The truth, however, is that every May, I feel like a bit of a hanger-on. Mother's Day is both mine and not mine. I am a mother, but I did not become one in the usual way. My partner Jane is Hannah's birth mother. I am Hannah's other mother, bearing no biological relation to her, but having been wholly present in calling her into being. I celebrate Mother's Day because it is the day designated for mothers and that is what I am, mostly. But not exactly.

Most women become mothers when they give birth. Sure, we can argue, in a pleasantly inclusive way, about what makes a mother a mother. But the reality is that pregnancy and birth are the yardstick most people use. Some women become mothers when they adopt a child. Some women become mothers through marriage. Some take less traveled roads.

When did I become a mother? The day Jane and I first began to imagine having a child, some ten years before we actually did? The evenings I curled up next to Jane on the couch and felt Hannah's kicks in her belly? Or the day I sat in the chair next to the exam table as the sonographer moved the ultrasound transducer across Jane's belly and announced, "It's a girl"? Was it when I spent hours assembling Hannah's crib and bouncy seat, wondering all the while what it would be like to hold her? Or was it at 3:00 a.m. on the morning of March 19 when I saw and touched Hannah for the first time and felt the earth shift on its axis?

Technically, of course, it was none of those times. Technically, I became a mother when I adopted Hannah, about a week after the official Mother's Day, when Hannah was two months old. Until that point, according to the law, she was Jane's alone. Legally, I was as closely related to Hannah as the bagger at the grocery store. And since we used anonymous sperm – who knows? – maybe less. A few states are now starting to acknowledge that gay and lesbian couples who plan together to have a child, prepare together for the child, and work together to take care of the child are both – imagine! – parents. But it is still only a few states and the rest of us, if we are lucky, make do with second parent adoptions and a stack of papers spelling out our connections to our children. Others go on faith, usually because they live in a state where there is no other choice, asserting their motherhood in action, but with no legal proof to back them up.

On that May day, nearly seven years ago, I stood before a judge in Minneapolis, holding Hannah to my chest in a way that I hoped looked indisputably maternal, and affirmed that I understood and accepted the full responsibilities of legal adoption. Yes, I understood that it meant I would have physical and financial responsibility for her. Yes, I understood that, if Jane and I broke up, Hannah could still lay claim to support from me. Yes, I understood that it meant we were each other's for life. In some ways, Adoption Day, as I call it, is more my Mother's Day. It is the day when I officially became Hannah's and she became mine.

"I don't understand how you can have two moms," Hannah's friend Jacob says to her one day as I am driving him home. They have spent the past ten minutes sitting in the back seat of the car giggling about the "M word" – marriage, to those not seven years old – a word they both find so noxious and hilarious that even to utter it brings on fits of giggles.

"I just do," Hannah says in her that's-the-way-it-is voice.

"Do you have a father?" Jacob asks.

"Nope," says Hannah. "Never have and never will."

"Oh," says Jacob and they go back to giggling.

I am both Hannah's mom and her other mom, same and different. Much of what I do on a daily basis looks very similar – ok, identical – to what other moms do and some of it looks like what fathers do. I get her up in the morning and take her to school. I meet her when she gets off the bus. I sit with her while she does her homework. I run next to her while she finds her balance on roller blades. I bring her a bowl of Frosted Mini Wheats for a bedtime snack.

But I remain convinced that being a nonbiological lesbian mom is something distinct unto itself. This may just be my reluctance to become either a dad-substitute or to be subsumed with Jane into one big maternal figure. But I think it's also a desire to name this experience, to assert that it is its own way of being in the world.

Every year, I think about how to celebrate Adoption Day. Should Hannah and Jane and I go for a hike? Go to the corner grill and drink milkshakes? Play Monopoly? Bond? And every year, it slips by us, trumped by the giddiness of spring in Minnesota. Perhaps this is appropriate; like the experience of being a nonbiological lesbian mom, it is both its own unique thing and it is just another day.