The most unusual thing about our family is not that our sons have two mommies. Here in Vermont, that rarely raises eyebrows nowadays. No, here's the part people have trouble bending their minds around: Our sons are part of a connected tribe of 40 half-siblings, scattered all across the country, all conceived with sperm from the same anonymous donor.

When my wife and I were planning our family, we picked an open-ID donor — one who agrees that his identity can be disclosed to offspring at age 18. We didn't have any intention of seeking contact with him ourselves, but we thought our children should have the option if they wanted.

When our firstborn was about 18 months old, we called our sperm bank to start trying for another baby. But we learned that twins born with our donor's sperm had been reported to have a genetic condition. That meant we'd have to talk to the bank's genetic counselor and sign some informed consent waivers before we could place our order.

Unfortunately, the counselor had just embarked on a two-week vacation. 

There we were, reeling, having just learned that (a) somewhere out there, our son had twin half-siblings, (b) something was "wrong" with them genetically, and (c) no one could tell us any details — for two weeks. Patience is not one of my virtues, so I went online and discovered that our sperm bank offered private forums to connect with other families with offspring by the same donor. An exploratory message quickly led to an invitation to a closed Facebook group of two dozen other mamas, all raising kids by a man none of us have ever met.

Initially, it was disorienting to discover that not only did these other families exist, but that there were so many of them. In 2012, there were 20 families; that number has since swelled to 27. Our 40 children — we call them "diblings," or donor siblings — range in age from 6 months to 7 years. There are 28 boys and 12 girls, all being raised in families headed by two women or by single mothers by choice.

Those first few weeks after making contact were mind-boggling. We spent hours poring over pictures of the other kids, marveling over the strong resemblance these children shared with our son. We exchanged a flurry of getting-to-know-you messages, discovering other similarities: how these kids love music and all have the same eyebrows, pointy elf ears and concave toenails; how most of them were early talkers and late walkers; how they all hate eggs. Everybody was so welcoming. And the "genetic problem" that had us so worried was reassuringly explained by the twins' mother — her sons were born premature, had pyloric stenosis, resolved through surgery, and were recovering just fine. When I got pregnant with our second son, I was in good company; two other mamas were pregnant at the same time.

Over time, our connection has expanded beyond Facebook. Our families exchange holiday cards, birth announcements and hand-me-downs. The kids write each other letters and share pictures and videos on the Facebook group. We have a meet-up every summer. The location changes to make it easier for our far-flung group to reunite, though not everyone can make it every year. This June, we're looking forward to spending a week on Long Island with nine other families. We've rented a big house with a pool and a play structure.

Our 3-year-old keeps asking when it will be "the hot times" (summer) so we can go see the diblings. As the kids get older and their contact becomes less parent-directed, I look forward to watching them develop and nurture their own relationships.

In hindsight, though we hadn't planned to contact donor families, I'm grateful for our connection. Now our sons know from their earliest memories that they have a donor dad and donor siblings, and there's no drama surrounding "When should we tell them?" or "How should we tell them?" They don't have to feel alone or isolated because their family doesn't look like their school friends' — they know 38 other kids out there who also have two moms or just one mom and no dad.

Yet for me, the best and most surprising blessing is not the diblings, but their mamas. Before stumbling into this community, I could not have imagined how much these women would come to mean to me. These special ladies are my first resource for parenting advice or commiseration when the "threenager" is getting on my last nerve.

And our sisterhood goes well beyond our common parenting struggle. We support each other through job loss and divorce, health scares and work frustrations. We celebrate each other's pregnancies, promotions, home purchases and other triumphs. We offer each other a safe place to share secrets and vent feelings.

I don't know if our kids will want contact with the donor when they come of age. I can only imagine that his reaction to the discovery that he's spawned this tribe of diblings will likely both mirror and completely dwarf my own shock at how numerous we are. If and when the time comes, I'm confident our group will open our hearts to welcome him into this wild and wonderful extended family with love and gratitude, because he's the one who made it all possible.