms aligned with the gender assigned to them at birth (none passed).

“Once your child meets the outer world, which may be day care, or preschool, or grandparents — it's pretty much impossible to maintain a gender-free state,” Lise Eliot, professor of neuroscience at the Chicago Medical School and author of “Pink Brain, Blue Brain,” said in an email. “And depending on how conventional your community is, you could be setting your child up for bullying or exclusion.”

Parents like the Sharpes understand these realities — but they’re determined to shield their children from them for as long as possible.

Deciding to raise a ‘theyby’

The Sharpes, both mechanical engineers in their early 30s, say their decision to raise their twins without designated genders evolved from a mix of research and personal experience. When Julia found out she was pregnant, she felt conflicted about learning the sex of the twins. As a female engineer in a male-dominated profession, she understood the constraints of gender expectations firsthand.

“It's taken a lot of work for me to feel confident in my designs and my suggestions, and to really stand up for myself,” she said.

At first, Nate didn’t understand why Julia wanted to wait to find out the babies’ sex. But after the couple began researching how stereotypes affect a child’s development, he changed his mind.

“We read about how from when they're 20-week fetuses, they're already starting to be gendered, and people are calling the little girls ‘princesses,’ and buying certain things for different children,” Julia said. “We wanted to prevent that, so that's how it started. And then about a couple weeks before they were born, Nate just said, ‘What if we didn't tell people ever?’”